Answer the following

Define:

a) Atomic mass

b) Relative atomic mass

c) Gram molecular mass

d) Mole.

e) Valency of an element

(a) Atomic mass- Atomic mass is defined as the sum of a number of protons and neutrons normally found in the nucleus of an atom of a given element. It is measured in u (unified mass) or amu (atomic mass unit).

Ex-The atomic mass of Oxygen (O) is 16. It is the sum of 8 protons and 8 neutrons present in its nucleus.

(b) Relative atomic mass- Relative atomic mass (A_r) is a dimensionless quantity. It is defined as the ratio of the average mass of atoms of an element of a given sample to the 1/12 of the atomic mass of a carbon-12 atom.

Ex- Relative atomic mass of Aluminium (Al)

=

=

=

= 26.98 u

(c) Gram molecular mass- Gram molecular mass of a substance is its relative molecular mass expressed in grams.

Ex- Gram molecular mass of Aluminium (Al) = 26.98g

(d) Mole- A mole of a substance is defined as the amount of substance containing a same number of atoms or molecules as there are atoms in exactly 12g of carbon-12.

(e) Valency of an element- Valency is the combining capacity of an element. It is the number of electrons an atom of an element can gain or loss.

If valence electrons are 4 or less then the valency of an atom is equal to the valence electrons. And if valence electrons are more than 4, the valency is equal to 8 minus the number of electrons in the outer most shell.

Ex- Sodium has electronic configuration 2, 8, 1. So its valency is 1 because it has 1 valence electrons. Oxygen has electronic configuration 2, 6. So its valency is 2 because it has 6 valence electrons.