Structure of Atom

History of Development of Atomic Model

John Dalton: 1st atomic model

J.J. Thompson: Discover electron

Rutherford: Discover proton

Neils Bohr: Exist of energy shell in atom

James Chadwick: Discover neutron

Modern Atomic Model

1. The central nucleus consists of protons and neutrons. It containing almost all the mass of the atom.
2. The nucleus of an atom is very small compared to the size of the atom.
3. The electrons are orbiting outside the nucleus in the electron shells.
4. The electrons are moving in electron shells at a very high speed and we cannot determine the position of the electrons at a particular time.

Subatomic Particles

1. An atom contains three types of subatomic particles: proton, neutron and electron.
2. The proton and neutron form the nucleus at the centre of an atom.
3. The electron moves around the nucleus at a very high speed.
4. The nucleus is positively charged because of the presence of protons, which are positively charged. The neutrons are neutral.

Proton Number

The proton number (Z) represent the number of protons found in the nucleus of an atom.

Nucleon Number

The nucleon number (A), also called atomic mass number or mass number, is the number of protons plus the number of neutrons in an atomic nucleus.

Isotopes

Isotopes are atoms of certain elements which have the same number of protons but different number of neutrons in the nucleus of the atoms.

Similarities of Isotopes

1. Isotopes of an element have same number of proton in the nucleus.
2. Isotopes of an element show same chamical properties.

Diferences of Isotopes

1. Isotopes of an element have different number of neutron in the nucleus.
2. Isotopes of an element have different physical properties.

Arrangement of Electrons

1. The way in which the electrons are distributed in the shells of an atom is called the electron arrangement or electron configuration of the atom.
2. The first shell can hold up to two electrons. This is called a duplet.
3. The second shell can hold up to eight electrons. This is called an octet.
4. The third shell can hold up to eighteen electrons.