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2.3-2.5Isotopes and Atomic Weighs

24Mg 25Mg 26Mg 12 12 12Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Chapter 2 Atoms and Molecules

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Isotopes

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

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

Isotopes

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A nuclear 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.

Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol:

mass number 23 Na

atomic number 11

Nuclear Symbol

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From the nuclear symbol, we can determine the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom.

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-

Information from Nuclear Symbols

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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 ______ ______ ______

Learning Check

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12C 13C 14C 6 6 6

protons 6 p+ 6 p+ 6 p+

neutrons 6 n 7 n 8 n

electrons 6 e- 6 e- 6 e-

Solution

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Write the nuclear 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- ___________

Learning Check

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A. 8 p+, 8 n, 8 e- 16O

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B. 17p+, 20 n, 17e- 37Cl 17

C. 47p+, 60 n, 47 e- 107Ag 47

Solution

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Learning Check

1. Which of the following pairs are isotopes of the same element?2. In which of the following 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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Solution

B. 12X 14X

6 6

Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.

C. 15X 16X

7 8

An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.

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Isotopes of Magnesium

In naturally occurring magnesium, there are three isotopes.

24Mg 25Mg 26Mg 12 12 12

Isotopes of Mg

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Isotopes of Sulfur

A sample of naturallyoccurring sulfur containsseveral isotopes with thefollowing abundances

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

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

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Relative Masses of Atoms

Use atomic weights of the elements to calculate molecular weights (MW) of compounds

Relative masses provide a simple way of comparing the masses of atoms. Ex. Mass of neon atoms is 20.18 and Mass of calcium atoms is 40.08.

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Ca atom massNe atom mass

= 40.0820.18

= 1.986 Mass of Calcium is 2x than Ne

He atom mass H atom mass

= 4.0031.008

= 3.971 Mass of Helium is 4x than Hydrogen

The exact relationship between two masses calculated

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Definitions

Atomic mass unit (amu) is a unit used to express the relative masses of atoms.

Atomic Weight is the mass of an average atom of an element expressed in atomic mass units

Molecular Weight is the relative mass of a molecule expressed in atomic mass units and calculated by adding together the atomic weights of the atoms in the molecule.

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Calculating the atomic weight of compounds

MW = CnHmOk

MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)

H2O the MW is

MW = 2(at. Wt. H) + 1(at. Wt. O)

MW = 2(1.008 u) + 1(15.996 u)

MW = 18.012 u or 18.01 u for water

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Use atomic weighs from the periodic table inside the front cover to determine the molecular weight of urea, CH4N2O, the chemical form in which much nitrogenous body waste is excreted in the urine.

Learning Check

a. 58.02 u

b. 62.25 u

c. 60.06 u

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Solution

MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)

Urea chemical formula is CH4N2O,

MW = 1(12.01 u) + 4(1.008) + 2(14.01) + 1(16.00)

MW = 60.062 u or rounded off 60.06 u

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Atomic 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.

Na22.99

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Isotopes of Some Elements and Their Atomic Mass

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

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Atomic 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.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Using the periodic table, specify the atomic mass of

each element.

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check

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Using the periodic table, specify the atomic mass of

each element:

A. calcium 40.08 amu

B. aluminum 26.98 amu

C. lead 207.2 amu

D. barium 137.3 amu

E. iron 55.85 amu

Solution