Atomic Number: Definition, Isotopes and Isobars

Atomic Number Definition

An atom consists of protons, neutrons, and electrons. While electrons revolve around the nucleus, protons and neutrons are inside the nucleus. The number of protons in an atom determines its atomic number. Every atom of an element has the same number of protons and hence the same atomic number. In other words, the atomic number denotes the number of protons. The atomic number is a property of an element by which they are defined. It helps predict the element’s properties and thus facilitates arranging the various elements on the periodic table according to their atomic number represented as Z. Atomic Number (Z) = Number of protons Hydrogen with 1 proton has an atomic number of 1 while oxygen with 8 protons has an atomic number of 8.

Mass Number of Elements

The mass of an atom is the total number of its protons and neutrons. The collective term of protons and neutrons is nucleons. Thus, the mass of an atom is defined as the total number of protons and neutrons that are inside its nucleus. For example, oxygen has 8 protons and 8 neutrons, so we can calculate its mass number as 16u. Aluminium with 13 protons and 14 neutrons makes its mass number 27u. The mass number is denoted by ‘A’. So, it is written as – Mass number (A) = Number of nucleons = Number of protons + Number of Neutrons While writing the notation of an atom, the position of mass number and atomic number must be specific. Both these numbers are written on the left side with the mass number at the top and the atomic number at the bottom of the symbol. For example, oxygen has an atomic number of 8 and a mass number of 16. It will be written as, For example, oxygen is written as 168O.

Isotopes and Isobars

The term isotope is derived from Greek words – isos and topos which means equal and place. So, isotope means ‘the same place’. All isotopes of a single element will be placed in the same place in the periodic table. On the other hand, the isobar came from Greek words – isos and baros, which means equal and weight, respectively. Isobars mean, ‘The equal weight’.

What are Isotopes?

Isotopes differ in their atomic mass but have the same atomic number. We can say that isotopes have the same number of protons but different numbers of neutrons in their nucleus. The isotopes of hydrogen namely, protium, deuterium and tritium. Their atomic number is 1 and their mass number is 1, 2, and 3 respectively. Their respective atom notations are, ¹¹H, ²¹H and ³¹H. Several elements have isotopes, the most common being carbon – ¹²⁶C, ¹³⁶C and ¹⁴⁶C. Isotopes have the same chemical properties but have different  physical properties and atomic masses. Thus, isotopes are defined as the different atoms of the same element, having the same atomic number but differing in their mass numbers.

What are Isobars?

When different atoms of different elements have the same atomic mass but a different atomic number, they are called isobars. For example, atoms of calcium and argon, both have the same mass number 40u but different atomic numbers of 20 and 18 respectively. Thus, they are isobars with the same number of nucleons but different number of protons.