1 mole of an element 'X' contains 6.022 x 10²³ atoms
Now weight of 1.66 x 10²⁰ atoms = 16.26 x 10^-3 g
Mass of  6.022 x 10²³ atoms = \(\frac { 16.26\times { 10 }^{ -3 } }{ 1.66\times { 10 }^{ 20 } } \) x 6.02 x 10²³ = 59 g
The atomic mass of element 'X' is 59 u

(i) Percentage of oxygen = \(\frac { 0.144 }{ 0.24 } \times 100\) = 60
Percentage of boron = \(\frac { 0.096 }{ 0.24 } \times 100\)= 40
(ii) 16 g oxygen = 1 mole
0.144 g oxygen = \(\frac { 0.144 }{ 16 } \) = 0.009 mole

Since M has a valancy of +1
Chemical formula of chloride of M = MCl
Chemical formula of bicarbonate of M = MHCO₃
Chemical formula of sulphide of M = M₂S

The atomic mass of an element is the relative mass of its atom as compared with the mass of a particular atom of carbon taken as 12 units.Thus the atomic mass of an element indicates the number of times one atom of element is heavier than 1/12th of \({ 12 }_{ C }\) atom or (C-12).For example, the atomic mass of oxygen is 16 which indicates that an atom of oxygen is 16 times heavier than 1/2th of a \({ 12 }_{ C }\) atom. One atomic mass unit (u) = 1th/12 the mass of \({ 12 }_{ C }\) atom.

An atom is a building block of an element.It is very very small, so small that millions of atoms when stacked would hardly make a layer as thick as thin sheet of paper.The radius of an atom of hydrogen is 10^-10 m or 10^-1 nm, one molecule of water is 1 nm.Compared to it grain of sand is 10^-4 m.

23 gram atomic unit or 23 gram sodium (1 mole) = 6.022 x 10²³ atoms
100 gram atomic unit or 100 gram sodium (1 mole) = \(\frac { 6.022\times { 10 }^{ 23 }\times 100 }{ 23 } =2.617\times { 10 }^{ 24 }\)atoms
again 56 gram atomic unit or 56 grams iron (1 mole) =  6.022 x 10²³ atoms
100 gram atomic unit or 100 grams iron = 1.075 x 10²³ atoms
Thus 100 g of carbon has more atoms than 100g of iron.

1 mole = atomic mass = 6.022 x 10²³ atoms
Now mass of one atom of M = 3.05 x 10^-22
Now mass of 6.022 x 10²³ atoms of M =  3.05 x 10^-22 x  6.022 x 10²³ g = 183.7 g
Mass of 1 mole or atomic mass of M = 183.7 g
This could be tungeten (W)

1. Metals exists as atomic crystals and do not form molecules.Generally, metals and other elements like carbon and silicon do not have single molecular structure but consists of a very large indefinite number of atoms bonded together.A form of carbon bonded with sixty atoms is called buckminsterfullerene
2. Non-metals exist as molecules containing one or more atoms.For example, argon exists as Ar and nitrogen as N₂
3. Compounds containing non-metals only exist as molecules.For example, compound of nitrogen and hydrogen is a ammonia molecule.Compound of carbon  and oxygen is a carbon dioxide molecule.
4. Compounds containing metals and non-metals exist as ions.For example, compound of sodium and chlorine consists of positivity charged sodium ions and negatively charged chloride ions. 

(i) Mass of 1 mole CO₂ = 12 + 32 = 44 u
Mass of 5 moles CO₂ = 44 x 5 = 220 u
Mass of 1 mole of H₂O = 2 + 16 = 18 u
Mass of 5 moles of H₂O = 5 x 18 = 90 u
(ii) 40 g calcium make = 1 mole
240 g calcium make = \(\frac { 240 }{ 40 } \) = 6 moles
24 g magnesium make = 1 mole
240 g magnesium make = \(\frac { 240 }{ 24 } \) = 10 moles
Ratio of 240 g calcium and 240 g magnesium in moles = \(\frac { 6 }{ 10 } \) or 3 : 5

A polyatomic ion is a group of atoms carrying a charge (positive or negative).
For example, ammonium ion (NH4+)
hydroxide ion (OH^-)
carbonate ion (CO₂³-)
sulphate ion