# 2.2.5: Average Atomic Mass - Biology We are searching data for your request:

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The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.

Learning Objectives

• Calculate the average atomic mass of an element given its isotopes and their natural abundance

## Key Points

• An element can have differing numbers of neutrons in its nucleus, but it always has the same number of protons. The versions of an element with different neutrons have different masses and are called isotopes.
• The average atomic mass for an element is calculated by summing the masses of the element’s isotopes, each multiplied by its natural abundance on Earth.
• When doing any mass calculations involving elements or compounds, always use average atomic mass, which can be found on the periodic table.

## Key Terms

• mass number: The total number of protons and neutrons in an atomic nucleus.
• natural abundance: The abundance of a particular isotope naturally found on the planet.
• average atomic mass: The mass calculated by summing the masses of an element’s isotopes, each multiplied by its natural abundance on Earth.

The atomic number of an element defines the element’s identity and signifies the number of protons in the nucleus of one atom. For example, the element hydrogen (the lightest element) will always have one proton in its nucleus. The element helium will always have two protons in its nucleus.

## Isotopes

Atoms of the same element can, however, have differing numbers of neutrons in their nucleus. For example, stable helium atoms exist that contain either one or two neutrons, but both atoms have two protons. These different types of helium atoms have different masses (3 or 4 atomic mass units ), and they are called isotopes. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is called the mass number. This is because each proton and each neutron weigh one atomic mass unit (amu). By adding together the number of protons and neutrons and multiplying by 1 amu, you can calculate the mass of the atom. All elements exist as a collection of isotopes. The word ‘isotope’ comes from the Greek ‘isos’ (meaning ‘same’) and ‘topes’ (meaning ‘place’) because the elements can occupy the same place on the periodic table while being different in subatomic construction. ## Calculating Average Atomic Mass

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

Average atomic mass = f1M1 + f2M2 +… + fnMnwhere f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

The average atomic mass of an element can be found on the periodic table, typically under the elemental symbol. When data are available regarding the natural abundance of various isotopes of an element, it is simple to calculate the average atomic mass.

• For helium, there is approximately one isotope of Helium-3 for every million isotopes of Helium-4; therefore, the average atomic mass is very close to 4 amu (4.002602 amu).
• Chlorine consists of two major isotopes, one with 18 neutrons (75.77 percent of natural chlorine atoms) and one with 20 neutrons (24.23 percent of natural chlorine atoms). The atomic number of chlorine is 17 (it has 17 protons in its nucleus).

To calculate the average mass, first convert the percentages into fractions (divide them by 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

Average atomic mass of chlorine = (0.7577 ⋅⋅ 35 amu) + (0.2423 ⋅⋅ 37 amu) = 35.48 amu

Another example is to calculate the atomic mass of boron (B), which has two isotopes: B-10 with 19.9% natural abundance, and B-11 with 80.1% abundance. Therefore,

Average atomic mass of boron = (0.199⋅⋅10 amu) + (0.801⋅⋅11 amu) = 10.80 amu

Whenever we do mass calculations involving elements or compounds (combinations of elements), we always use average atomic masses.

## How Many Protons, Neutrons, and Electrons in an Atom?

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The three parts of an atom are positive-charged protons, negative-charged electrons, and neutral neutrons. Follow these simple steps to find the number of protons, neutrons, and electrons for an atom of any element.

### Key Takeaways: Number of Protons, Neutrons, and Electrons

• Atoms are made of protons, neutrons, and electrons.
• Protons carry a positive electrical change, while electrons are negatively charged, and neutrons are neutral.
• A neutral atom has the same number of protons and electrons (charges cancel each other out).
• An ion has an unequal number of protons and electrons. If the charge is positive, there are more protons than electrons. If the charge is negative, electrons are in excess.
• You can find the number of neutrons if you know the isotope of the atom. Simply subtract the number of protons (the atomic number) from the mass number to find the remaining neutrons.

## Average Atomic Mass Worksheet Answer Key

Click on pop out icon or. 1 three isotopes of silicon occur in nature. Pin On Customize Design Worksheet Online

### Using the equation below we can calculate the average atomic mass for carbon. Average atomic mass worksheet answer key. 1 rubidium is a soft silvery white metal that has two common isotopes 85rb and. Calculating average atomic mass worksheet. Silicon 28 92 23 27 97693 amu silicon 29 4 68 28 97649 amu silicon 30 3 09 29 97377 amu calculate the average atomic mass for the three isotopes of silicon.

The atomic mass of boron is 10 811. C 12011 average mass isotope i x mass isotope isotope 2 mass 2. Calculate the average atomic mass of iodine.

The average atomic mass is the weighted average of all the isotopes of an element. 722 x 85 61 37 278 x 87 24 186 85 556. Atomic number and mass number answer.

The average atomic mass worksheet is a very effective method to learn about the properties of every element that exists in the world. Average atomic mass pogil key pdf. It works by providing a computer based formula that you can input into the form.

What is its average atomic mass. Average atomic mass worksheet. Displaying all worksheets related to atomic number and mass number answer.

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The average atomic mass is calculated from the relative abundance and the masses for these two isotopa. 75 x 133 99 75 20 x 132 26 4 05 x 134 total 132 85 amu average atomic mass determine the average atomic mass of the following mixtures of isotopes. 10 average atomic mass t pdf created date.

Worksheets are chemistry work atomic number and mass number atomic structure atomic numbers practice 1 chapter 2 atoms and atomic molar mass work and key chemistry average atomic mass work he sai. This allows you to see the results of the formula and predict its results based on the information provided in the form. Iodine is 80 127i 17 126i and 3 128i.

Lithium 6 is 4 abundant and lithium 7 is 96 abundant. If the abundance of 85rb is 72 2 and the abundance of 87rb is 27 8 what is the average atomic mass of rubidium. Therefore boron 11 is more abundant because the mass number is closer to the atomic mass.

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## 7.2 The Periodic Table

As early chemists worked to purify ores and discovered more elements, they realized that various elements could be grouped together by their similar chemical behaviors. One such grouping includes lithium (Li), sodium (Na), and potassium (K): These elements all are shiny, conduct heat and electricity well, and have similar chemical properties. A second grouping includes calcium (Ca), strontium (Sr), and barium (Ba), which also are shiny, good conductors of heat and electricity, and have chemical properties in common. However, the specific properties of these two groupings are notably different from each other. For example: Li, Na, and K are much more reactive than are Ca, Sr, and Ba Li, Na, and K form compounds with oxygen in a ratio of two of their atoms to one oxygen atom, whereas Ca, Sr, and Ba form compounds with one of their atoms to one oxygen atom. Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) also exhibit similar properties to each other, but these properties are drastically different from those of any of the elements above.

Dimitri Mendeleev in Russia (1869) and Lothar Meyer in Germany (1870) independently recognized that there was a periodic relationship among the properties of the elements known at that time. Both published tables with the elements arranged according to increasing atomic mass. But Mendeleev went one step further than Meyer: He used his table to predict the existence of elements that would have the properties similar to aluminum and silicon, but were yet unknown. The discoveries of gallium (1875) and germanium (1886) provided great support for Mendeleev’s work. Although Mendeleev and Meyer had a long dispute over priority, Mendeleev’s contributions to the development of the periodic table are now more widely recognized (Figure 1). Figure 1. (a) Dimitri Mendeleev is widely credited with creating (b) the first periodic table of the elements. (credit a: modification of work by Serge Lachinov credit b: modification of work by “Den fjättrade ankan”/Wikimedia Commons)

By the twentieth century, it became apparent that the periodic relationship involved atomic numbers rather than atomic masses. The modern statement of this relationship, the periodic law, is as follows: the properties of the elements are periodic functions of their atomic numbers. A modern periodic table arranges the elements in increasing order of their atomic numbers and groups atoms with similar properties in the same vertical column (Figure 2). Each box represents an element and contains its atomic number, symbol, average atomic mass, and (sometimes) name. The elements are arranged in seven horizontal rows, called periods or series, and 18 vertical columns, called groups. Groups are labeled at the top of each column. In the United States, the labels traditionally were numerals with capital letters. However, IUPAC recommends that the numbers 1 through 18 be used, and these labels are more common. For the table to fit on a single page, parts of two of the rows, a total of 14 columns, are usually written below the main body of the table. Figure 2. Elements in the periodic table are organized according to their properties.

Many elements differ dramatically in their chemical and physical properties, but some elements are similar in their behaviors. For example, many elements appear shiny, are malleable (able to be deformed without breaking) and ductile (can be drawn into wires), and conduct heat and electricity well. Other elements are not shiny, malleable, or ductile, and are poor conductors of heat and electricity. We can sort the elements into large classes with common properties: metals (elements that are shiny, malleable, good conductors of heat and electricity—shaded yellow) nonmetals (elements that appear dull, poor conductors of heat and electricity—shaded green) and metalloids (elements that conduct heat and electricity moderately well, and possess some properties of metals and some properties of nonmetals—shaded purple).

The elements can also be classified into the main-group elements (or representative elements) in the columns labeled 1, 2, and 13–18 the transition metals in the columns labeled 3–12 and inner transition metals in the two rows at the bottom of the table (the top-row elements are called lanthanides and the bottom-row elements are actinides Figure 3). The elements can be subdivided further by more specific properties, such as the composition of the compounds they form. For example, the elements in group 1 (the first column) form compounds that consist of one atom of the element and one atom of hydrogen. These elements (except hydrogen) are known as alkali metals, and they all have similar chemical properties. The elements in group 2 (the second column) form compounds consisting of one atom of the element and two atoms of hydrogen: These are called alkaline earth metals, with similar properties among members of that group. Other groups with specific names are the pnictogens (group 15), chalcogens (group 16), halogens (group 17), and the noble gases (group 18, also known as inert gases). The groups can also be referred to by the first element of the group: For example, the chalcogens can be called the oxygen group or oxygen family. Hydrogen is a unique, nonmetallic element with properties similar to both group 1 and group 17 elements. For that reason, hydrogen may be shown at the top of both groups, or by itself. Figure 3. The periodic table organizes elements with similar properties into groups.

Click on this link for an interactive periodic table, which you can use to explore the properties of the elements (includes podcasts and videos of each element). You may also want to refer to Figure 1above that shows photos of all the elements.

## Average Atomic Mass Pogil Answers Model 4

Isoto e natural abundance on earth 0 0 atomic mass am u 16 00. Most elements have more than one naturally occurring isotope. This Is How My Chemistry Teacher Teaches Dimensional Analysis This Is Going To Be An Intere Dimensional Analysis Physical Science Lessons Chemistry Teacher

### Show all of your work and check your answer against the mass listed on the periodic table. Average atomic mass pogil answers model 4. Page 1 of 4. Pogil average atomic mass names. Consider the natural abundance information given in model 2.

Show all of your work and check your answer against the mass listed on the periodic table. Helium has two naturally occurring isotopes helium 3 and helium 4. Kindly say the average atomic mass pogil answers is universally compatible with any devices to read if your library doesn t have a subscription to overdrive or.

Show all of your work and check your answer against the mass listed on the periodic table. 24 25 26 3. Answer 78 99 23 9850 amu 10 00 24 9858 amu 11 01 25 9826 amu 100 78 99 23 9850 amu 100 10 00 24 9858 amu 100 11 01 25 9826 amu 100 15.

How are masses on the periodic table determined. Isotope information is provided below. Isoto e natural abundance on earth 0 0 atomic mass am u 16 00 160 170 180.

Our books collection hosts in multiple locations allowing you to get the most less latency time to download any of our books like this one. Answer 24 mg 8 25 mg 1 26 mg 1 model 2 natural abundance information for magnesium isotope natural abundance on earth atomic mass amu 24 mg 78 99 23 9850 25 mg 10 00 24 9858 26 mg 11 01 25 9826 7. Displaying average atomic mass pogil key pdf.

Use one of the methods in model 3 that gave the correct answer for average atomic mass to calculate the average atomic mass for oxygen. Use one of the methods in model 3 that gave the correct answer for average atomic mass to calculate the average atomic mass for oxygen. Isotope information is provided below.

Use one of the methods in model 3 that gave the correct answer for average atomic mass to calculate the average atomic mass for oxygen. What are the mass numbers of the naturally occurring isotopes of magnesium shown in model 1. Isotope natural abundance on earth atomic mass amu 160 99 76 15 9949.

Consider the natural abundance information given in model 2. For the methods in model 3 that gave the correct answer for average atomic mass show that they are mathematically equivalent methods. Average atomic mass pogil answers instantly.

Do all the atoms of magnesium in model 1 have the same atomic mass and explain why. What is the atomic number for this element. No because some of the atoms have different masses than.

Isotope information is provided below. This activity will help you answer the question. As you learned previously the atoms of those isotopes have the same atomic number number of protons making them belong to the same element but they have different mass numbers total number of protons and neutrons giving them different.

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### Extended Data Fig. 1 Purification and characterization of BM2(1–51).

a, Amino acid sequences of the TM domain of AM2 and BM2. The conserved proton-selective histidine and the gating tryptophan are shown in red. The other pore-lining heptad a and d residues are polar in BM2 and hydrophobic in AM2 (blue). b, SDS-PAGE gel showing Ni 2+ -affinity purification of SUMO-BM2. The flow through contains all soluble cellular proteins with low affinity for Ni 2+ . The column was washed with 50 mM imidazole, and SUMO-BM2 (18 kDa band) was eluted in two fractions at >90% purity with 300 mM imidazole. c, Analytical reverse-phase HPLC chromatogram of BM2 before (black) and after (red) protease cleavage of the SUMO tag to give native BM2 at an elution time of 11.2 min. d, MALDI mass spectrum of purified BM2(1-51), showing excellent agreement between the observed mass and the theoretical mass. e, Circular dichroism spectrum of BM2 in 0.5% n-dodecylphosphocholine solution at pH 7.5. Spectral deconvolution indicates 60% α-helicity and 40% disordered or turn structures. f, LC-MS total ion chromatogram of purified 4- 19 F-Phe5, 4- 19 F-Phe20 labeled synthetic BM2(1-51), showing excellent purity. g, Deconvolution of extracted ion chromatogram of purified 4- 19 F-Phe5, 4- 19 F-Phe20 BM2. The measured molecular weight is in excellent agreement with the expected molecular weight.

### Extended Data Fig. 2 Resonance assignment and inter-residue correlations of membrane-bound BM2 at pH 4.5.

a, Representative strips of the NCACX (orange) and NCOCX (blue) regions of the 3D NCC spectrum to obtain sequential resonance assignment. The spectrum was measured at Tsample = 280 K. b, Representative F2-F3 planes of the 3D CCC spectrum, showing various inter-residue correlations (assigned in red) that restrain the structure. The spectrum was measured using spin diffusion mixing times of 41 ms and 274 ms, at Tsample = 280 K. c, 2D 13 C- 13 C TOCSY spectrum with 7.7 ms mixing, collected at Tsample = 290 K. Residues 43–51 are dynamic and exhibit chemical shifts indicative of random coil conformation. d, 1D 13 C cross-polarization (CP) spectrum preferentially detects immobilized residues while the 13 C INEPT spectrum preferentially detects highly dynamic residues. These 1D spectra were measured at Tsample = 280 K.

### Extended Data Fig. 3 Resonance assignment and inter-residue correlations of membrane-bound BM2 at pH 7.5.

a, 2D 13 C- 13 C correlation spectrum with 55 ms CORD spin diffusion, measured at Tsample = 280 K. b, Representative F2-F3 strips from the 3D CCC spectrum, showing various inter-residue correlations (assigned in red) that restrain the structure. The spectrum was measured using spin diffusion mixing times of 41 ms and 274 ms, at Tsample = 280 K. c, 2D 13 C- 13 C TOCSY spectrum with 7.7 ms mixing, collected at Tsample = 290 K. Residues 43–51 are dynamic and exhibit chemical shifts indicative of random coil conformation. d, 13 C CP spectrum preferentially detects immobilized residues while the 13 C INEPT spectrum preferentially detects highly dynamic residues. These 1D spectra were measured at Tsample = 280 K.

### Extended Data Fig. 4 Secondary structure of BM2 in the closed (high pH) and open (low pH) states.

a, Cα (black) and Cβ (magenta) secondary chemical shifts at pH 7.5 and pH 4.5. b, Chemical-shift derived (ϕ, ψ) torsion angles at pH 7.5 (black) and pH 4.5 (orange). At both pH, the TM domain is α-helical while the cytoplasmic tail is mostly disordered. In addition, a short β-strand segment is present at low pH. c, Helical wheel representations of residues 48-63 in AM2 and the corresponding residues 29-44 in BM2. Hydrophobic residues are colored green, polar residues black, positively charged residues blue, and negatively charged residues red. AM2 has a separate hydrophobic face and a hydrophilic face, indicative of an amphipathic helix, while BM2 has alternating polar and non-polar residues, consistent with a β-strand conformation. d, Static 31 P spectra of BM2-containing POPE membrane at high and low pH and POPC/POPG membranes at low pH, all measured at a sample temperature of 303 K. At high pH the POPE membrane consists of

35% hexagonal phase. At low pH BM2 converts most of the POPE membrane to the hexagonal phase, but retains the lamellar form for the POPC/POPG membrane. Green dashed line is a superposition of 35% of the pH 4.5 POPE spectrum and 65% of the pH 4.5 POPC: POPG spectrum.

### Extended Data Fig. 5 BM2 has similar conformations in POPE and POPC:POPG bilayers.

a, 2D 13 C- 13 C CORD spectra of BM2 in the two lipid membranes at low pH. b, 2D 15 N- 13 C correlation spectra of BM2 in the two lipid membranes at low pH. The POPE sample was measured at Tsample = 290 K for the 2D NC spectrum and 280 K for the 2D CC spectrum, while the POPC: POPG sample was measured at Tsample = 270 K to account for the lower phase transition temperature of this membrane. The lipid bilayers of both samples were in the gel phase, as assessed by 1 H spectra of the sample. Both spectra were measured under 14 kHz MAS on an 800 MHz spectrometer. c, Chemical shift differences between the POPE and POPC:POPG samples at low pH. Residues in the α-helical TM domain and the β-strand do not show significant chemical shift differences.

### Extended Data Fig. 6 Measurement of BM2 helix orientation using rotationally averaged 15 N- 1 H dipolar couplings.

a,b, N-H DIPSHIFT data of the tripeptide formyl-MLF, measured at Tsample = 315 K using (a) 15 N detection and (b) 13 C detection. The dipolar-doubled version of DIPSHIFT is used in these experiments. The 15 N-detected DIPSHIFT data were analyzed using the total intensities from the centerband and sidebands. The 13 C-detected N-H couplings used a 15 N- 13 C TEDOR mixing time of 2.11 ms. The 13 C-detected N-H couplings are 0.9 times the 15 N-detected values, indicating incomplete powder averaging. This scaling factor was included in determining the BM2 orientation from 13 C-detected N-H dipolar couplings. c, Calculated 15 N- 1 H dipolar waves as a function of the helix tilt angle. An 18-residue ideal α-helix with (ϕ, ψ) angles of (-65˚, -40˚) were tilted from an external axis by 0°–30°. The 15 N- 1 H dipolar couplings show the expected sinusoidal oscillations with a periodicity of 3.6 residues. The amplitude and offset of the dipolar wave indicate the helix tilt angle. d, Reduced χ 2 values of the measured and simulated 15 N- 1 H dipolar couplings of membrane-bound BM2 at high and low pH. The minimum χ 2 value is found at a tilt angle of 14˚ for high-pH BM2 and 20˚ for low-pH BM2. The ±2˚ uncertainty represents one standard deviation.

### Extended Data Fig. 7 13 C- 19 F REDOR data for measuring interhelical distances of BM2 at high pH (black curves and filled symbols) and low pH (orange curves and open symbols).

The high pH data were measured at a sample temperature (Tsample) of 273 K, while the low pH data were measured at 261 K. Additional high-pH data measured at Tsample = 261 K (red symbols in some of the panels) are indistinguishable from 273 K data, confirming that the protein is immobilized at both temperatures. a, N-terminal residues that are dephased by 4F-Phe5. b, C-terminal residues whose dephasing is attributed to 4F-Phe20. All sites show less dephasing for the low-pH sample than the high-pH sample, indicating longer distances for the open channel. P4 has negligible dephasing at low pH. c, Representative χ 2 as a function of 13 C- 19 F distance, showing the extraction of the best-fit distances and uncertainties. d, Aromatic region of representative 13 C- 19 F REDOR spectra of BM2 at high pH. The difference spectrum (∆S) shows no dephasing for the 119-ppm W23 Cε3/ζ3/η2 peak (blue dashed line), indicating that 4F-Phe20 of the neighboring helix is far from these indole carbons. e, This is consistent with a W23 rotamer of t90 (χ1 = −125°, χ2 = 98°) but inconsistent with the mt rotamer (χ1 = -80°, χ2 = -177°).

### Extended Data Fig. 8 HxxxW motif rotamers and comparison of the closed BM2 structures in lipid bilayers versus detergent micelles.

a, Structural ensembles of H19 and W23 in the conserved HxxxW conduction motif at high pH (left) and low pH (right). The H19 χ1 is trans but the χ2 is not constrained well by experimental data. W23 predominantly adopts the t90 rotamer in both closed and open structural ensembles. b,c, Comparison of the high-pH BM2 TM structure in lipid bilayers versus in detergent micelles. b, Solid-state NMR structure determined here in POPE membranes. c, Solution NMR structure determined in DHPC micelles 1 .

### Extended Data Fig. 9 Hydration of membrane-bound BM2.

a, Aliphatic region of the 13 C spectra measured with 100 ms (black) and 2 ms (red) 1 H polarization transfer from water to the protein, measured at Tsample = 273 K. The low-pH protein shows higher intensities, indicating higher water accessibility. b, Aromatic region of the 13 C spectra also show significantly higher water-transferred intensities at low pH than high pH. c, Water-to-protein polarization transfer curves for various residues. The buildup rates are faster at low pH (orange) than at high pH (black). d, 1D 15 N CP spectra of the H19 and H27 side chains of BM2 at high and low pH, measured at Tsample = 280 K. The imidazole 15 N signals are shifted 8–9 ppm downfield at low pH compared to high pH, indicating increased protonation of the histidines. e, Control 2D 13 C- 13 C correlation spectrum, measured using a 1 H- 1 H spin diffusion time of 100 ms to allow water magnetization to equilibrate with the protein. The spectra were measured at Tsample = 273 K.

### Extended Data Fig. 10 Pulse sequences of key 2D and 3D correlation experiments used for determining the structures of closed and open BM2 channels.

a, 3D CCC experiment. The first 13 C spin diffusion period is short to obtain intra-residue correlations while the second is long to obtain inter-residue cross peaks. b, Water-edited 2D CC experiment. A selective 90˚ pulse excites the water 1 H magnetization, a 1 H T2 filter removes the rigid protein magnetization, then the water magnetization is transferred to the protein. Filled and open rectangles indicate 90° and 180° pulses, respectively. c, 3D NCC experiment involving an out-and-back 15 N- 13 C TEDOR period followed by 13 C spin diffusion. The experiment simultaneously detects NCACX and NCOCX correlations. d, Frequency-selective 13 C- 19 F REDOR for distance measurements. e, 3D NC-resolved N-H dipolar-doubled DIPSHIFT experiment for measuring helix orientations.

An average atomic mass is defined as the average mass of all isotopes within a given substance.

How to calculate average atomic mass?

First, determine the fractional percent of each isotope in the substance

For example, chlorine has two major isotopes. 1 with 75.77 percent of atoms and 1 with 24.23 percent of atoms. These two percentages would be the fractional percents of those isotopes.

Using the same example above, this would be 35 and 37 amu respectively.

Calculate the average atomic mass using the information from steps 1 and 2 and the formula above.

The average atomic mass is the average mass of all of the isotopes that make up a substance.

The fractional percent is the total percentage of a particular isotope in a substance.

## Calculating Atomic Mass

You can calculate the atomic mass (or average mass) of an element provided you know the relative abundance (the fraction of an element that is a given isotope), the element's naturally occurring isotopes, and the masses of those different isotopes. We can calculate this by the following equation:

[ ext = left( \%_1 ight) left( ext_1 ight) + left( \%_2 ight) left( ext_2 ight) + cdots]

Look carefully to see how this equation is used in the following examples.

Example (PageIndex<1>): Boron Isotopes

Boron has two naturally occurring isotopes. In a sample of boron, (20\%) of the atoms are (ce)-10, which is an isotope of boron with 5 neutrons and mass of (10 : ext). The other (80\%) of the atoms are (ce)-11, which is an isotope of boron with 6 neutrons and a mass of (11 : ext). What is the atomic mass of boron?

Boron has two isotopes. We will use the equation:

[ ext = left( \%_1 ight) left( ext_1 ight) + left( \%_2 ight) left( ext_2 ight) + cdots onumber]

• Isotope 1: (\%_1 = 0.20) (Write all percentages as decimals), ( ext_1 = 10)
• Isotope 2: (\%_2 = 0.80), ( ext_2 = 11)

Substitute these into the equation, and we get:

[ ext = left( 0.20 ight) left( 10 ight) + left( 0.80 ight) left( 11 ight) onumber]

The mass of an average boron atom, and thus boron's atomic mass, is (10.8 : ext).

Example (PageIndex<2>): Neon Isotopes

Neon has three naturally occurring isotopes. In a sample of neon, (90.92\%) of the atoms are (ce)-20, which is an isotope of neon with 10 neutrons and a mass of (19.99 : ext). Another (0.3\%) of the atoms are (ce)-21, which is an isotope of neon with 11 neutrons and a mass of (20.99 : ext). The final (8.85\%) of the atoms are (ce)-22, which is an isotope of neon with 12 neutrons and a mass of (21.99 : ext). What is the atomic mass of neon?

Neon has three isotopes. We will use the equation:

[ ext = left( \%_1 ight) left( ext_1 ight) + left( \%_2 ight) left( ext_2 ight) + cdots onumber]

• Isotope 1: (\%_1 = 0.9092) (write all percentages as decimals), ( ext_1 = 19.99)
• Isotope 2: (\%_2 = 0.003), ( ext_2 = 20.99)
• Isotope 3: (\%_3 = 0.0885), ( ext_3 = 21.99)

Substitute these into the equation, and we get:

[ ext = left( 0.9092 ight) left( 19.99 ight) + left( 0.003 ight) left( 20.99 ight) + left( 0.0885 ight) left( 21.99 ight) onumber]

The mass of an average neon atom is (20.17 : ext)

The periodic table gives the atomic mass of each element. The atomic mass is a number that usually appears below the element's symbol in each square. Notice that the atomic mass of boron (symbol (ce)) is 10.8, which is what we calculated in Example (PageIndex<1>), and the atomic mass of neon (symbol (ce)) is 20.8, which is what we calculated in Example (PageIndex<2>). Take time to notice that not all periodic tables have the atomic number above the element's symbol and the mass number below it. If you are ever confused, remember that the atomic number should always be the smaller of the two and will be a whole number, while the atomic mass should always be the larger of the two and will be a decimal number.

Chlorine has two naturally occurring isotopes. In a sample of chlorine, (75.77\%) of the atoms are (ce)-35, with a mass of (34.97 : ext). Another (24.23\%) of the atoms are (ce)-37, with a mass of (36.97 : ext). What is the atomic mass of chlorine?

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