1. Matter

2. Classification

3. Atoms Z is the atomic number A is the mass number = number of protons
= number of electrons
A is the mass number
= no. of protons + no. of neutrons

4. Atoms atomic number of Cl = 17 mass number = 35 = 17 protons
= 17 electrons
mass number = 35
= no. of protons + no. of neutrons
= 17 + no. of neutrons
 no. of neutrons = 35 – 17 = 18

5. Electronic arrangement and electronic diagram
Electronic arrangement of or
= 2, 8, 7
Electronic diagram Cl

6. Isotopes
Isotopes are atoms of the same element with equal number of protons but different number of neutrons.
OR Isotopes are atoms of the same element with equal atomic number but different mass number.
Isotopes have similar chemical properties because they have the same number of outermost shell electrons.
Isotopes have different physical properties because their atomic masses are different.

7. Atomic mass The actual mass of one atom =
It is meaningless to remember such value. Moreover, it is found that mass of an electron is just of the mass of a proton.
As a result, the mass of an atom is mainly due to mass of nucleus, i.e. total mass of protons and neutrons.

8. Relative isotopic mass
Scientist assigned the mass of an atom
= atomic mass unit (a.m.u.)
Carbon-12 consists of 6 protons and 6 neutrons in the nucleus, 12 sub-atomic particles totally.
As a result, relative mass of proton = relative mass of neutron = 1 a.m.u.

9. Relative isotopic mass=
Relative isotopic mass of
= 79 a.m.u. (79 sub-atomic particles in the nucleus)

10. Relative isotopic mass
Elements contain isotopes and each isotope has its own abundance.
Usually, we mention the names of reagents required in a reaction without specifying which isotopes.
E.g. 23g of sodium burns in chlorine to form sodium chloride. What is the mass of chlorine gas required for the complete reaction?
As a result, we have to find the weighted average of chlorine isotopes.

11. Relative atomic mass
Due to the presence of isotopes, atomic mass of an element should not be an integer. It should be the weighted means of the isotopes.
Relative atomic mass =

12. Isotopes Relative abundance = 75% : 25% = 3 : 1
= : 1
Relative atomic mass of Cl
= 35 x 75% + 37 x 25%
= 35.5 a.m.u.

13. Isotopes
If the relative atomic mass of Ga = 69.8, find the relative abundance of each isotope.
Let the relative abundance of and be x% and (100 – x)% respectively
69(x%) + 71(100-x)% = 69.8
x = 60

14. Isotopes
Ga has two isotopes and If the relative abundance of is 60% and the relative atomic mass of Ga = Find the value of x.
69(60%) + x(40)% = 69.8
x = 71