Class 8 Science Lesson 5 – Inside the Atom | Questions and Answers

 

CLASS 8 • SCIENCE

🔬 5. Inside the Atom

Exercise Questions and Answers

📖 Exercise – Answer the Following

Question 1(a)

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

✅ Answer

Thomson's Atomic Model	Rutherford's Atomic Model
According to Thomson, an atom is a positively charged sphere in which electrons are embedded.	According to Rutherford, an atom has a small, dense and positively charged nucleus at its centre.
There is no nucleus in Thomson's model.	A nucleus is present at the centre of the atom.
Positive charge is distributed throughout the atom.	Almost all the positive charge and mass are concentrated in the nucleus.
Electrons are embedded in the positively charged sphere.	Electrons are present outside the nucleus and revolve around it.
It does not explain the presence of empty space in an atom.	It shows that most of the atom is empty space.

💡 In short: Thomson proposed that electrons are embedded in a positively charged sphere, whereas Rutherford proposed that electrons revolve around a small, dense nucleus.

Question 1(b)

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

✅ Answer

Valency: Valency of an element is the combining capacity of an atom. It is related to the number of electrons an atom loses, gains or shares to achieve a stable electronic configuration.

Valence electrons: The electrons present in the outermost shell of an atom are called valence electrons.

📌 Relationship between valence electrons and valency:

• If the number of valence electrons is 1, 2, 3 or 4, the valency is generally 1, 2, 3 or 4 respectively.
• If the number of valence electrons is 5, 6 or 7, the valency is generally 8 minus the number of valence electrons.
• Noble gases generally have valency 0.

Element	Electronic Configuration	Valence Electrons	Valency
Sodium (Na)	2, 8, 1	1	1
Chlorine (Cl)	2, 8, 7	7	1
Oxygen (O)	2, 6	6	2

Question 1(c)

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

✅ Answer

Mass number: The mass number of an atom is the sum of the number of protons and neutrons present in its nucleus.

Mass Number (A) = Number of Protons + Number of Neutrons

Atomic number: The atomic number is the number of protons present in the nucleus of an atom. It is represented by Z.

🧪 For Carbon:

• Number of protons = 6
• Number of electrons = 6
• Number of neutrons = 6

Atomic Number (Z) = Number of Protons = 6

Mass Number (A) = 6 + 6 = 12

⭐ Therefore: The atomic number of carbon is 6 and its mass number is 12.

Question 1(d)

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

✅ Answer

The particles present inside an atom are called subatomic particles. The three main subatomic particles are electrons, protons and neutrons.

Particle	Electrical Charge	Approximate Mass	Location
⚡ Electron	Negative (−)	Very small; about 1/1836 of proton mass	Outside the nucleus
➕ Proton	Positive (+)	Approximately 1 u	Inside the nucleus
⚪ Neutron	Neutral (0)	Approximately 1 u	Inside the nucleus

⚡ Electron

1. Electrons are present outside the nucleus.
2. They have a negative electrical charge.
3. Their mass is very small compared with that of a proton or neutron.

➕ Proton

1. Protons are present in the nucleus.
2. They have a positive electrical charge.
3. Their mass is approximately 1 u.

⚪ Neutron

1. Neutrons are present in the nucleus.
2. They have no electrical charge.
3. Their mass is approximately 1 u.

⭐ Quick Revision

Atomic Number = Number of Protons

Mass Number = Protons + Neutrons

Neutral Atom: Protons = Electrons

2. Give scientific reasons

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

Answer:

An atom contains electrons, protons and neutrons. Protons and neutrons are present in the nucleus and have almost all the mass of the atom. The mass of electrons is very small and can be neglected. Therefore, almost all the mass of an atom is concentrated in the nucleus.

💡 Remember:
Protons + Neutrons → Almost all the mass → Nucleus

2(b). Atom is electrically neutral.

Answer:

An atom contains positively charged protons and negatively charged electrons. In a neutral atom, the number of protons is equal to the number of electrons. Therefore, the positive and negative charges cancel each other and the atom is electrically neutral.

Number of protons (+) = Number of electrons (−)

💡 Remember:
Equal positive and negative charges → Atom is neutral

2(c). Atomic mass number is a whole number.

Answer:

Atomic mass number is the sum of the number of protons and neutrons present in the nucleus. Since the numbers of protons and neutrons are whole numbers, their sum is also a whole number. Therefore, the atomic mass number is a whole number.

Mass number (A) = Number of protons + Number of neutrons

Example:

Carbon has 6 protons and 6 neutrons.

Mass number = 6 + 6 = 12

💡 Remember:
Protons + Neutrons = Mass number

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

Answer:

The nucleus is positively charged and electrons are negatively charged. There is an electrostatic force of attraction between the nucleus and electrons, which keeps the electrons bound to the atom. Also, electrons occupy specific energy levels around the nucleus. Therefore, atoms are stable.

💡 Remember:
Positive nucleus + Negative electrons → Force of attraction → Electrons remain bound → Atom is stable

⭐ Quick Revision

1. Mass of atom:
Almost all the mass is concentrated in the nucleus.

2. Electrical neutrality:
Number of protons = Number of electrons.

3. Mass number:
Protons + Neutrons = Mass number.

4. Stability of atom:
Electrostatic attraction keeps electrons bound to the nucleus.

3. Define the Following Terms

a. Atom

An atom is the smallest particle of an element that retains all the chemical properties of that element.

b. Isotope

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

c. Atomic Number

The atomic number of an element is the number of protons present in the nucleus of an atom. It is denoted by Z.

d. Atomic Mass Number

The atomic mass number is the total number of protons and neutrons present in the nucleus of an atom. It is denoted by A.

Formula: A = Number of protons + Number of neutrons

e. Moderator in Nuclear Reactor

A moderator is a substance used in a nuclear reactor to slow down the fast-moving neutrons produced during nuclear fission, thereby helping to sustain the chain reaction.

Examples: Heavy water and graphite.

4. Draw a neat labelled diagram.

a. Ruthrford's scattering experiment

b. Thomson's atomic model

c. Diagramatic sketch of electronic 
configuration of Magnesium (Atomic number 12)

d. Diagramatic sketch of electronic 
configuration of Argon (Atomic number 18)

5. Fill in the Blanks

1. Electron, proton, neutron are the types of particles in an atom.
2. An electron carries a negative charge.
3. The electron shell nearest to the nucleus is the K shell.
4. The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of magnesium is the M shell.
5. The valency of hydrogen is one. Therefore, the valency of Fe is three as per the formula Fe₂O₃.

6. Match the Pairs

Group ‘A’

a. Proton
b. Electron
c. Neutron

Group ‘B’

i. Negatively charged
ii. Neutral
iii. Positively charged

Answers

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

7. Deduce from the Datum Provided

Datum	To Deduce	Answer
2311Na	Neutron number	12
146C	Mass number	14
3717Cl	Proton number	17

Working

a. Sodium (Na):
Neutron number = Mass number − Atomic number
= 23 − 11
= 12

b. Carbon (C):
The upper number represents the mass number.
Therefore, mass number = 14

c. Chlorine (Cl):
The lower number represents the atomic number (proton number).
Therefore, proton number = 17