Question

Mercury has an atomic number of 80. How many electrons are there in an isotope of mercury $204$?

Answer 1

Hint: According to Dalton’s theory the atom is the most basic form of matter but the atom contains various subatomic particles like the electrons and protons. The atomic structure of the atom depends on the electronic configuration in which the electrons are arranged inside the atoms. This electronic configuration is responsible for various properties that the atom shows.

Complete step by step answer:
Mercury $204$ indicates that the mass number of this isotope of mercury is $204$. The mass number of the atom is the summation of the number of protons and neutrons.
The atomic number of the element is the number of protons that are present in the atom. So to get the number of electrons in the element we just need to decrease the number of protons from the summation amount of mass number
From the above discussion we get
Number of electron $$=$$ Mass number $-$ atomic number
Number of electrons $=204-80=124$

Note: The atom consists of electrons which are present inside the atoms in specific orbits or shells. These electrons are present in the shells according to the energy. The distribution of the electron is similar to how the planets move in our solar system.
The electrons are placed in shells according to the energy. These shells are also given naming convention which helps in the description of an electron that is present in the atomic shells.
All the electrons in the atom have some quantum numbers which are used to describe its location and thus also help in inferring the properties that the element is going to show.