1

Atomic Structure

What is an atom?

The smallest particle of an

element that retains

properties of that element.

“Gedanken”

2

Atomic Theory

Democritus vs. Dalton

•500 BC •1808 AD

•Pure

thought

•Observation

+ experiment

Matter is like “Legos.” It is not continuous! Weird.

3

Dalton’s Atomic Theory

Elements composed of atoms

Atoms of element are identical*

Compounds: simple ratios of

atoms (H2O)

Chem reaction: rearrangement,

not creation, of atoms

4

Atomic Size

A penny has 2.4 x 1022 Cu atoms= 24 000 000 000 000 000 000 000.

Vs.

Earth’s population of 6 x 109

= 6 000 000 000.

5

Subatomic Particles

Atom comprised of 3 particles:

• electron e-

• proton p+

• neutron n

6

ElectronJ. J. Thomson: 1897

7

ProtonE. Goldstein: 1886

Atoms are neutral, so there must

be a + particle: proton (p+).

p+ is 1840x more massive than e-

p+e-

8

NeutronJ. Chadwick: 1932

Neutron: neutral particle nearly

same mass as proton.

Why did it take so long to

discover the neutron?

9

Mass Units

Mass p+= 1.673 x 10-24 g

Mass n = 1.675 x 10-24 g

Definition: “atomic mass unit”

1 amu = 1/12th mass of carbon

atom that contains 6 p+ & 6 n

~1.0

amu

10

Subatomic Particles

ChargeMass

(g) (amu)

p+ +1 1.673 x 10-24 1

n 0 1.675 x 10-24 1

e- -1 9.11 x 10-28 1

1840

11

Atomic StructureE. Rutherford: 1911

12

E. Rutherford: 1911Expected Actual

13

Conclusions:

Most of atom is empty space

Mass concentrated in nucleus

Nucleus is positively charged

E. Rutherford: 1911

14

Nucleus:Small & Massive

e-

e-

e-p+

n

15

Nucleus:Small & Massive

Analogy

16

Gedanken

What keeps you

from falling through

a wall?

Electrons (on outside of atom)

determine chemical properties.

17

Atoms of Different Elements

If all atoms have the same

subatomic particles, what makes

a carbon atom different from an

oxygen atom?

18

Atomic Number: Z

Z = number of protons

Z determines element

Periodic Table

is arranged by Z

19

Periodic Table

20

Mass Number

Mass number = # p+ + # n

(in nucleus)

Recall: nearly

all mass of

an atom is in

the p+ and n.

21

Atomic Symbols

N14mass number (#p+ + #n)

7atomic number (#p+)

# p+ = 7

# n = 14 – 7 = 7

# e- = 7 (atom is neutral)

22

Atomic Symbols

Shorthand:

N14

7= Nitrogen-14

He4

2= He-4

23

Atoms: First 4 Elements

H 1 1 1 1 0

He 2 4 2 2 2

Li 3 7 3 3 4

Be 4 9 4 4 5

Symbol At. # Mass# p+ e- n

24

Atoms: You try!

Symbol At. # Mass # p+ e- n

7 7

9 10

39 19

59 27

C 8

25

Isotopes

Atoms with same number of p+

(the same element) but different

number of n.

C12

6and C

14

6

6 n 8 n

26

Isotopes

Neon-20 Neon-21 Neon-22

10 p+ 10 p+ 10 p+

10 n 11 n 12 n

10 e- 10 e- 10 e-

What are the same and different

for isotopes of an element?

27

Atomic Mass

Why aren’t masses on

the Periodic Chart

whole numbers?

e.g. Chlorine mass listed as

35.453 amu

28

Atomic Mass

Periodic Table lists weighted

average mass of naturally

existing isotopes.

e.g. Cl-35 abundance 75.8 %

Cl-37 abundance 24.2 %

29

Calculating Atomic Mass

Cl-35: x 35.0 amu = 26.5 amu75.8%

100%

total = 35.5 amu

For each isotope take the fractional

abundance times mass. Then add.

Cl-37: x 37.0 amu = 9.0 amu24.2%

100%

Atomic Mass: Try It

30

Calculate the atomic mass of

silicon give the following data:Isotope Abundance Mass(amu)

Si-28 92.2% 27.98

Si-29 4.7% 28.97

Si-30 3.1% 29.97

31

Periodic Table

Mendeleev: mid-1800s

arranged known

elements by

properties and mass.

now arranged by Z

(Moseley, 1913)

http://www.wimp.com/thegenius/

32

Periodic Table

Rows Periods

period numbers: 1, 2, 3 etc

Columns Groups or Families

group numbers: 1, 2, 3, etc.

33

Periodic TableMetals & Nonmetals

Metals

•high luster

•high electrical conductivity

•ductile

•malleable

34

Periodic Table

Nonmetals

•nonlustrous

•poor conductors of electricity

•brittle

35

Periodic Table

Metalloids

B, Si, Ge, As, Sb, & Te

computer chips

36

Periodic Table

Representative Elements

Transition Elements

Inner Transition Elements

37

Periodic Table: Groups

Group 1. Alkali Metals

Group 2. Alkaline Earth Metals

Group 17. Halogens

Group 18. Noble or Inert Gases

38

Diatomic Elements

“Have No Fat”

Hydrogen H2Fluorine F2

Oxygen O2

Nitrogen N2

Chlorine Cl2

Bromine Br2

Iodine I2

39

States of the Elements

At STP:

which elements are liquids?

which elements are gases?

which elements are solids?

40

Warm-up

41

Name the subatomic particles

that make up the atom.

Where is each particle located

within the atom?

Warm-up

42

What were the four points of Dalton’s

atomic theory?

Fill in the chart:

Particle Mass Discoverer Location

Which particle is the most important in

chemistry?

43

Warm-up: •Define atomic no. & mass no.•Fill in the chart.

Symbol At.

No.

Mass

No.

p+ e- n

8 8

16 17

48 22

Al 15

Warm-up

44

Lithium has two isotopes, Li-6 with a

mass of 6.02 amu that is 7.42%

abundant and Li-7 with a mass of

7.02 amu that is 92.58% abundant.

•Calculate the atomic mass of lithium.

Define atomic mass

and mass number.