What is Atomic Mass? Atomic mass is represented as the average weight of an atom and also recognized as atomic weight. An atom is composed of three subatomic particles, i.e., protons, neutrons, and electrons, as electrons are very light in weight, so the atomic mass considered as the mass of a total number of neutrons and protons present in the nucleus of an atom. The atomic number and the mass number of an element is usually written in the following way: The helium atom has a mass number of 4 and atomic number of 2. Example; What is the mass number and atomic number of the? Solution: Mass number = 31, atomic number = 15. Number of neutrons = mass number – atomic number. Example: How many neutrons.

a·tom·ic mass num·ber

atomic mass number

a·tom·ic mass num·ber

Contents

• Atomic number and Mass number
• Isotopes

Atom

An atom is the smallest particle of an element which can take part in chemical reaction. Atom consists of three fundamental particles i.e. proton, neutron and electron. Atoms of same elements are similar in properties whereas atoms of different elements are different in properties. Example:- ‘H’ represent the atom of hydrogen.

Proton is positively charged and electron is negatively charged particle. In an atom, number of protons = number of electrons. Hence, the net charge present in an atom is zero i.e. a free atom is chargeless.

Atomic number and Mass number

Atomic number :

• Atomic number is the number of protons present in an atom.
• The modern periodic table is arranged in order of increasing atomic number.

Mass number and Atomic mass :

• Mass number is the sum of the number of protons and the number of neutrons present in an atom. It is a whole number.

Mass no. of an atom = No. of protons + No. of neutrons

• Atomic mass is the average mass of the all of the isotopes of that element. It is a decimal number.
• For example: Hydrogen has three isotopes – ₁H¹, ₁H² and ₁H³ having mass number 1, 2 and 3 respectively. Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium. Therefore the atomic mass of hydrogen is 1.00784 amu.
• The atomic mass of an element element is measured in atomic mass unit (amu, also known as Daltons ‘ D’or unified atomic mass unit ‘u’).
• 1amu = 1.66 x 10^-24 grams. 1gm = 6.022 x 10²³ amu ( i.e. Avogadro’s number).

Here,

• Atomic number = Number of protons = Number of electrons = 13
• Mass number = No. of protons + No. of neutrons
• No. of neutrons = Mass number – No. of protons = 27-13 = 14.

Atomic mass of first 20 elements

Isotopes

Atoms of the same element having same atomic number but different mass number (atomic mass/weight) are called isotopes. For example:

Isotopes of hydrogen :

There are three isotopes of hydrogen:

1. Protium or ordinary hydrogen
2. Deuterium or heavy hydrogen
3. Tritium or radioactive hydrogen.

Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium.

Isotopes have different physical properties since they differ in their mass number.

They have same chemical properties since their electronic configuration is same. However, they differ in the rate of chemical reaction. For example, D₂ reacts with Cl₂ about 13 times slower than H₂ does. The different in rate of reaction due to difference in mass of the atoms of the same element is called isotope effect.

Some other examples of isotopic elements :

Isobars

Atoms of different elements having different atomic number but same mass number are called isobars. For example :

₁₈Ar⁴⁰, ₁₉K⁴⁰ and ₂₀Ca⁴⁰

Isotones

Atoms of different elements having different atomic number and mass number but same number of neutrons are called isotones. For example :

₆C¹⁴, ₇N¹⁵ and ₈O¹⁶

Objective questions and their answers

1. Which of the following is known as heavy hydrogen?

a. Protium c. Tritium

b. Deuterium d. Para hydrogen

2. Which of the following is known as radioactive hydrogen?

a. Protium c. Tritium

b. Deuterium d. Para hydrogen

3. Least abundant isotope of hydrogen is:

a. Protium c. Tritium

b. Deuterium d. Heavy hydrogen

4. Diamond and graphite are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

5. ₆C¹⁴ and ₈O¹⁶ are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

6. ₆C¹⁴ and ₇N¹⁴ are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

7. All particles residing inside the nucleus of an atom are termed as:

a. Protons c. Electrons

b. Neutrons d. Nucleons

Atomic And Mass Number For Argon

8. What makes the atomic mass fractional ?

a.Prerence of isotopes

b. Number of unpaired electrons

How To Find The Atomic Number

c. Spherical shape

d. Quantum number.

9. Which of the following are not isotopes:

a. ₁H¹ and ₁H³

b. ₁₈K⁴⁰ and ₂₀Ca⁴⁰

c. ₆C¹⁴ and ₇N¹⁴

d. Both b and c.

10. Charge present in the nucleus of an atom is :

a. Positive c. Chargeless

b. Negative d. Both +Ve and -Ve

11. Molecular weight of heavy water is :

a. 16 c. 20

b. 18 d. 22

Answers :

1. b 2. c 3. c

4. d [Note : different forms of same element having different properties are called allotropes]

5. c 6. b 7. d

8. a 9. d 10. a

Atomic And Mass Number Chart

11. c Note :Heavy water– Deuterium oxide (D₂O) is called heavy water. It’s molecular weight is 20 and boiling paint is 101.5⁰C and melting point is 3.8⁰C.

Atomic And Mass Number Of Carbon

References

Atomic Number And Mass Worksheet

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.