Question

An isotope of helium is represented by the symbol ${}_2^{3}He$. How many protons, neutrons and electrons are there in an atom of the isotope?

• A. $1$ proton, $2$ neutrons, $1$ electron
• B. $1$ proton, $3$ neutrons, $2$ electrons
• C. $2$ protons, $1$ neutron, $2$ electrons
• D. $2$ protons, $3$ neutrons, $2$ electrons

Answer 1

The correct option is C $2$ protons, $1$ neutron, $2$ electrons

${}_2^{3}He$ :

Ideal representation of an element : ${}_Z^{A}X$

Here, $Z=$ atomic number $=$number of protons

$A=$ mass number $=$ number of protons $+$ number of neutrons

Also, in an atom number of protons $=$ number of electrons

So, for ${}_2^{3}He$ :

Number of protons $=$ $Z=$ $2$

Number of electrons $=$ number of protons $=$ $2$

number of neutrons $=$ $A$ $-$ $Z$ $=$ $3$ $-$ $2$ $=$ $1$