Question

An atom of phosphorus has the symbol ${}_{15}^{31}\text{P}$. How many protons, electrons and neutrons are present in an atom of phosphorus?

• A. Protons-15, electrons-15, neutron-16
• B. Protons-15, electrons-15, neutron-15
• C. Protons-15, electrons-16, neutrons-16
• D. Protons-16, electrons-16, neutrons-15

Answer 1

The correct option is A Protons-15, electrons-15, neutron-16

$\bullet$ An element is expressed as ${}_Z^A\text{X}$, where Z represents atomic number, A represents atomic mass and X stands for the atomic symbol of the given element.

$\bullet$ Phosphorus is expressed as ${}_{15}^{31}\text{P}$, hence, 15 is its atomic number and 31 the mass number.

$\bullet$ The atomic number gives the number of protons present in an atom. For a neutral atom, it also represents the number of electrons.

$\bullet$ Therefore, no. of protons = no. of electrons = 15

$\bullet$ Mass number represents the sum of the number of protons and neutrons present in the nucleus of an atom.

$\bullet$ Thus, no. of neutrons = mass no. - no. of protons = 31-15 = 16.