What is The Composition of Nucleus and Atomic Mass?

CBSE Board students must learn about atomic mass in detail to solve questions that are asked in exams. An Atomic Mass Unit (amu) is a unit of mass to determine atomic and molecular weights. The Atomic Mass Unit is defined as one-twelfth of the mass of an unbound atom of carbon-12 in its ground state. This unit allows scientists to compare the masses of different atoms and molecules. One amu is approximately equal to $1.66053906660 \times {10}^{-24}$ Kilograms.

A nucleus of an atom is composed of a tightly packed arrangement of protons and neutrons. Protons and neutrons are two heavy particles in an atom, and therefore, 99.9% of the mass is concentrated in nucleus. Protons possess a net positive charge and therefore the nucleus of atom is positively charged as a whole. The negatively charged electron revolves around the central nucleus. In the JEE Main exam and IIT JAM exam, questions based on the composition of nucleus will not be asked but based, on the properties and entities of nucleus, questions may be asked.  

Since mass concentration at the nucleus of an atom is immense, the nuclear force that holds protons and neutrons together is also large. Protons are in close vicinity to each other within the tiny nucleus, and the electrostatic forces of repulsion also act within the nucleus. Nuclear energy depends on the release of the energy which is trapped within the nucleus of atom. The total number of protons within nucleus is equal to the number of electrons that revolve around nucleus. Therefore, the atom is electrically neutral.

As a CUET exam aspirant, you must know about the terminologies like Isotopes and nucleus. Isotopes are variants of chemical element that have the same number of protons in their nuclei but with different number of neutrons. Difference in number of neutrons mean that isotopes of same element have different atomic masses.  An isotope has the following characteristics:

• Same atomic number: Isotopes have same atomic number since they have the same number of protons.
• Different mass numbers: The mass number of isotope is the sum of number of protons and neutrons in nucleus.
• Chemical properties: Isotopes of an element have almost identical chemical properties because they have same number of electrons that determines chemical behaviour.
• Physical properties: Physical properties of isotopes can be different. For instance, isotopes can have different boiling points, densities and melting points because of the varying masses.
• Stability: Some isotopes do not undergo radioactive decay which means they are stable. On the other hand, some isotopes are radioactive, which means they are unstable and decay over time. 
• Natural abundance: Many elements in the nature exist as mixture of isotopes. The relative abundance of each isotope may vary.

Let us understand how to calculate the average atomic atoms:

• First, it is important to determine the isotopes of elements that you are studying. Isotopes have the same number of protons, but they have different number of neutrons.
• After that, it is important to obtain the atomic mass of every isotope which is given in atomic mass units.
• Now, you need to find the relative abundance of each isotope. This is mostly given as percentage.
• After that, you need to convert the percentage abundances to decimals by dividing each by 100.
• Multiply mass of every isotope by its relative abundance in the decimal form.
• Add up all the contributions from all isotopes to get average atomic mass.

The formula to calculate the average atomic mass is:

Average Atomic Mass = (Mass of Isotope₁ × Abundance₁) + (Mass of Isotope₂ × Abundance₂) + … + (Mass of Isotopeₙ × Abundanceₙ)

Let us take a look at the following table to understand the diffrence between atomic mass and mass number for students who are planning to take the IISER exam in the near future:

Let us take a look at some of the important questoons related to atomic mass and composition of nucleus: