(i) In atoms, electrons revolve around the nucleus in certain special or permissible orbits known as discrete orbits or shells or energy levels.
(ii) While revolving in these discrete orbits the electrons do not radiate energy.
(iii) The circular orbits are numbered as 1, 2, 3, 4, ... or designated as K, L, M, N, shells. These numbers are referred to as principal quantum numbers (n)
(iv) K shell (n = 1) is closest to the nucleus and is associated with lowest energy. L, M, N, .... etc are the next higher energy levels. As the distance from the nucleus increases the energy of the shells also increases.
(v) The energy of each orbit or shell is a fixed quantity and the energy is quantized.
(vi) As the distance from the nucleus increases, the size of the orbits also increases.
(vii) Maximum number of electrons that can be accommodated in an energy level is given by 2n² where n is the principal quantum number of the orbit.
(viii) When an electron absorbs energy, it jumps from lower energy level to higher energy level.
(ix) When an electron returns from higher energy level to lower energy level, it gives off energy.

In 100g of water, the weight of hydrogen = 11.11 g
The weight of oxygen = 100 – 11.11 = 88.89 g
In 100g of sulphur dioxide, the weight of sulphur = 50 g
Weight of oxygen = 100 – 50 = 50 g
The ratio between the weight of oxygen and Hydrogen is 88.89:11.11 i.e. 8:1 .....   (1)
In hydrogen sulphide, the weight of sulphur = 94.11 g
The weight of hydrogen = 100 – 94.11 = 5.89 g
The ratio between the weight of sulphur and hydrogen is 94.11: 5.89 ie. 16: 1  ..... (2)
The two ratios 1 and 2 are related as 8/1: 16/1 (or) 1: 2
These are simple multiples of each other. The ratio between the weight of sulphur (32) and oxygen (16) which combine separately with the weight of Hydrogen (2) supports the law of reciprocal proportions

Uses of radioactive isotopes:
1. Uranium 235 is used as fuel in nuclear reactor.
2. Cobalt 60 is used in the treatment of cancer.
3. Iodine 131 is used in the treatment of goitre.
4. C-14 is used in carbon dating.

CH_4(g) +2O_2(g) → CO_2(g) +2H₂O_(g)
1             2           1              2
volume volumes  volumes volumes
1x           2x          1x          2x
50cm³ 50cm³  50cm³    50cm³
50cm³ 100cm³ 50cm³ 100cm³ 
Volume of oxygen used = 100 cm³
Volume of carbon dioxide formed = 50 cm³ 

1. Laws of conservation of mass:
In a chemical change the total mass of the reactant is equal to the total mass of the products.
2. Law of definite proportion:
"A pure chemical compound prepared by any method consists of the same elements combined together in the fixed proportion by mass".
3. Law of multiple proportion: 
When two elements A and B combine together to form more than one compound, then masses of A which separately combines with a fixed mass of B are in simple ratio.
4. Laws of Reciprocal proportion:
"If two elements combine separately the same weight of a third element, the ratios of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.
5. Gay Lussac's law of combining volumes:
Whenever gases react together, the volumes of the reacting gases as well as the products bear a simple whole number ratio, provided all the volumes are measured under similar conditions of temperature and pressure.

The elements in the increasing order of atomic no.
Helium, Boron, Oxygen, Neon, Sodium, Magnesium, Calcium, Silicon, Sulphur.

Atomic number of calcium is 20. When it loses 2 electrons Ca²⁺ is formed.
\(Ca\quad -\quad 2e\quad \longrightarrow \quad C{ a }^{ +2 }\)
2,8,8,2      2,8,8
According to Bohr and Bury rule the outermost shell cannot have more than 8 electrons. So Ca²⁺ has completely filled outer shell.

Beryllium.