Question

An atom has an atomic number of $79$ and an atomic mass of $197$ . Once this atom enters into an ionic bond, it has how many protons, neutrons, and electrons?

Answer 1

With the help of atomic number, we can find an element. The atomic number of the elements is unique. Each element has its own fixed atomic number. However, the mass number of the element may vary.

Complete answer:
It is given that the atomic number $Z$ is $79$ , that means there are $79$ positively charged nuclear particles, $79$ protons in the nucleus. If the atom is neutral then it will contain $79$ electrons. Now we will write the electronic configuration of this atomic number which is:
$Z = \;[Xe]\;4{f^{14}}5{d^9}6{s^2}$
From the electronic configuration, it is clear that the element is in the $d$ block as the last electron is in the $d$ orbital. After knowing its block we will check its period and group. So here the last electron is in $5d$ thus its period is six. To find the group we have to add the $ns$ and $(n - 1)d$ hence we get $11th$ group. From this information it is clear that the element is gold.
The atomic number of gold is $79$ , thus it has $79$ protons. And if the element is in its neutral state then it has the same number of electrons as to neutralize the whole atom. So, the number of electrons is $79$ . We know that the number of neutrons is equal to the difference of atomic mass and number of protons. As it is given that atomic mass is $197$ , so the number of neutrons is:
$n = {A_m} - p$
Where, $n$ is the number of neutrons, ${A_m}$ is atomic mass, $p$ is the number of protons.
$n = 197 - 79$
$\Rightarrow n = 118$
Thus, the number of protons is $79$ , the number of electrons is $79$ and the number of neutrons is $118$ .

Note:
Remember that the atom has a unique atomic number but not atomic mass; it can vary because of isotopes. Atomic number is the number of protons present in the atom whereas the atomic mass is the sum of the number of protons and neutrons present in the nucleus of the atom.