When two elements A and B combine together to form more than one compound, the masses of A which separately combines with the fixed mass of B are in simple ratio. This law was proposed by John Dalton.

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 

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.

(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.