## 27 November 2017

### 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 centre 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 = AAlZ = Number of protons + Number of neutornsAlNumber of protons

• Mass of aluminium ( A ) is 26
• Atomic number of aluminium ( Z ) is 13
• Therefore the number of neutron in the aluminium = 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 proton and 11 numbers of electrons.
• It has 2 electrons in first orbit, 8 electrons in second orbit and only one electrons in the last or third orbit. List of atomic number

### 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 1st orbit electrons < Energy of 2nd orbit electrons
• Energy of 2nd orbit electrons < Energy of 3rd orbit electrons
• Energy of 3rd orbit electrons < Energy of 4th 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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