LecturePLUS Timberlake1 Atoms and Elements The Atom Atomic Number and Mass Number Isotopes.

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Atoms and Elements

The Atom

Atomic Number and Mass Number

Isotopes

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The Atom

An atom is the smallest particle that retains the

characteristics of an element

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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

The Parts of the AtomAtoms are composed

of three subatomic particles:

• Protons have a positive charge (+)

• Electrons carry a negative charge (-)

• Neutrons are electrically neutral.

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Subatomic Particles

Particle Symbol Charge RelativeMass

Electron e- 1- 0

Proton p+ + 1

Neutron n 0 1

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Location of Subatomic Particles

10-13 cm

electrons

protons

neutrons

10-8 cm

nucleus

The Structure of the Atom The protons and

neutrons, each with a mass of about 1 amu are found in the tiny, dense nucleus.

The electrons are located outside the nucleus.

Models of the Atom

9

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Atomic Number

Counts the number

of

protons

in an atom

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Atomic Number on the Periodic Table

11

Na

Atomic Number

Symbol

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All atoms of an element have the same number of protons

11

Na

11 protons

Sodium

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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

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Periodic Table

1 2 3 4 5 6 7 8

11 Na

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Learning Check

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

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Solution

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

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Number of Electrons

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

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Mass Number

Counts the number

of

protons and neutrons

in an atom

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Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

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Subatomic Particles in Some Atoms

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-

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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

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Learning Check

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 _______ _______ _______

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Solution

12C 13C 14C 6 6 6

#p 6 6 6

#n 6 7 8

#e 6 6 6

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Learning Check

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

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Solution

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

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Learning Check

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- ___________

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Solution

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

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

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

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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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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

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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.

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Atomic Mass on the Periodic Table

11

Na

22.99

Atomic Number

Symbol

Atomic Mass

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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

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Atomic Mass

Atomic mass is the weighted average mass of all the atomic masses of all the isotopes of that atom.

Weighted Average

• You have two types of coins: pennies and quarters.The pennies have a mass of 5g.The quarters  have a mass of 15g.57% of the coins are pennies, 43% of the coins are quarters.What is the average mass of your coins?

34

Weighted Average – Atomic Mass

• You have two isotopes of Boron: 10B and 11B.The 10B has a mass of 10amu.The 11B has a mass of 22 amu.57% of the atoms are 10B, 43% of the atoms are 11B.What is the average mass of your atoms?

35

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Example of an Average Atomic Mass

Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine.

35 x 75.5 = 26.4 100

35.5

37 x 24.5 = 9.07

100

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Problem 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 ________

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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__

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Calculating Atomic Mass

Add :

Percent (%) abundance of each isotope

Multiplied by the

Mass of each isotope of that element

Weighted average = mass isotope1(%) + mass isotope2(%) + …

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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

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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?

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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

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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?

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

Determine Y in terms of X X + Y = 100

Y = 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

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Collect X terms

10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

-1.0 X = -20

-1.0 - 1.0

X = 20 % 10B

Y = 100 - X

% 11B = 100 - 20%

= 80% 11B

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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%

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Solution AT8

2) 70%Solution

X + Y = 100

Y = 100 - X

Solve for X:X (62.9) + (100 - X )(64.9) = 63.5

100 100

Multiply through by 100

62.9X + 6490 - 64.9X = 6350

-2.0 X = -140

X = 70%