Short Answer

A mass number is the total number of protons and neutrons present in the nucleus of an atom. Since electrons have very little mass, only protons and neutrons contribute to the mass of an atom. The mass number helps identify different isotopes of an element because isotopes have the same number of protons but different numbers of neutrons.

The mass number is represented by the symbol A. For example, carbon-12 has a mass number of 12 (6 protons + 6 neutrons), while carbon-14 has a mass number of 14 (6 protons + 8 neutrons). Mass number plays an important role in atomic structure and nuclear chemistry.

Detailed Explanation :

Mass Number

The mass number is one of the most important concepts in atomic structure. It helps us understand how atoms differ in mass and how isotopes of an element are formed. Every atom contains three basic particles—protons, neutrons, and electrons. Among these, protons and neutrons are heavy and stay inside the nucleus, while electrons are light and move outside the nucleus. Because electrons have very small mass, they do not contribute significantly to the overall mass of an atom.

The mass number is the sum of the number of protons and neutrons in the nucleus. These two particles are collectively known as nucleons. The mass number tells us how heavy an atom is relative to others and helps identify different forms of the same element.

The mass number is represented by A in atomic notation.

Definition of Mass Number

The mass number of an atom is defined as:

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

For example:

• A helium atom has 2 protons and 2 neutrons → mass number = 4
• A nitrogen atom has 7 protons and 7 neutrons → mass number = 14

This simple formula helps calculate the mass of any atom when the number of protons and neutrons is known.

Why Mass Number Is Important

The mass number provides valuable information:

1. Tells the total nucleons in the nucleus
   Both protons and neutrons contribute mass, giving the atom its weight.
2. Helps identify isotopes
   Isotopes have the same atomic number (same number of protons) but different mass numbers due to different numbers of neutrons.

Example:

1. • Carbon-12 → mass number 12
   • Carbon-14 → mass number 14
2. Used in writing nuclear symbols
   The mass number is written at the top left of an element’s symbol (A/Z notation).

Example:
\,^{14}_{6}C represents carbon-14.

1. Helps understand nuclear reactions
   Nuclear processes involve changes in mass number when neutrons or protons are added or removed.
2. Important for calculating atomic mass
   The mass number gives an approximate value of the atomic mass because 1 proton = 1 amu and 1 neutron = 1 amu.

Difference Between Mass Number and Atomic Number

Many students get confused between atomic number and mass number. Understanding the difference is important:

• Atomic number (Z) = number of protons
• Mass number (A) = protons + neutrons

Atomic number identifies the element, while mass number tells how heavy its atom is.

Examples to Understand Mass Number

Example 1: Hydrogen

Hydrogen has 1 proton and 0 neutrons.
Mass number = 1 + 0 = 1
Its isotope, deuterium, has:
1 proton + 1 neutron → mass number = 2

Example 2: Oxygen

Oxygen has 8 protons and 8 neutrons.
Mass number = 8 + 8 = 16

Another isotope of oxygen:
8 protons + 10 neutrons → mass number = 18

Example 3: Sodium

Sodium has 11 protons and 12 neutrons.
Mass number = 11 + 12 = 23

These examples show how mass number changes depending on the number of neutrons.

Mass Number and Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. Because mass number depends on neutrons, it is used to name isotopes.

For example:

• Chlorine-35 → mass number 35
• Chlorine-37 → mass number 37

Both are chlorine because they have 17 protons, but their mass numbers differ.

How Mass Number Helps in Nuclear Chemistry

Mass number plays a key role in nuclear reactions:

• When a neutron is added → mass number increases
• When a neutron is lost → mass number decreases
• During radioactive decay, mass number may change

Scientists use mass number to track these changes and understand nuclear stability.

Conclusion

A mass number is the total number of protons and neutrons in the nucleus of an atom. It represents how heavy the atom is and helps differentiate isotopes of the same element. Represented by the symbol A, the mass number is essential for writing nuclear symbols, understanding atomic structure, and studying nuclear reactions. It remains a crucial concept in both basic and advanced chemistry.