HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BCDemocritus develops the idea of atoms

He got the idea that everything was made of small bits by looking at sand on the beach. He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he calledATOMAATOMA

(greek for indivisible)

HISTORY OF THE ATOMHISTORY OF THE ATOM

• 340 BC• Aristotle thought that

everything was made of four “elements”: Earth, Air, Fire and Water

• Aristotle was wrong, but his idea was accepted for the next 2000 years.

Aristotle’s competing idea

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808John DaltonRevived Democritus’ idea

Dalton said that all matter was made up of tiny spheres that were somehow different from each other, resulting in different elements. He called these tiny spheres

ATOMSATOMS

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John ThomsonDiscovered the electron

While working with cathode rays, Thomson discovered that they were made of tiny, identical and negatively charged particles which were later calledELECTRONSELECTRONS

HISTORY OF THE ATOMHISTORY OF THE ATOM

Thomson developed the idea that atoms were made up of electrons scattered unevenly in a soup of positive charge to balance the electron's negative charge.

1904 Thomson creates a new model of the atom.

like plums surrounded by pudding.

PLUM PUDDINGMODEL

HISTORY OF THE ATOMHISTORY OF THE ATOM1910 Ernest Rutherford discovers the

nucleus• His team fired alpha

particles at a piece of gold foil which was only a few atoms thick.

• They found that although most of the particles passed straight through, About 1 in 10,000 hit something.

• It was as if you fired a cannon at a sheet of tissue paper and the cannonball bounced back.

HISTORY OF THE ATOMHISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the particles passed through the foil, a small number were deflected and some even bounced straight back.

helium nuclei

HISTORY OF THE ATOMHISTORY OF THE ATOM

• Rutherford’s new evidence led him to propose a new model with a central nucleus.

• He suggested that the positive charge was all in the central nucleus, instead of spread out like in Thomson’s model.

• It was this small, massive, positively charged nucleus that deflected the particles in the experiment.

HISTORY OF THE ATOMHISTORY OF THE ATOM1913 Niels Bohr creates a new model of the

atom. Bohr studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's model by adding that the electrons were in orbits around the nucleus, like planets going around the sun. Each orbit could only hold a set number of electrons.

Bohr’s Atom

electrons in orbits

nucleus

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ 1

-1

No charge

1

1

1/1800

Mass

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

Remember that models like this are not drawn to scale!

ATOMIC NOTATIONATOMIC NOTATION

the number of protons AND neutrons in the nucleus

the number of protons in the nucleusHeHe44

22Atomic number

Mass number

To find the number of neutrons, you have to subtract the atomic number from the mass number.

More About the Atomic Number

• The atomic number is also the ID number of the element.

• Only helium has atomic number 2. • If a nucleus has 26 protons, it

must belong to an iron atom.• The Periodic Table is arranged by

atomic number

What about the electrons?

• The electrons weigh almost nothing. • They are held by the nucleus by

electrical attraction.• Atoms are electrically neutral, which

means that the number of electrons in a neutral atom must be the same as the atomic number: number of electrons = number of protons

ATOMIC STRUCTUREATOMIC STRUCTURE

Electrons are arranged in Energy Levels

or Shells around the nucleus of an atom.

• first level a maximum of 2 electrons

• second level a maximum of 8

electrons

• third level a maximum of 8

electrons

ATOMIC STRUCTUREATOMIC STRUCTURE

There are two ways to represent the atomic

structure of an element or compound;

1. Electronic Configuration2. Bohr 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;

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 DIAGRAMSBOHR DIAGRAMS

With Bohr diagrams, elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the energy levels. For example:

Nitrogen N XX X

X

XX

X N 7

14

BOHR DIAGRAMSBOHR DIAGRAMS

Draw the Bohr diagrams for the following elements;

O Cl8 17

16 35a) b)

O

XX

X

X

X

X

X

X

Cl

X

X

X

X X

XX

X

X

X

X

X

X

XX

X

X

X

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.