Atomic symbol of an element
An atom of every element has an atomic number and a mass number. To represent both these entities for an element, the chemical or atomic symbol of the element is written with the atomic number at its bottom–left in the subscript and the mass number is written on its top–left in the superscript.
For example, we represent one type of chlorine atom in the following manner:
Atomic symbol of chlorine
What are Isotopes?
Isotopes are defined as the atoms of the same element having the same number of protons but different number of neutrons. In other words, isotopes are the atoms of the same element that have the same atomic number but different mass number.
For example, some chlorine atoms have 20 neutrons instead of 18 in their nucleus and are represented in the following manner:
Isotopes of chlorine
What are Isobars?
Atoms of different elements having different atomic numbers but the same mass number are called isobars.
Consider the examples of five different elements: sulphur, potassium, calcium, chlorine and argon. In each case, the number of protons and neutrons is different. But the sum of the protons and neutrons is the same.
| Element | Number of protons | Number of neutrons | Mass number (protons + neutrons) |
| Sulphur | 16 | 24 | 40 |
| Chlorine | 17 | 23 | 40 |
| Argon | 18 | 22 | 40 |
| Potassium | 19 | 21 | 40 |
| Calcium | 20 | 20 | 40 |