Write down the electronic configuration of P 15 31 .

Mass number of the element = 31 Atomic number of the element = 15 Atomic number is a total number of protons and neutrons present in the nucleus of an atom. In neutral atom, total number of protons is equal to total number of electrons. Therefore, number of electrons = 15 Its electronic configuration is 2 , 8 , 5 .

Draw the picture of an atom N 7 14 .

In the given atom, the atomic number of the element Nitrogen is 7 . The number of protons is equal to number of electrons. Hence, its electronic configuration is 2 , 5 . The two electrons occupy the inner shell and the five electrons occupy the valence shell.

Chlorine has isotopes, Cl 17 35 and Cl 17 37 in the ratio 75 : 25 . Calculate the atomic mass of Chlorine.

The atomic mass of an element represents the weighted average of all the of that element occurring naturally. The mass number of the isotopes are 35 and 37 respectively. The atomic mass of Chlorine from the given isotopes is, = 35 × 75 + 37 × 25 100 = 2625 + 925 100 = 3550 100 = 35 . 5 u

Pick up the isotopes: X 8 16 ; Y 9 19 ; Z 10 20 ; X 8 17 ; X 8 18

Isotopes are the species having same atomic number but different mass number. From the given species, X 8 16 ; X 8 17 ; X 8 18 are the isotopes of element X . It has same atomic number 8 and different mass number 16 , 17 and 18 respectively. These isotopes differ in the number of neutrons present in their nucleus.

Identify the isotones: X 17 37 ; Ca 20 40 ; K 19 39 ; Cl 17 35 ; Ar 18 40

Atoms of different element with different atomic number but with same number of neutrons are called as isotones. From the given species, X 17 37 ; Ca 20 40 ; K 19 39 are isotones because they have equal number of neutrons. X 17 37 = 37 - 17 = 20 Ca 20 40 = 40 - 20 = 20 K 19 39 = 39 - 19 = 20