CHAPTER 4 – THE ATOM

John Dalton’s Atomic Theory (19th Century)

1) All matter is composed of small particles called atoms

2) All atoms of an element are identical in size, mass, and chemical properties

3) Atoms cannot be created, divided into smaller particles or destroyed

4) Different atoms combine in simple whole number ratios to form compounds

5) In chemical reactions, atoms are separated, combined, or rearranged

FALSE

FALSE

Rule #4

+

ELEMENT A ELEMENT B COMPOUND of A and B

Rule #5

+

What is an atom?

• Defn – smallest particle of an element that retains properties of the element

How many copper atoms can fit on a penny?

29,000,000,000,000,000,000,000 atoms(29 x 1021)

Electron

• Discovered by J.J. Thomson (1909) using a cathode ray tube (CRT)

• Electron – negative charged particle of matter (-1)

- has no relative mass

Nucleus

• Discovered by Ernest Rutherford (1911) using Gold Foil Experiment

• Nucleus – dense, positively charged region in center of atom

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Composition of Nucleus

a) Proton – positive (+1) charged particle of nucleus

b) Neutron – neutral (0) charged particle of nucleus

Particle Symbol Location ChargeRelative

Mass

Electron e- around nucleus -1 0

Neutron n0 nucleus 0 1

Proton p+ nucleus +1 1

What makes an atom of one elementdifferent from an atom of another element ?

Atomic Number

• Defn – number of protons in an atom

• How is periodic table arranged?by increasing atomic number

• How many electrons do atoms have?# electrons = # protons

atomic number identifies atoms

reading the periodic table

H

Hydrogen

1

1.008

Element

Atomic number

SymbolAtomic mass

Mass Number

• Defn – total number of protons and neutrons

NOT SAME AS ATOMIC MASS!!!!!!

H

Hydrogen

1

1.008

Cannot be found on periodic table

NO NO NO NO NO!!!!

Mass Number

• Remember this formula to find # neutrons:

MANM – A = N (Mass #) – (Atomic #) = Neutrons

Refresh!!!!

• I’ve given you 3 numbers:

1) Atomic number2) Atomic mass3) Mass number

The ones with the word “ATOMIC” is on the periodic table

Not on periodictable

What’s in a cheeseburger?

Isotope• Defn – atoms with same number of

protons, but different number of neutrons

HYDROGEN - 1 HYDROGEN - 2 HYDROGEN - 3

= neutrons= protons

= electrons

3 Different Hydrogen IsotopesHYDROGEN – 1

(protium)

1 proton0 neutrons1 electron

HYDROGEN – 2(deuterium)

HYDROGEN – 3(tritium)

1 proton1 neutron1 electron

1 proton2 neutrons1 electron

Ways to write isotopes

1) Hyphen notation (2 parts)

name of element – # OR symbol – #

ex: carbon-14 OR C-14 chlorine-35 OR Cl-35 hydrogen-3 OR H-3

MASSNUMBER

Ways to write isotopes

2) Nuclear symbol (3 parts)

C14

6elementsymbol

mass number

atomic number

Ex problem

K

Potassium

19

39.10

# protons

# neutrons

# electrons

Nuclearsymbol

Potassium-39 Potassium-40 Potassium-41

19

19

1919

19

2220 21

19

K KK39

19

41

19

40

19

Mass of individual atoms

• Atoms have very very tiny masses to work with (~10-27 kg)

• Instead we use a relative mass – atomic mass unit (amu)

• Atomic Mass Unit (amu) – 1 amu ≈ mass of 1 proton or 1 neutron

amu’s and subatomic particles

particle Mass (amu)

e-

p+

n0

0.000549

1.007276

1.008665

0

1

1

Atomic Mass

• Defn – weighted average mass of all isotopes of an element

• Where can it be found?found on periodic table K

Potassium

19

39.10

Ex problem: Calculate atomic mass of chlorine.

Isotope mass (amu) percent abundance

35Cl 35 75.770%37Cl 37 24.230%

What do you expect the average to be?About 35 amu

Actual = 35.453 amu

Which isotope is most abundant?Ar-38, Ar-40, or Ar-41?

Ar

Argon

18

39.948

Ar-40