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Atomic Number and Mass Number

  • Last Updated : 21 Sep, 2021

The structure of an atom comprises protons, neutrons and electrons which provide the mass and charge of the atoms. The nucleus comprises protons and neutrons, with the electron orbiting around that. Because atoms are electrically neutral, the quantity of positively charged protons and negatively charged electrons must be the same. Also since neutrons have no effect on the charge, their quantity is unrelated to the number of protons and can vary even amongst atoms of the same element.

Before understanding the concept of Atomic Number and Mass Number of an Element, Let’s discuss the three main elementary particles of an atom as:

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  • Electrons: Electrons are the negatively charged sub-atomic particles of an atom. The mass of an electron is negligible. The symbol for an electron is e. Electrons are extremely small and are found outside the nucleus.
  • Protons: Protons are the positively charged particles that are present in the nucleus of a hydrogen atom, that is protons are the positively charged sub-atomic particles of an atom. The mass of a proton is negligible. The symbol for an electron is p+. Electrons are extremely small and are found outside the nucleus.
  • Neutrons: Neutrons are subatomic particles that are one of the primary constituents of atomic nuclei. Neutrons are denoted by n or no. Neutrons don’t have any charge associated with them. They have a mass that is slightly greater than that of a proton. Neutrons and protons are collectively called nucleons.

Following are some important properties of electron, proton and neutron:

Particle

Charge

Mass of the particle

Symbol

Location in an atom

Electron

-1.6 × 10-19 Coulomb



9.11 × 10-31 kg

e

Outside the nucleus

Proton

+1.6 × 10-19 Coulomb

1.673 × 10-27 kg

p+

In the nucleus

Neutron

No charge

1.675 * 10-27 kg

n or no

In the nucleus

Atomic Number 

The number of protons that a chemical element has in its centre (nucleus) is called the atomic number. The mass number is determined by the number of protons and neutrons combined. 

Atomic numbers and mass numbers are always whole numbers as they are obtained by counting protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom. 

Mathematically, the atomic number is defined as.

Atomic Number (Z) = No of protons (p+)

The mass number reports the mass of the atom’s nucleus in atomic mass units (a.m.u).

Properties of the Atomic Number:

  1. The total number of protons in the nucleus of an atom gives us the atomic number of an atom.
  2. It is represented with the letter ‘Z.’
  3. Atoms of different elements have different atomic numbers. For example, all carbon atoms have the atomic number of 6, whereas all atoms of Oxygen have an atomic number of 8.
  4. This term was first introduced by Henry Gwyn-Jefferies Moseley.

Mass Number

Rutherford discovered that an atom’s nucleus, which is made up of protons and neutrons, contains the vast bulk of its mass. The total number of protons and neutrons in an atom is known as the mass number.

Properties of the Mass Number:



  1. The number of protons and neutrons combined give us the mass number of an atom.
  2. It is represented using the letter ‘A.’ For example, an atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is 12.
  3. While the number of protons remains the same in all atoms of an element, the number of neutrons can vary. Thus, atoms of the same element can have different mass numbers, and these are called isotopes.

Mathematically, the mass number is defined as.

Mass Number (A) = No of Protons + No of Neutrons

The mass number and the atomic number of different elements of the periodic table are as follows:

Element

Symbol

Atomic Number (Z)

Mass Number (A)

Number of Protons

Number of Electrons

Number of Neutrons (A-Z)

Valency

Hydrogen

H

1

1

1

1

0

±1

Helium



He

2

4

2

2

2

0

Lithium

Li 

3

7

3

3

4

+1

Beryllium

Be

4

9

4

4

5

+2

Boron

B

5

11

5

5

6

+3

Carbon

C

6

12

6

6

6

+4

Nitrogen



N

7

14

7

7

7

-3

Oxygen

O

8

16

8

8

8

-2

Fluorine

F

9

19

9

9

10

-1

Neon

Ne

10

20

10

10

10

0

Sodium

Na

11

23

11

11

12

+1

Magnesium

Mg

12

24

12

12

12

+2

Aluminium

Al

13



27

13

13

14

+3

Silicon

Si

14

28

14

14

14

+4

Phosphorus

P

15

31

15

15

16

-3

Sulphur

S

16

32

16

16

16

-2

Chlorine

Cl

17

35

17

17

18

-1

Argon

Ar

18

40

18

18

22

0

Potassium

K

19

39

19

19

20

+1

Calcium

Ca

20

40

20

20

20



+2

Valency and its difference from atomic number (A) and mass number (Z)

  1. The electrons present in the outermost shell of an atom are known as the valence electrons and their combining capacity to react and form molecules with atoms of the same or different elements is known as valency of the atom.
  2. Atoms of elements, having a completely filled outermost shell show little chemical activity and their combining capacity or valency is zero.
  3. For example, the number of electrons in the outermost shell of hydrogen is 1, and in magnesium, it is 2. Therefore, the valency of hydrogen is 1 as it can easily lose 1 electron and become stable. On the other hand, that of magnesium is 2 as it can lose 2 electrons easily and also attain stability.
  4. When we want to find out the valency, we look at electrons in the outermost shell but when we want to know the atomic number or the mass number, we look at the total number of protons and neutrons.

Sample Questions

Question 1: Define atomic number and mass number.

Answer: 

The number of protons that a chemical element has in its centre (nucleus) is called atomic number. 

Atomic Number (Z) = No of protons

The mass number is determined by the number of protons and neutrons combined. 

Mass Number (A) = No of Protons + No of Neutrons

Question 2: Why are atomic numbers and mass number whole numbers?

Answer: 

Atomic number and mass number are always whole numbers as they are obtained by counting protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom. 

Question 3: Why is atomic number denoted by Z?

Answer: 

The atomic number symbol, Z, stands for “Zahl,” meaning German number. The symbol Z denoted an element’s place in the periodic table before 1915.

Question 4: What is the valency of an atom?

Answer: 

The electrons present in the outermost shell of an atom are known as the valence electrons and its combining capacity to react and form molecules with atoms of the same or different elements is known as valency of the atom.

Question 5: What is the difference between valency and mass number?

Answer:

When we want to find out the valency, we look at electrons in the outermost shell but when we want to know the atomic number or the mass number, we look at the total number of protons and neutrons.




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