Physical Science:

An atom is the basic building block of matter. It defines the structure of elements. The name "atom" is taken from the Greek which means indivisible. It was once thought that atoms were the smallest things in nature and could not be further divisible. However, we now know that atoms are made up of very tiny particles, electrons, protons, and neutrons. The number of protons in a nucleus of an atom determines the name of the element. Atoms bond together to form molecules and make up all matter in the universe. Atoms play a crucial role in chemical reactions. The study of atoms is called the atomic physics. Many discoveries had been made throughout the \(20^{th}\) century to understand the structure of atom and its behavior.

The Nucleus is a central part of an atom that contains positively charged particles called protons and neutrons. Protons and neutrons are collectively called nucleons. The number of protons in an atom is caled an atomic number. Atomic number determines the element to which the atom belongs. The nucleus is surrounded by negatively charged electrons which are orbiting in shells. The atomic number and electronic arrangement in the shells determine the properties of an element. The size of nucleus is very small compared to the size of the atom, but it contains most mass of the atom. Therefore it is a highly dense part of the atom. The strong nuclear force holds the protons and neutrons together in the nucleus. Nucleus undergoes many changes such as when two nuclei combine together to form a large nucleus, it is called a fusion process. But when a large nucleus splits into two or more nuclei it is called a fission process. The study of atomic nuclei is called nuclear physics. Atoms of the same element are different called isotopes of the element. The isotopes of an element are atoms with the same number of protons but different number of neutrons in their nucleus.