In this theory, Neil Bohr’s atomic structure, definitions of matter, molecules, atoms, nucleus, atomic weight, extra nucleus, electrons in orbit, atomic number and energy of an electron are explained.

Atomic structure

• The scientist Neil Bohr explains the atomic structure. 
• According to Neil an atom consists of positive charged nucleus and negative charged electrons revolve in circular orbit. 
• The electrons can revolve around the nucleus in certain orbits. 
• Each orbit consists of certain energy level. 
• As the distance between nucleus and orbit increases, energy of electrons increases. 
• If the energy is given to an electron, the electron lifts to higher energy level. This is called as excitation.

Matter

• The matter is defined as anything which occupies space and weight such as wood, copper, water, air etc.

Molecule

• The matter is made up of very small particles known as molecules.

Atoms

• It is smallest into which matter can be divided without release of charged particles. It is smallest unit of matter.

Nucleus

• The nucleus of an atom is mainly consists of two particles protons and neutrons. 
• The proton has positive charge whereas the neutron has no charge. The mass of neutron is same as that of proton. 
• Therefore the nucleus has positive charge.

Atomic weight

• It is sum of weight of protons and weight of neutrons.
• Atomic weight = weight of protons + weight of neutrons
• A = Z + N
• According to electron theory, the atoms of all elements composed mainly two parts namely nucleus at the center and electrons surrounding it. 
• The nuclear symbol consists of three parts, symbol of element, atomic number and atomic mass. 
• Let us try to understand it.

²⁶Al₁₃ = ^A Al _Z = ^{Number of protons + Number of neutrons} Al _{Number of protons}

Mass of aluminum (A) is 26

Atomic number of aluminum (Z) is 13

Therefore the number of neutron in the aluminum = A – Z = 26 – 13

= 13

Extra nucleus (Electrons)

• It is outer part of an atom which contains electrons only. 
• The charge of an electron is equal and opposite to that of charge of proton. The number of electron is equal to number of proton. 
• Therefore we can say that an atom is neutral.
• Charge of an electron = 1.602 × 10 ^{– 19} Coulomb
• Mass of an electron = 9.0 × 10 ^{– 19} Kg
• Radius of an electron = 1.9 × 10 ^{– 15} Meter
• The ratio of charge of an electron to the mass of an electron is 1.77 × 10¹¹ Coulomb per kg. 
• It means that mass of an electron is very small as compared to its charge. The electron is greatly affected by electric or magnetic field.

Electrons in orbit

• The electrons of an atom around nucleus revolve in different orbits. 
• The number of electrons in any orbit is equal to 2n².

Where n is orbit number

Number of electrons in first orbit = 2(1)² = 2

                                Second orbit = 2(2)² = 8

                                   Third orbit = 2(3)² = 18

• The number of electron in the last orbit should not greater than 8
• Any orbit should not contain more than 18 electrons. 
• The atomic number of silicon is 14. It means that 2 electrons lies in first orbit, 8 electrons in second orbit and remaining 4 electrons in the third or last orbit.

Atomic number

• It is defined as the number of proton is equal to number of electrons.

        Atomic number = Number of proton

                                  = Number of electrons

• The atomic number of sodium is eleven (11), it means that it has 11 number of protons and 11 number of electrons. 
• It has 2 electronics in first orbit, 8 electrons in the second orbit and only one electron in the last or third orbit. List of number is given here.

https://education.jlab.org/itselemental/index_num.html

Energy of an electron

• Each electron consists of kinetic energy due to its motion and potential energy due to charge of electron. 
• As the distance between nucleus and electron increases, the energy of an electron also increases. Therefore
• Energy of 1^st orbit electrons < Energy of 2^nd orbit electrons
• Energy of 2^nd orbit electrons < Energy of 3^rd orbit electrons
• Energy of 3^rd orbit electrons < Energy of 4^th orbit electrons
• It means that electrons of first orbit consists lowest energy whereas the electrons of last orbit have highest energy. 
• The electrons of last orbit play important role to determine electrical, chemical and physical properties of a material.

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