S.MORRIS 2006

Where did it all begin?

The word “atom” comes from the

Greek word “atomos” which means

indivisible.

The idea that all matter is made up of

atoms was first proposed by the

Greek philosopher Democritus in the 5th

century B.C.

HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

he pounded up materials in his pestle and

mortar until he had reduced them to

smaller and smaller particles which he

called

ATOMAATOMA

(greek for indivisible)

HISTORY OF THE ATOMHISTORY OF THE ATOM

1803 John Dalton

suggested that all matter was made up of

tiny spheres that were able to bounce

around with perfect elasticity and called

them

ATOMSATOMS

Dalton’s Atomic Theory (1808)1. Elements are composed of extremely small particles

called atoms.

3. Atoms of different elements can combine in simple whole number ratios to form compounds.

4. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

2.1

2. All atoms of a given element are identical. The atoms of one element are different from those of any other element

8 X2Y16 X 8 Y+

2.1

Law of Definite Proportions

• The ratio of mass of elements in a compound is always the same

• Every Water molecule will contain 16g of oxygen and 2 g of hydrogen

2

2.1

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

found that atoms could sometimes eject a

far smaller negative particle which he

called an

ELECTRONELECTRON

HISTORY OF THE ATOMHISTORY OF THE ATOM

Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge

1890

like plums surrounded by pudding.

PLUM PUDDING

MODEL

HISTORYHISTORY OF THE ATOMOF THE ATOM

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out his

famous experiment.

they fired Helium nuclei at a piece of gold

foil which was only a few atoms thick.

they found that although most of them

passed through. About 1 in 10,000 hit

1910

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

2.2

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

2.2

HISTORY OF THE ATOMHISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the helium nuclei passed

through the foil, a small number were deflected and, to

their surprise, some helium nuclei bounced straight back.

helium nuclei

HISTORY OF THE ATOMHISTORY OF THE ATOM

Rutherford’s new evidence allowed him to propose a

more detailed model with a central nucleus.

He suggested that the positive charge was all in a

central nucleus. With this holding the electrons in place

by electrical attraction

However, this was not the end of the story.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits. Rather like planets orbiting the

sun. With each orbit only able to

contain a set number of electrons.

Chadwick’s Experiment (1932)

H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

+ 9Be 1n + 12C + energy

neutron (n) is neutral (charge = 0)

n mass ~ p mass = 1.67 x 10-24 g2.2

Subatomic Particles

Particle Mass

(g) Charge

(Coulombs) Charge (units)

Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1

Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1

Neutron (n) 1.67 x 10-24 0 0

mass p = mass n = 1840 x mass e-

2.2

Bohr’s Atom

electrons in orbits

nucleus

Atoms

• # of protons & neutrons

• The basic unit of Matter

• The smallest particle of an element that retains the properties of that element.

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

All About Atoms

Particle

Proton

Neutron

electron

Charge

+ charge

- charge

No charge

1amu

1 amu1/1836

Mass

ATOMIC STRUCTUREATOMIC STRUCTURE

Represents the number of protons in an atom

Never changes

P+ equal to the number of e-

the number of protons and neutrons in an atomNeutrons equal mass # - atomic #

HeHe22

44 Mass Number

Atomic number

number of electrons = number of protons

Ions

Charged particles due to the loss or gain of electrons

2) Atoms that are called cations lose e- thus becoming positive

1) Atoms that are called anions gain e- thus become negative

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

2.5

Isotopes

• Atoms with the same atomic number but different mass number

• Atoms having the same number of protons but different numbers of neutrons

• Average Atomic Mass of an element is the weighted average of an element’s naturally occurring isotopes

2.3

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?Isotope Maker

2.3

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

# OF NEUTRONS = mass number – atomic number

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

2.3

Bohr Model of the Atom

• Electrons travel around the nucleus in one of several orbits/shells/ nrg levels

• Principal energy level is designated by a quantum number (n)

• Quantum number is the same as period on the periodic table

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

2.4

ATOMIC STRUCTUREATOMIC STRUCTURE

Electrons are arranged in Energy Levels

or Shells around the nucleus of an atom.

• first shell maximum of 2 electrons

• second shell maximum of 8

electrons

• third shell max of 18 electrons

• fourth shell max of 32 electrons

ATOMIC STRUCTUREATOMIC STRUCTURE

There are two ways to represent the atomic

structure of an element or compound;

1. Electronic Configuration

2. Electron dot diagrams

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

With electronic configuration elements are

represented numerically by the number of

electrons in their shells and number of shells. For

example;

N

Nitrogen

14

7

2 in 1st shell

5 in 2nd shell

configuration = 2 , 5

2 + 5 = 7

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

Write the electronic configuration for the following elements;

Ca O

Cl Si

Na20

40

11

23

8

17

16

35

14

28B

11

5

a) b) c)

d) e) f)

2,8,8,2 2,8,1

2,8,7 2,8,4 2,3

2,6

Valence

• Valence shell is outermost occupied energy level and is the same as the period number in the periodic table

• Valence electrons are the electrons in the outer energy level of an atom

DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS

With Dot & Cross diagrams elements and

compounds are represented by Dots or Crosses to

show electrons, and circles to show the shells. For

example;

Nitrogen N XX X

X

XX

X

N7

14

DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS

Draw the Dot & Cross diagrams for the following elements;

O Cl8 17

16 35a) b)

O

X

XX

X

X

X

X

X

Cl

X

X

X

X X

X

XX

X

X

X

X

X

XX

X

X

X

SUMMARYSUMMARY

1. The Atomic Number of an atom = number of

protons in the nucleus.

2. The Atomic Mass of an atom = number of

Protons + Neutrons in the nucleus.

3. The number of Protons = Number of Electrons.

4. Electrons orbit the nucleus in shells.

5. Each shell can only carry a set number of electrons.