Question

How many neutrons can an atom have?

Answer 1

Every atom consists of subatomic particles that keep them stable. The major subatomic particles that are known to people are – electron, proton and neutrons. The electron is the negatively charged particle, while the proton is the positively charged particle. The neutron is a neutral particle that holds the stability of the atoms. The combination of protons and neutrons together in an atom is called its nuclei.
We also know that the atomic mass number of an element is nothing but the sum of the number of protons (that is the atomic number of the element) and the number of neutrons.

Complete step by step solution:
We know that each element in the periodic table has unique characteristics and features. One among the features that are unique for each element is its atomic number that is determined by the number of protons in the atom $\left( Z \right)$ and the atomic mass number of the element $\left( A \right)$ .
The atomic mass number of an element is nothing but the sum of the number of protons and the number of neutrons in the atom. The combination of protons and neutrons is the nucleus and thus the protons and neutrons can be called the nucleons.
The electronic configuration of an element is affected by the charge on the nucleus which is decided by the number of protons in the atom. The neutrons being neutral does not seem to affect the electronic configuration of the atom. Since it has no effect in the atom, there is no specified limit for the number of neutrons that an atom can have.
However, if the number of neutrons exceeds $1.5$ times the number of protons then the nucleus will be considered radioactive or unstable.

Note:
Thus, for an atom $Ca_{20}^{40}$ with atomic number 20 and mass number 40 the number of neutrons will be,
$A - Z = 40 - 20 = 20$
Thus, calcium has 20 neutrons and is a stable compound with $\dfrac{N}{Z} = \dfrac{{20}}{{20}} = 1$ .