Atomic Number and Mass Number, Isotopes and Isobars

Atomic number and mass number both are main characteristics of an atom of the element. If you know the atomic and mass number of an atom of the element, then you can easily calculate the number of neutrons present in the nucleus of the atom. Neutral atom consists of the same number of protons and electrons. By knowing atomic number and mass number of an element we can predict many properties of the element. 


What is the Atomic Number of an Element? 

As we have already discussed atomic structure in other articles so you would be knowing that now an atom is composed of electrons, protons and neutrons. Electrons revolve around the nucleus of an atom while protons and neutrons are present in the nucleus of an atom. Number of protons present in an atom of an element determines its atomic number. All atoms of the same element consist of the same number of protons or we can say all atoms of the same element have the same atomic number. Atomic number represents the number of protons in an atom. Atomic number is a characteristic property of an element. Actually, elements are defined by the number of protons they have. By knowing the number of protons in an element we can predict its various properties. This is the reason all elements are arranged according to their atomic number (or number of protons) in the periodic table. We can understand the importance of atomic numbers by Modern Periodic Law which was given by H.G.J. Moseley around 1913. The modern periodic table is based on this rule. Modern periodic law states that “Physical and Chemical properties of an element are periodic functions of their atomic numbers.” Atomic number of an element is denoted by ‘Z’. 


Thus, the atomic number of an element is the number of protons present in the nucleus of its one atom. In short it can be represented as –

Atomic Number (Z) = Number of protons


For example, a hydrogen atom has one proton in its nucleus, so its atomic number is 1. While an oxygen atom has eight protons in its nucleus, so its atomic number is 8. First eighteen elements are given below with their atomic number, symbol, and number of protons for your better understanding –


Element 

Symbol 

Atomic Number 

Number of Protons 

Hydrogen 

1

1

Helium 

He 

2

2

Lithium 

Li 

3

3

Beryllium 

Be 

4

4

Boron 

5

5

Carbon 

C

6

6

Nitrogen 

7

7

Oxygen 

8

8

Fluorine 

F

9

9

Neon 

Ne 

10

10

Sodium 

Na 

11

11

Magnesium 

Mg 

12

12

Aluminium 

Al 

13

13

Silicon 

Si 

14 

14 

Phosphorous 

15

15

Sulphur 

16 

16 

Chlorine 

Cl 

17 

17 

Argon 

Ar

18 

18 


What is the Mass Number of Elements? 

As we know, the mass of an atom of any element is concentrated at its nucleus. So, the mass number of an atom depends on the composition of its nucleus. It means the mass number of an atom depends on the number of protons and neutrons present in the nucleus of that atom. Protons and neutrons together known as nucleons. Thus, the mass number can be defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example, oxygen has 8 protons and 8 neutrons, so its mass number is 16u. while aluminium has 13 protons and 14 neutrons, so its mass number is 27u. Mass number of an atom is denoted by ‘A’. In short it can be written as –


Mass number (A) = Number of protons + Number of neutrons = Number of nucleons


First eighteen elements are given below with their symbol, number of protons, number of neutrons and mass number for your better understanding –


Element 

Symbol 

Number of Protons 

Number of Neutrons 

Mass number 

Hydrogen 

1

0

1u

Helium 

He 

2

2

4u

Lithium 

Li 

3

4

7u

Beryllium 

Be 

4

5

9u

Boron 

5

6

11u

Carbon 

C

6

6

12u

Nitrogen 

7

7

14u

Oxygen 

8

8

16u

Fluorine 

F

9

10

19u

Neon 

Ne 

10

10

20u

Sodium 

Na 

11

12

23u

Magnesium 

Mg 

12

12

24u

Aluminium 

Al 

13

14

27u

Silicon 

Si 

14 

14

28u

Phosphorous 

15

16

31u

Sulphur 

16 

16

32u

Chlorine 

Cl 

17 

18

35u

Argon 

Ar

18 

22

40u


Most of the students make mistakes while writing notation of an atom. So, as we are discussing mass number and atomic number of atoms, let us give you a brief about how to write notation of an atom correctly. While writing the notation of an atom of an element, its mass number and atomic number are written on the left of the symbol of the element. More precisely, mass number should be written at the top left and atomic number at the bottom left of the symbol of element. The notation of an atom is represented below –

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Or 

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Where X = Symbol of element 

A = Mass number 

Z = Atomic number 

For example, oxygen is written as 168O. 


Isotopes and Isobars 

The term isotope is formed from the two Greek words – isos and topos. Isos means equal and topos means place, thus isotope means ‘the same place’. Same place has been assigned to all isotopes of an element in the periodic table. On the other hand, the term isobar is made up of two Greek words – isos and baros. Isos means equal and baros means weight. Thus, isobar means ‘The equal weight’. 


Let us discuss isotopes and isobars separately in detail.  


What are Isotopes? 

Those atoms of an element which have the same atomic number, but different mass numbers are called isotopes of that element. Thus, we can say isotopes of an element have the same number of protons but different number of neutrons. For example, hydrogen has three isotopes, namely, protium, deuterium and tritium. Their atomic number, mass number, number of protons and neutrons are given below with their atomic notation in tabular form –



Number of protons 

Number of neutrons 

Atomic number (Z) 

Mass number (A)

Atomic notation 

Protium 

1

0

1

1

11H

Deuterium 

1

1

1

2

21H

Tritium 

1

2

1

3

31H


Many elements have various isotopes such as carbon also has two isotopes 612C and 614C. Each isotope of element is pure substance. Isotopes of an element possess the same chemical properties but different physical properties. Thus, isotopes can be defined as the atoms of the same element, having the same atomic number but different mass numbers. 


What are Isobars? 

Atoms of different elements with different atomic numbers but the same mass number are called isobars. For example, atoms of calcium and argon possess the same mass number which is 40u while different atomic numbers 20 and 18 respectively. Thus, we can say in isobars the number of nucleons is the same but the number of protons are different. 


This ends our coverage on the topic “An introduction to Atomic number and Mass number, Isotopes and Isobars”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.