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Mass Number of an atom is the total number of protons and neutrons present in an atom. We know that an atom consists of electrons, protons, and neutrons but the mass of the atom is contributed by protons and neutrons as the mass of electrons is very low hence it doesn’t contribute to the mass of an atom. In this article, we will learn what is the mass number, the mass number definition, the mass number formula, and the mass number of some commonly known elements.

What is Atomic Number?

Atomic number of an element is the total number of protons that are present in the atom. The atomic number of an atom explains the properties of an element, it is represented using the letter “Z”. All the elements are arranged in the periodic table on the basis of their atomic number.

For example,

  • Atomic number of oxygen is 8
  • Atomic number of carbon is 6

We represent the atomic number as,

  • 7Z (Atomic Number of Nitrogen)

What is Mass Number?

Mass Number of an atom also called Atomic Mass Number is the total number of protons and neutrons present in an atom. Rutherford in his gold foil experiment concluded that the mass of the atom is concentrated in a small region called the Nucleus which is positively charged. Later on, it was found that the nucleus consists of two particles Protons and Neutrons. Neutrons and Protons are together called nucleons. Proton is a positively charged subatomic particle while Neutron is a neutral particle. The mass of Proton and Neutron is the same and is equal to 1.67 ⨯10-27 kg. Compared to Proton and Neutron, an electron is 1000 times lighter as the mass of an electron is 9.1 ⨯ 10-31 kg

Mass Number Definition

Mass Number is defined as the sum of the total number of protons and neutrons present in the nucleus of atoms. It gives the idea of how heavier is the atom of an element.

Mass Number Formula

Mass Number is represented using the letter ‘A’ and we know that it is the total number of protons and atoms in an atom. Hence, the Mass Number Formula is given as:

Mass Number (A) = Number of Protons + Number of Neutrons

Mass Number Example

Some examples of the Mass Number are mentioned below:

Example 1: Hydrogen Atom has 1 proton, and no neutron hence the mass number of hydrogen is 1.

Example 2: Helium has 2 Protons and 2 Neutrons, hence, the mass number of Helium is 4.

Example 3: Sodium has 11 Protons and 12 Neutrons, hence the mass number of sodium is 23.

Mass Number Properties

The Properties of Mass Number are mentioned below:

  • Mass Number is sum total of the numbers of protons and neutrons present in an atom.
  • The symbol of Mass Number is A.
  • Mass Number for an Element can be different for different atoms of the same element. These are called isotopes. Example C-12, C-13 and C-14
  • Mass Numbers for two elements can be the same for atoms of two different elements. Such a pair of atoms are called isobars. Examples include Ar-40 and Ca-40 both have the same mass number but different atomic numbers as they are different elements.

How to Find Mass Number?

Mass Number of an atom is the total number of protons and neutrons present in the mass number. Mass Number can be calculated using the following steps:

Step 1: First look for the Atomic Number of the element as the Atomic Number is equal to the mass number.

Step 2: Now look for the number of neutrons in the atom.

Step 3: Add the Number of Protons i.e. Atomic Number and the number of neutrons to get the Mass Number of the element.

Calculation of Number of Neutrons

The formula used for calculating Mass Number can be modified to calculate the number of protons. We know that mass number is the sum of the number of protons i.e. Atomic Number and the Number of Neutrons. Hence, the Number of Neutrons can be calculated by subtracting Atomic Number from Mass Number.

Number of Neutrons = Mass Number – Number of Protons

OR

Number of Neutrons = Mass Number – Atomic Number

Mass Number of Elements

As we have already learned that Mass Number of an Element is the sum total of the number of protons and neutrons. Mass Number of elements is denoted by the letter ‘A’. Let’s learn the mass number of some elements.

Mass Number of Hydrogen

Hydrogen is the very first element in the modern periodic table. Hydrogen is the most abundant element found in the universe. Hydrogen is denoted by the letter ‘H’. The atomic number of Hydrogen is 1. It means it has one proton. But it has no neutrons. Hence, the mass number of hydrogen is 1.

Mass Number of Lithium

Lithium is the first element of the second period in the modern periodic table. The Atomic Number of Lithium is 3. The number of neutrons present in Lithium is 4. Hence, the Mass Number of Lithium is 7.

Mass Number of Carbon

Carbon is one of the most important in the periodic table. It is the main element found in any organic compound. The atomic carbon of Carbon is 6, hence the number of protons in carbon is 6. The number of neutrons in carbon is 6. Hence the mass number of Carbon is 12.

Mass Number of Nitrogen

Nitrogen is the most abundant gas found in the atmosphere. Nitrogen is denoted by the letter ‘N’. The atomic number of Nitrogen is 7. Hence, the number of protons in Nitrogen is 7. The number of neutrons in Nitrogen is also 7. Thus, the Mass Number of Nitrogen is 14.

Mass Number of Oxygen

Oxygen is an important non-metallic element. Oxygen supports life and combustion. Oxygen is denoted by the letter ‘O’. The atomic number of oxygen is 8. Hence, it has 8 protons. Oxygen has also 8 neutrons, hence the mass number of Oxygen is 16.

List of Mass Numbers of First 20 Elements

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)

Hydrogen

H

1

1

1

1

0

Helium

He

2

4

2

2

2

Lithium

Li 

3

7

3

3

4

Beryllium

Be

4

9

4

4

5

Boron

B

5

11

5

5

6

Carbon

C

6

12

6

6

6

Nitrogen

N

7

14

7

7

7

Oxygen

O

8

16

8

8

8

Fluorine

F

9

19

9

9

10

Neon

Ne

10

20

10

10

10

Sodium

Na

11

23

11

11

12

Magnesium

Mg

12

24

12

12

12

Aluminum

Al

13

27

13

13

14

Silicon

Si

14

28

14

14

14

Phosphorus

P

15

31

15

15

16

Sulfur

S

16

32

16

16

16

Chlorine

Cl

17

35

17

17

18

Argon

Ar

18

40

18

18

22

Potassium

K

19

39

19

19

20

Calcium

Ca

20

40

20

20

20

Atomic Number and Mass Number

Atomic Number is the number of protons in an atom while Mass Number is the total number of protons and neutrons in an atom. Atomic Number is denoted by the letter ‘Z’ and the Mass Number is denoted by the letter ‘A’. Atomic Number and Mass Number of an element are different however this is not the case always. In the case of Hydrogen, the Atomic Number and Mass Number both are equal to 1.

In general, the mass number is generally larger than the Atomic Number as the mass number takes account into both numbers of protons and the number of neutrons. Thus Atomic Number differs from the Mass Number by the number of neutrons present in the atom.

Difference between Mass Number(A) and Atomic Number(Z)

The difference between valency, A, and Z is discussed below:

  • 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 other atoms of the same or different elements is known as the valency of the atom.
  • The valence shell of the atom is the last shell in which electrons fill. It can accommodate 8 electrons and after that, it became chemically inactive and its valency becomes zero.
  • Thus the valency of an atom is defined as the number of atoms shared by the valance shell of the electron to achieve its octet state.
  • Mass number (A) is the number of nucleons i.e. protons and neutrons that any nucleus of the atom has.
  • Atomic number (Z) is the number of protons that any atom has.

Representation of an Atom

The atom of any element is represented by using the English Alphabet and its notation is discussed in the image below,

Structure-of-Atom

Atomic Mass vs Mass Number

Atomic Number and Mass Number both indicate the mass of an atom but they have some differences between them. Let’s learn the difference between Atomic Mass and Mass Number through the following table:

Atomic Number

Mass Number

It is the weighted average of the mass of an atom in the natural stage which also includes its isotopes

It is the total number of protons and neutron present in an atom

Atomic Mass takes into account all the isotopes that exist

Mass number is calculated differently for different isotopes

Atomic Mass can be fractional or in decimal

Mass Number is always a whole number

Unit of Atomic Mass is atomic mass unit.

Mass Number has no units

Atomic Mass is the same for all the atoms of an element regardless

Mass Number is different for different isotopes

Also, Check,

Solved Examples on Mass Number

Example 1: Find the number of electrons, protons, and neutrons of an atom with Atomic Number (Z) 13 and Mass Number (A) 26.

Solution:

Given,

Z = 13 and A = 26

  • Atomic Number = Number of Protons = Z =13
  • Number of Electrons = Number of Protons = 13
  • Mass Number = Number of Neutrons = A – Z = 26 – 13  = 13

Example 2: Find the number of electrons, protons, and neutrons of an atom with atomic number(Z) 19 and mass number(A) 39.

Solution:

For the atom given above,

Number of Protons = Z 
                                = 19

Number of Electrons = Number of Protons 
                                  = Z 
                                  = 19

Number of Neutrons = A – Z
                                 = 39 – 19
                                  = 20

Atomic Number and Mass Number – FAQs

1. Define Mass Number.

Mass number is determined by the number of protons and neutrons combined inside the nucleus of the atom. It is represented by the symbol A.

  • Mass Number (A) = Number of Protons + Number of Neutrons

2. How to find Mass Number of an Element?

Mass Number can be obtained by adding the total number of protons and neutrons present in the nucleus of an atom.

3. Why are Mass Numbers Whole Numbers?

Atomic Number and Mass number represent the number of sub-atomic particles that are present inside an atom and as we know that subatomic particles are not present in fractions but are rather always in whole numbers i.e. 1/2 electrons, 2/3 protons, etc do not exist. Thus, atomic numbers and mass numbers are always whole numbers.

4. What is Difference between Atomic Number and Mass Number?

The atomic number is the number of protons present in an atom while the mass number is the number of protons and neutrons present in an atom.

5. What is Mass Number of Sodium?

The mass number of Sodium is 23.

6. What is Mass Number of Oxygen?

The mass number of Oxygen is 16.



Last Updated : 20 Nov, 2023
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