•Atom is the smallest particle that can exist independently.•Every element is made of atoms.• All atoms of same element are identical to each other.•Atoms of different elements are different from each other.

•Atom consist of three sub–atomic particlesi.Protonsii.Neutronsiii.Electrons

Electron- Negatively

charged particles having mass

1/1840

Proton- Positively charged particles

having mass 1

Neutron- Neutral particles having

mass 1

•Nucleon number of an element is the total number of protons and neutrons in its atom.

•Proton number of element is the number of protons present in its atom.

Nucleon Number

Proton Number

•Atoms of same element having different number of neutrons.

•Isotopes have same proton number (Z) but different nucleon number or mass number (A)

•Isotopic abundance means the amount in percentage of a isotope present naturally.

•Normal hydrogen contains all the three isotopes of hydrogen.

•Average atomic mass of an element is the average of nucleon number or mass number of all its isotopes according to their isotopic abundance

(Nucleon number of 1st isotope × isotopic abundance) + (Nucleon number of 2nd isotope × isotopic abundance)

100

•Average atomic mass of hydrogen is 1.008 units•Average atomic mass of chlorine is 35.5 units

•Relative atomic mass of an element is the ratio of the average mass of an atom to 1/12th the mass of atom

•Relative atomic mass of hydrogen is 1.008 units•Relative atomic mass of chlorine is 35.5 units

First shell can contain 2

electrons (a duplet)

Second shell can contain 8 electrons (an

octet)

Electrons revolve around the nucleus in orbits just like planets revolve around the sunEach orbit have a particular energy, so they are called energy levels or shells.

Valence electrons are the number of electrons present in the last shell of an atom of element

How many electrons are present in the shells.

In periodic table elements are arranged in order of increasing proton number

Vertical columns are called “groups”. Group number indicates the number of valence electrons. Each element in the group contains equal number of valence electrons.

Horizontal rows are called “periods”. Period number indicates the number of shell. Each element in the period have same number of shells.