Atomic Theory

Democritus

• 440 B.C.E• All matter is made up of atoms,

which are indivisible• he pounded up materials in his pestle

and mortar until he had reduced them to smaller and smaller particles which he called

ATOMOSATOMOS (Greek for indivisible)

Dalton•In the early 1800’s, the

English chemist John Dalton did a number of experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Atomic Theory•All elements are composed

of atoms. Atoms are solid small spheres and indivisible particles.

•BILLIARD BALL MODEL

continued...

•Atoms of the same element are exactly alike.

H H

continued...

•Atoms of different elements are different.

O H

continued...

•Compounds are formed by the joining of atoms of two or more elements.

H H O

Was he right?•Dalton’s atomic theory of

matter became one of the foundations of chemistry.

•BUT it didn’t explain attraction of repulsion of objects.

J.J.Thomson•In 1897, the work of an English

scientist J. J. Thompson provided the first hint that atoms were made up of even smaller particles

•Atoms are divisible…Dalton was wrong!

Thompson’s Model•He stated that atoms are

made up of positively (+) and negativelynegatively (-) charged particles

And...•He thought the positive (+)

and negative (-)negative (-) particles were s p r e a d o u t in atoms likeplums in pudding or raisins in a bun.

Questions...•What kind of particles did Thompson find in atoms?

•What did his idea of an atom look like?

Was Thomson Right?•Not really…there are positively (+) and negativelynegatively (-) charged particles inside atoms.

•BUT...

Rutherford’s Model•In 1908, Rutherford discovered

the nucleus.•He stated that atoms have a

small, dense, positively (+) charged center called a nucleus.

.

Alpha Particle ScatteringThe Nuclear Atom 26/04/23

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An atom

Alpha Source

+

Rutherford’s ModelThe Nuclear Atom 26/04/23

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He suggested that all of the atom’s positive charge, together with most of its mass, is concentrated in the centre.

Alpha particles which travel close to the nucleus are strongly deflected. The degree of deflection depends on how close it approaches.

Rutherford’s ModelThe Nuclear Atom 26/04/23

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The nucleus must be very small in comparison to the atom.

This will account for the vast majority making it through unaffected.

So...•Particles are not spread out like

plums in pudding…•Positive particles (protons) are

in the center (nucleus).•Where are the negativenegative particles (electrons)?

Bohr’s Model•In 1913, Niels Bohr

proposed an improvement to Rutherford’s Model:

•Electron’s move in definite orbits around the nucleus, like planets around the sun.

+

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Are we there yet?•Bohr’s model was used for a

long time…

•BUT...

Modern Atomic Model

•An atom has a small, positively charged nucleus surrounded by a large region in which there are enough electrons (-) to make the atom neutral (0).

P

E

E

E

E

E

E

E

E

E

E

E

PPPPPPPP P

PP

E

Bohr’s Atom

electrons in orbits

nucleus

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ ve charge

-ve charge

No charge

1

1

nil

Mass

ATOMIC STRUCTUREATOMIC STRUCTURE

the number of protons in an atom

the number of protons and neutrons in an atom

HHee

22

44 Atomic mass

Atomic number

number of electrons = number of protons

HeliumChemical Name

Chemical Symbol

ATOMIC STRUCTUREATOMIC STRUCTURE

Electrons are arranged in Energy Levels

or Shells around the nucleus of an atom.

• first shell a maximum of 2 electrons

• second shell a maximum of 8

electrons

• third shell a maximum of 8

electrons

• forth shell a maximum of 8

electrons

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example;

NNitrogen

7

14

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

Bohr Rutherford DiagramsBohr Rutherford Diagrams

With Bohr Rutherford diagrams, elements and compounds are represented by Dots to show electrons, and circles to show the shells. For example;

Nitrogen N OO O

O

OO

O N 7

14

Bohr Rutherford DiagramsBohr Rutherford Diagrams

Draw the Bohr Rutherford Diagrams for the following elements;

O Cl8 17

16 35a) b)

O

OO

O

O

O

O

O

O

Cl

O

O

O

O O

OO

O

O

O

O

O

O

OO

O

O

O

SUMMARYSUMMARY1. 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.