MAK LABS — Std 8 General Science Chapter 5 Inside the Atom

1Answer the following.

(a) What is the difference in the atomic models of Thomson and Rutherford?

Thomson’s atomic model	Rutherford’s atomic model
Positive charge is spread throughout the atom.	Positive charge is concentrated in the nucleus at the centre.
Electrons are embedded in the positive sphere.	Electrons revolve around the nucleus.
There is no nucleus.	There is a nucleus at the centre.
The atom has no empty space.	Most of the atom is empty space.

(b) What is meant by valency of an element? What is the relationship between the number of valence electrons and valency?

The number of chemical bonds formed by an atom of an element is called its valency.

If the valence electrons are 4 or less, valency = number of valence electrons. Example: Na (2, 8, 1) has valency 1.
If the valence electrons are 4 or more, valency = 8 − number of valence electrons. Example: Cl (2, 8, 7) has valency 8 − 7 = 1.

(c) What is meant by atomic mass number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.

The total number of protons and neutrons in the nucleus of an atom is called its atomic mass number (A).

The nucleus of carbon has 6 protons and 6 neutrons.

Atomic number (Z) = Number of protons = 6
Mass number (A) = Protons + Neutrons = 6 + 6 = 12

(d) What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.

The particles smaller than an atom are called subatomic particles.

Proton: It has positive charge (+1e). Its mass is 1 u. It is in the nucleus.
Neutron: It has no charge. Its mass is 1 u. It is in the nucleus.
Electron: It has negative charge (−1e). Its mass is negligible. It revolves around the nucleus in shells.

2Give scientific reasons.

(a) All the mass of an atom is concentrated in the nucleus.

The mass of an atom is due to protons and neutrons. The mass of electrons is negligible. Protons and neutrons are present in the nucleus. Hence, all the mass of an atom is concentrated in the nucleus.

(b) Atom is electrically neutral.

The number of protons in the nucleus is equal to the number of electrons. The positive charge on protons balances the negative charge on electrons. Hence, an atom is electrically neutral.

(c) Atomic mass number is a whole number.

Atomic mass number is the total number of protons and neutrons. These particles cannot be in fractions. Hence, the atomic mass number is a whole number.

(d) Atoms are stable though negatively charged electrons are revolving within it.

Electrons revolve only in definite stable orbits. Their energy remains constant in an orbit. So they do not fall into the nucleus. Hence, atoms are stable.

3Define the following terms.

(a) Atom: The smallest particle of an element which retains its chemical identity in all physical and chemical changes is called an atom.

(b) Isotope: Atoms of the same element having the same atomic number but different mass numbers are called isotopes. E.g. ¹²₆C, ¹³₆C, ¹⁴₆C.

(c) Atomic number: The number of protons in the nucleus of an atom is called its atomic number (Z).

(d) Atomic mass number: The total number of protons and neutrons in the nucleus of an atom is called its atomic mass number (A).

(e) Moderator in nuclear reactor: The substance used to reduce the speed of neutrons in a nuclear reactor is called a moderator. E.g. graphite, heavy water.

4Draw a neat labelled diagram.

(a) Rutherford’s scattering experiment

Rutherford’s α-particle scattering experiment with a thin gold foil.

(b) Thomson’s atomic model

Thomson’s plum pudding model of the atom.

(c) Diagrammatic sketch of electronic configuration of Magnesium (Atomic number 12)

Magnesium: 12 electrons — K = 2, L = 8, M = 2.

(d) Diagrammatic sketch of electronic configuration of Argon (Atomic number 18)

Argon: 18 electrons — K = 2, L = 8, M = 8 (complete octet).

5Fill in the blanks.

(a) Electron, proton, neutron are the types of subatomic particles in an atom.

(b) An electron carries a negative charge.

(c) The electron shell K is nearest to the nucleus.

(d) The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of magnesium is M.

(e) The valency of hydrogen is ‘one’ as per the molecular formula H₂O. Therefore valency of ‘Fe’ turns out to be three as per the formula Fe₂O₃.

6Match the pairs.

Group ‘A’	Group ‘B’
a. Proton	iii. Positively charged
b. Electron	i. Negatively charged
c. Neutron	ii. Neutral

7Deduce from the datum provided.

Datum	To deduce	Answer
²³₁₁Na	Neutron number	23 − 11 = 12
¹⁴₆C	Mass number	14
³⁷₁₇Cl	Proton number	17

5. Inside the Atom

Exercises