Class 9, Chemistry Chapter 1. Structure of Atom notes

What is a molecule?

Molecule is the smallest particle of a substance which has all the properties of that substance. Molecules are made up of atoms.   

Dalton's atomic theory

John Dalton put forward his atomic theory based on logical thinking. 

Who established that there are two types of charges in substances?

            Sir Humphry Davi

Who is known as father of electricity?

Ans:

Michael Faraday

Who proposed the law of electrolysis?

Ans:

Michael Faraday

Who invented discharge tube (vacuum tube)?

Ans:

Heinrich Geissler

What did Julius Plucker’s experiment reveal?

Julius Plucker passed electric current through gasses at very low pressure in discharge tube. He observed a glow of light on the sides of glass tube. When a magnet is placed near the glow the position of the glow is changed. This indicated the presence of electric charge in the rays emerged from the gases in discharge tube.

Who discovered X-rays?

Ans:

Wilhelm Roentgen

What were the observation of J J Thomson about cathode rays?

—  Cathode rays are made up of negatively charged particles.

—  These particles possess mass and energy.

—  All the gases gave the same negative particles irrespective of the nature of the gas.

—  These particles are smaller than the atoms and are part of atoms.

—  The negatively charged particle in the atom is the electron

What were the inferences of Rutherford about atom from his experiments?

—  Major portion of an atom is empty.

—  There is a small part inside the atom where all the positive charges concentrated.

—  This central region of an atom is called a nucleus.

—  Particles responsible for the positive charge are protons.

—  The mass of a proton is equal to the mass of a Hydrogen atom.

—  Predicted the presence of chargeless particles in the nucleus.

Inferences of James Chadwick about atom.

 

—  There are neutral particles having mass equal to that of protons inside the nucleus.

—  These chargeless particles are neutrons.

—  The total mass of the atom is concentrated in the nucleus.

Discovery of sub atomic particles and scientists

Electron – J J Thomson

Proton – Earnest Rutherford

Neutron – James Chadwick
Features of subatomic particles

Atoms are electrically neutral. Why?

          Atoms have positively charged protons and negatively charged electrons. Each proton carries a positive charge and each electron carries a negative charge. Since the number of electrons and number of protons in a atom are equal, the atoms are electrically neutral.  

Explain the planetary model of atom suggested by Rutherford?

          The atom model suggested by Rutherford is known as planetary model of atom. The features of planetary model of atoms are as follows :

Ø  Atom has a center

Ø  Nucleus is at the center

Ø  Electrons revolve around the nucleus in shells                 

Why was the model of atom suggested by Rutherford eventually discarded?

Ans:

According to Rutherford’s model of atom, electron should eventually collapse into nucleus as they lose their energy while revolving within the field of attraction of the nucleus. But this does not happen in atoms. Rutherford failed to give an explanation to this doubt. So his model of atom was eventually discarded. 

Bohr model of the atom

Atomic number :- The total number of protons in an atom is called its atomic number. The letter Z is used to represent atomic number.

Mass number :- The total number of protons and neutrons in an atom is called mass number. This is represented by the letter A.

Principles of filling up of electrons in shells

 

2. Filling up of electrons in shells of higher energy happens only after the shells of lower energy are filled.

 3. The maximum number of electrons that can be accommodated in the outermost shell of an atom is 8

Isotopes

Atoms of the same element having same atomic number but different mass number are called isotopes

example: isotopes of Hydrogen are Protium,  Deuterium, and Tritium

          atomic number all these isotopes is 1

          mass number of Protium is 1

          mass number of Deuterium is 2

          mass number of Tritium is 3

 

Uses of isotopes

 

¢ Deuterium is used in atomic reactors

¢ Carbon-14 is used to determine the age of fossils and prehistoric objects.

¢ Phosphorous-31 is used as tracers for identifying the nutrient exchange in plants.

¢ Iodine-131 and Cobalt-60 are used in medical field for diagnosis and treatment of ailments like cancer and tumor.  

¢ Uranium-235 is used as fuel in atomic reactors.

Symbols of certain elements are given in the table. Complete the table.

Let us assess

Answer:

Ans:

a)Number of protons = 17

Number of electrons = 17

Number of neutrons = 35 – 17 = 18

b)Electronic configuration: K – 2,  L – 8, M – 7

 

c)

 

Ans:

a)Electronic configuration: K – 2, L – 8, M – 5

b) Atomic number of the atom = 15

c)Number of neutrons = 31 – 15 = 16

 

d)

Ans:

          Element A

Atomic number = 6

Mass number = 12

Electronic configuration of the atom: K – 2, L – 4

          Element B

Atomic number = 7

Mass number = 15

Electronic configuration of the atom: K – 2, L – 5

Element C

Atomic number = 6

Mass number = 14

Electronic configuration of the atom: K – 2, L – 4

 

b)A and C are isotopes. They are atoms of the same element having same atomic number and different mass number.