Chapter 3 Atoms and Elements 3.5 Atomic Number and Mass Number 1

Chapter 3 Chapter 3 Atoms and Atoms and Elements Elements

3.5Atomic Number and Mass Number

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Atomic NumberAtomic Number

The atomic number

• is specific for each element.

• is the same for all atoms of an element.

• is equal to the number of protons in an atom.

• appears above the symbol of an element.

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Atomic Number and ProtonsAtomic Number and Protons

Examples of atomic number and number of protons:

• Hydrogen has atomic number 1; every H atom has one proton.

• Carbon has atomic number 6; every C atom has six protons.

• Copper has atomic number 29; every Cu atom has 29 protons.

• Gold has atomic number 79; every Au atom has 79 protons.

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ExamplesExamples

State the number of protons in each. A. A nitrogen atom

1) 5 protons 2) 7 protons 3) 14 protons

B. A sulfur atom 1) 32 protons 2) 16 protons 3) 6 protons

C. A barium atom 1) 137 protons 2) 81 protons 3) 56 protons

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Electrons in An AtomElectrons in An Atom

An atom

• of an element is electrically neutral; the net charge of an atom is zero.

• has an equal number of protons and electrons. number of protons = number of

electrons

Aluminum has 13 protons and 13 electrons. The net

(overall) charge is zero. 13 protons (13+) + 13 electrons (13 -) = 0

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Mass NumberMass Number

The mass number

• represents the number of particles in the nucleus.

• is equal to the number of protons + the number of neutrons.

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Atomic ModelsAtomic Models

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ExamplesExamples

An atom of zinc has a mass number of 65.A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom? 1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that has

37 neutrons? 1) 37 2) 65 3) 67

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ExamplesExamples

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is

1) 14 2) 16 3) 34

C. The element is

1) Si 2) Ca 3) Se

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3.6 Isotopes and Atomic 3.6 Isotopes and Atomic MassMass

Isotopes

• are atoms of the same element that have different mass numbers.

• have the same number of protons, but different numbers of neutrons.

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Atomic SymbolAtomic Symbol

An atomic symbol

• represents a particular atom of an element.

• gives the mass number in the upper left corner and the atomic number in the lower left corner.

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Z

A

X

X A

Atomic number

Mass number

or Element symbol

Information from Atomic Information from Atomic SymbolsSymbols

The atomic symbol for a specific atom of an element

gives the

• number of protons (p+),

• number of neutrons (n),

• and number of electrons (e-).

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Information from Atomic Information from Atomic SymbolsSymbols

Examples of number of subatomic particles for atoms

Atomic symbol

16 31 65

O P Zn 8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n 8 e- 15 e- 30 e-

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ExamplesExamples

Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following:

12C 13C 14C 6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

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ExamplesExamples

Write the atomic symbols for atoms with the following subatomic particles:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

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ExamplesExamples

1. Which of the pairs are isotopes of the same element?

2. In which of the pairs do both atoms have 8 neutrons?

A. 15X 15X 8 7

B. 12X 14X 6 6

C. 15X 16X 7 8

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Isotopes of MagnesiumIsotopes of Magnesium

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TABLE 3.7

Isotopes of SulfurIsotopes of Sulfur

A sample of naturallyoccurring sulfur containsseveral isotopes with thefollowing abundancesIsotope %

abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02

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32S, 33S, 34S, 36S16 16 16 16

Atomic MassAtomic Mass

The atomic mass of an element

• is listed below the symbol of each element on the periodic table.

• gives the mass of an “average” atom of each element compared to 12C.

• is not the same as the mass number.

• has the smallest unit of amu

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Na22.99

Isotopes of Some Elements and Isotopes of Some Elements and Their Atomic MassTheir Atomic Mass

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Most elements have two or more isotopes that contribute to the atomic mass of that element.8

ExamplesExamples

Using the periodic table, specify the atomic mass ofeach element.

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

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Calculating Atomic MassCalculating Atomic Mass

The calculation for atomic mass requires the

• percent(%) abundance of each isotope.

• atomic mass of each isotope of that element.

• sum of the weighted averages.

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Average massor Atomic mass(amu)

= isotoptic mass 1 x % abundance + isotopic mass 2 x % abundance +......isotopic mass n x % abundance

Atomic MassesAtomic Masses

carbon-12: 98.89 % natural abundance 12 amu

carbon-13: 1.11 % natural abundance 13.0034 amu

Why is the atomic mass of the element carbon 12.01 amu?

= 12.01 amu

mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111)

= 11.87 amu + 0.144 amu

Atomic Mass for ClAtomic Mass for Cl

The atomic mass of chlorine is

• due to all the Cl isotopes.

• not a whole number.• the average of two

isotopes: 35Cl and 37Cl.

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Calculating Atomic Mass for Calculating Atomic Mass for ClCl

35Cl has atomic mass 34.97 amu (75.76%) and 37C

has atomic mass 36.97 amu (24.24%). • Use atomic mass and percent of each

isotope to calculate the contribution of each isotope to the weighted average. Atomic mass 35Cl x % abundance =Atomic mass 37Cl x % abundance =

• Sum is atomic mass of Cl is

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ExamplesExamples

Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.

What is the atomic mass of gallium?

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