Atomic mass

LecturePLUS Timberlake 1

The Atom

Atomic Number and Mass Number

Isotopes


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


Subatomic Particles

Particle Symbol Charge RelativeMass

Electron e- 1- 0

Proton p+ + 1

Neutron n 0 1


Location of Subatomic Particles

10-13 cm

electrons

protons

neutrons

10-8 cm

nucleus


Atomic Number

Counts the number

of

protons

in an atom


Periodic Table

Represents physical and chemical

behavior of elements

Arranges elements by increasing atomic

number

Repeats similar properties in columns

known as chemical families or groups


Periodic Table

1 2 3 4 5 6 7 8

11 Na


Atomic Number on the Periodic Table

11

Na

Atomic Number

Symbol


All atoms of an element have the same number of protons

11

Na

11 protons

Sodium


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


C. Barium



Solution AT 1

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

A. Nitrogen

2) 7 protons

B. Sulfur

2) 16 protons

C. Barium

3) 56 protons


Number of Electrons

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


Mass Number

Counts the number

of

protons and neutrons

in an atom


Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11


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-


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


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 _______ _______ _______


Solution AT 2

12C 13C 14C 6 6 6

#P __6___ _ 6___ ___6___

#N __6___ _ _7___ ___8___

#E __6___ _ 6___ ___6___


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


Solution AT 3

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30

B. Number of neutrons in the zinc atom

2) 35

C. What is the mass number of a zinc isotope

with 37 neutrons?

3) 67


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-___________


Solution AT 4

16OA. 8 p+, 8 n, 8 e- 8

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

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


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


Solution AT 5

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

1) 14

B. It has a mass number of3) 34

C. The element is1) Si

D. Another isotope of this element would be

3) 36X 14


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.


Atomic Mass

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


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 __________


Solution AT6

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

A. calcium _40.1 amu _

B. aluminum _27.0 amu _

C. lead _207.2 amu_

D. barium _137.3 amu_

E. iron _55.8 amu__


Calculating Atomic Mass

Percent(%) abundance of isotopes

Mass of each isotope of that element

Weighted average =

mass isotope1(%) + mass isotope2(%) + …

100 100


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%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3


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

atomic mass of gallium?


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


Finding An Isotopic Mass

A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?


Assign X and Y values:X = % 10B Y = % 11B

Determine Y in terms of XX + Y = 100Y = 100 - X

Solve for X:X (10.0) + (100 - X )(11.0) = 10.8

100 100

Multiply through by 10010.0 X + 1100 - 11.0X = 1080


Collect X terms 10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B - 1.0Y = 100 - X

% 11B = 100 - 20% = 80% 11B


Learning Check AT8

Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check periodic table for atomic mass)

1) 30% 2) 70% 3) 100%


Solution AT8

2) 70%

Solution

62.9X + 6490 = 64.9X = 6350

-2.0 X = -140

X = 70%

Technology

Atomic mass