1

The Atom

Atomic Number

Mass Number

Isotopes

Adapted from Timberlake’s CheModules (www. Karentimberlake.com/chemodule.htm)

2

Atomic Theory

Atoms are building blocks of elements

Similar atoms in each element

Different from atoms of other elements

Two or more different atoms bond in simple

ratios to form compounds

3

Subatomic Particles

Particle Symbol Charge RelativeMass

Electron e- 1- 0

Proton p+ + 1

Neutron n 0 1

4

Location of Subatomic Particles

electrons

protons

neutrons

nucleus

5

Atomic Number

Counts the number

of

protons

in an atom

6

Atomic Number on the Periodic Table

11

Na

Atomic Number

Symbol

7

All atoms of an element have the same number of protons

11

Na

11 protons

Sodium

8

Learning Check AT 1

State the number of protons for atoms of each of the following:

A. Nitrogen

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

B. Sulfur

1) 32 protons 2) 16 protons 3) 6 protons

C. Barium

1) 137 protons 2) 81 protons 3) 56 protons

9

Number of Electrons

An atom is neutral The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons

10

Mass Number

Counts the number

of

protons and neutrons

in an atom

*Mass number is not on the periodic table

11

Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

12

More Atomic Symbols

16 31 65

O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

13

Isotopes

Atoms with the same number of protons,

but different numbers of neutrons.

Atoms of the same element (same atomic

number) with different mass numbers

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37

14

Learning Check AT 2

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

12C 13C 14C 6 6 6

#P _______ _______ _______

#N _______ _______ _______

#E _______ _______ _______

15

Learning Check AT 3

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom

1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons?

1) 37 2) 65 3) 67

16

Learning Check AT 4

Write the atomic symbols for atoms with the following:

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

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

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

17

Learning Check AT 5

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is

1) 34X 2) 34X 3) 36X 16 14 14

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Masses of Atoms

A scale designed for atoms gives their small

atomic masses in atomic mass units (amu)

An atom of 12C was assigned an exact mass of

12.00 amu

Relative masses of all other atoms was

determined by comparing each to the mass of 12C

An atom twice as heavy has a mass of 24.00

amu. An atom half as heavy is 6.00 amu.

19

Atomic Mass

Sometimes listed as “atomic weight”

Listed on the periodic table

Gives the mass of “average” atom of each

element compared to 12C

Average atom based on all the isotopes and their

abundance %

Atomic mass is not a whole number

Na22.99

20

Learning Check AT6

Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

21

Calculating Average Atomic Mass

You need to know…

Percent(%) abundance of isotopes

Mass of each isotope

Weighted average =

(mass of isotope 1)(% abundance/100) +(mass

of isotope 2)(% abundance/100) + …

22

Atomic Mass of Magnesium

Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

(.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

23

Learning Check AT7

Gallium is a metallic element found in

small lasers used in compact disc players.

In a sample of gallium, there is 60.2% of

gallium-69 (68.9 amu) atoms and 39.8% of

gallium-71 (70.9 amu) atoms. What is the

average atomic mass of gallium?

24

Solution AT7

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga

100

Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for 71Ga

100

Atomic mass Ga = 69.7 amu