The structure of atoms around us comprises a systematic collection of nuclear particles. The negatively charged particles around the nucleus are called electrons. The neutrons and protons together constitute the nucleus of an atom. The electrons of the atom always surround this nucleus.

We can also define the atomic number of an atom as the total number of protons present in its specific nucleus. Neutral atoms are atoms that have an equal number of electrons and protons in them. Every atom gains or loses its electrons to increase its stability. The atoms that are of various elements have a different atomic structure. Every atom of an additional component has a different number of electrons and protons.

In earlier centuries, various scientists tried to work on the atomic structure. They attempted this study with the help of various atomic models. Every model has different advantages and disadvantages. The most appropriate atomic model was proposed by John Dalton, Niels Bohr, J.J. Thompson, and Ernest Rutherford.

John Dalton was an English chemist. He proposed that all the matter around us is composed of atoms. He also suggested that these atoms are indivisible as well as indestructible. According to Dalton, the same elements are identical, whereas the atoms of different elements are not the same in physical and chemical properties.

• Every matter around us is made of atoms.
• Atoms cannot be divided.
• The same type of element contains the same kind of atoms.
• Every element has a specific type of atom in it.
• Every atom has a particular mass number.
• All atoms undergo specific chemical reactions.
• You can neither create nor destroy an atom.

Daltons’ atomic theory also brought up laws of mass conservation, multiple proportions, and reciprocal proportions.

• Dalton’s atomic theory was not able to justify the presence of isotopes.
• The approach did not explain the structure of the atom adequately.
• The scientists were also able to discover that atoms are divisible.

In the 1900s, Sir Joseph John Thomson presented his atomic structure model. He also won the Nobel Prize for the discovery of negatively charged particles called electrons with these experiments. He worked on the investigation known as the experiment of cathode rays.

• Neutrons
• The mass of a proton and a neutron is approximately the same, i.e., 1.674×10-24. Neutrons do not have any charge on them. They are neutral.
• Protons
• Protons refer to the positively charged particles in the atom. The charge on a proton is 1e. 1e equals 1.602 × 10-19 Mass of a proton is 1.672×10-24. Protons are more decadent than electrons. The number of protons in an atom is equal to the atomic number of that specific element.
• Electrons
• The mass of an electron is around 9.1 × 10-31.Electrons do not contribute to the mass of an atom as they have a very negligible mass.The charge on an electron is -1e, i.e., approximately -1.602 × 10-19.

According to the given constraints, all the electrons are supposed to fill in the s, p, d, f orbitals:

1. Aufbau’s principle: According to this rule, the filling of electrons of an atom should take place according to the ascending order of energy of these orbitals:

• The orbitals with lower energy should fill first compared to the orbitals with higher energy levels.
• The orbital energy level is α(p + l) value if any two orbitals have a similar (n + l) value.
• Ascending order of energy levels is 1s, 2s, 2p, 3s, 3p, 4s, 3d,…

1. Hund’s rule of maximum multiplicity: According to this rule, in the case of the filling process of the same energy orbitals, all orbitals must be singly filled at first.
2. Pauli’s exclusion principle: According to this rule, no two electrons can have all four quantum numbers as the same value. However, if two electrons are present in the same state of energy, they must have opposite spin.

The atomic structure comprises the nucleus, protons, and neutrons. An atom also includes negatively charged particles called electrons which always revolve around the nucleus. Democritus was the first man who brought up the concept of atomic structure and quantum mechanics. He proposed for the first time that matter comprises atoms. Since then, many atomic models were proposed, leading to the discovery of subatomic particles and their properties.

1. What do you mean by subatomic particles?
2. Subatomic particles are the particles that comprise the atom. It refers to the electrons, protons, and neutrons.
3. What is the difference between the atomic structures of isotopes?
4. The atomic structures of isotopes have a different number of neutrons in their nucleus. It is also referred to as nucleon numbers.
5. What are isotopes?
6. Isotopes refer to the various types of elements with different neutrons and the same number of protons.
7. What do you mean by mass number?
8. The mass number of an atom is the sum of several neutrons and protons.
9. What is an atomic number of an atom?
10. The atomic number of an element is the number of protons.