Elements, when arranged in order of increasing atomic number, lead us to the classification known as the modern periodic table. The chemical and physical properties of the elements vary periodically when the elements are arranged in the order of their atomic numbers.

The number of shells in calcium can be determined by writing its electronic configuration. Calcium has an atomic number of 20, which means it has 20 electrons. There are 2 electrons in the first shell, 8 electrons in the second shell, another 8 electrons in the third shell, and 2 in the fourth (outermost) shell of calcium. 
Its electronic configuration can be written as 2, 8, 8, 2. The number of shells in calcium which are filled with electrons are 4.

Valency is the combining capacity of the element.
Nitrogen has atomic number 7. Electronic configuration of nitrogen can be written as:
$N=1s^2,2s^2,2p^3$
It has 5 electrons in its outermost shell and requires 3 more electrons in order to complete its L shell. Hence its valency is 3.

Since isotopes have same number of protons and electrons but different number of neutrons, they differ only in physical properties and not in chemical properties.

• Dobereiner arranged elements in groups of three, called triads, in the increasing order of their atomic masses. In a triad, the atomic mass of the middle element was roughly equal to the average of the atomic masses of the other two elements.


• From the given options, the only element that can form a Dobereiner's Triad, along with lithium and sodium is only pottassium.  


• If A, B and C are Dobereiner's Triads, $\text{Atomic mass of B}=\frac{\text{Atomic mass of A}+\text{Atomic mass of C}}{2}$