Question

Consider the following table in which the elements from A to E are hypothetical :

ELEMENT	PROTONS	NEUTRONS	ELECTRONS
A	9	10	9
B	17	18	17
C	10	10	10
D	6	6	6
E	17	20	17

Choose from the letters which represent :

1. an element with mass number 19
2. an element having atomic number 10.

Answer 1

$\mathrm{Mass}\mathrm{number}=\mathrm{no}.\mathrm{of}\mathrm{protons}+\mathrm{no}.\mathrm{of}\mathrm{neutrons}$

(i) Given $\mathrm{Mass}\mathrm{number}=19$

Element A has $\mathrm{Mass}\mathrm{number}=19$,

as the$\mathrm{no}.\mathrm{of}\mathrm{protons}=9$ and $\mathrm{no}.\mathrm{of}\mathrm{neutrons}=10$

Hence, A is the element with mass number 19.

$\mathrm{Atomic}\mathrm{number}=\mathrm{no}.\mathrm{of}\mathrm{protons}$

(ii) Given $\mathrm{Atomic}\mathrm{number}=10$

Element C has $\mathrm{Atomic}\mathrm{number}=10$, as the $\mathrm{no}.\mathrm{of}\mathrm{protons}=10$

Hence, C is the element having atomic number 10.

Therefore,

1. An element with mass number 19 = A
2. An element having atomic number 10 = C