ATOMIC STRUCTURE

DEVELOPMENT OF ATOMIC MODEL

Scientist Atomic Structure

Atomic Model

John Dalton

(1766-1844)

1808- suggested that matter is made of tiny particles which are indivisible and indestructible

An atom is like a miniature billiard ball

J.J Thompson

(1856-1940)

1897- put forward the “plum pudding” model of the atom. The atom is like a sphere with small electrons embedded in a positively charged mass

Plum Pudding Model

-discovery of electron

Scientist Atomic Structure

Atomic Model

Ernest Rutherford

(1871-1937)

1911- shows that: atom is spherical & has a centre or nucleus with a dense positive chargeProtons are the positively charged particles in the nucleusA system of electrons rotates around the nucleus much like the planets orbiting around the sun

Rutherford’s Solar System Model

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Scientist Atomic Structure

Atomic Model

Niels Bohr

(1885-1962)

1913- showed that:Electrons orbit in specific energy levels called shellsEach shell is capable of holding a certain number of electrons.

Bohr’s electrons orbiting in shells

James Chadwick

(1891-1974)

1932- discovered that:A particle in the nucleus which has no electric charge and called it neutronAll nuclei (except hydrogen) contain neutrons as well as protons

Atomic structure of an atom Atoms are made of three different types of subatomic particles called

protons, neutrons and electrons Protons and neutrons are found in the nucleus and they are known as

nucleons Nucleus is positively charged because the protons are positively charged

and the neutrons are electrically neutral Electrons rotate around the nucleus in a certain orbit called shells, just

like planets rotate around the sun

SUBATOMIC PARTICLES

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proton

neutronshell

electron

{nucleon

THE PROPERTIES OF SUBATOMIC PARTICLES

Subatomic particles

Proton Neutron Electron

Symbol p n e¯

Relative electric charge

+1 0 -1

Relative mass

1 1 _1__

1840

An atom is neutral when it has the same number of protons and electrons

PROTON NUMBER

The number of protons in an atom of an element

Proton number = number of protons

NUCLEON NUMBER

The sum of protons and neutrons in an atom element

Nucleon number = number of protons + number of neutrons

NUMBER OF NEUTRONS

the difference between the nucleon number and the proton number

A neutral atom has the same number of protons and the number of electrons

Different elements have different numbers of protons

Number of neutrons = nucleon number – proton number

The number of protons, electrons and neutrons in a particular atom are given in the Periodic Table by the nucleon number and proton number

A : proton numberZ :nucleon number

X: symbol of an element

The number of protons, electrons and neutrons can be determined from the nucleon number and proton number

A

X Z

Example: (a) Sulphur

the symbol of the element above shows that it is sulphur Nucleon number = 32 Proton number = 16 Number of neutron = So, sulphur atom has 16 protons, 16 electrons and 16

neutrons

32

S16

32-16 = 16

Example: (b) Aluminium

The symbol of the element above shows that it is aluminium

Nucleon number = 27 Proton number = 13 Number of neutron = Aluminium atom has 13 protons, 13 electrons and 14

neutrons

27

Al13

27 – 13= 14

Example: (c) Sodium

Number of protons =

Number of neutrons

So, number of electrons =

23

Na11

11

= 23 - 11

= 12

11

Exercises1. What is matter?

2. What are atoms, molecules and ions?

3. How does the state of matter change with the change in temperature?

4. How does the kinetic theory of matter explain the inter-conversion of the state of matter?

5. How was the atomic model developed?

6. What are the main subatomic particles of an atom?

7. What is proton number?

8. What is nucleon number?