Ch.5: Inside the Atom

Inside the Atom | Class 8 | Science | Chapter 5| Maharashtra State Board

Learn about the structure of matter and atoms – from Kanad’s ‘Paramanu’ to Dalton, Thomson, Rutherford, and Bohr’s atomic models. Understand subatomic particles, electronic configuration, valency, isotopes, and the role of nuclear reactors in energy production.

Questions & Answers

Q.1 Answer the following.

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

Ans.

Thomson said atom has positive charge spread everywhere with electrons embedded (plum pudding model). Rutherford showed atom has a small heavy positive nucleus in center and electrons revolve around it.

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

Ans.

Valency is the combining capacity of an atom.

• If valence electrons ≤ 4 → Valency = number of valence electrons.

• If valence electrons > 4 → Valency = 8 – number of valence electrons.

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

Ans.

Atomic mass number (A) = Protons + Neutrons.For carbon: Protons = 6 → Atomic number (Z) = 6.Mass number (A) = 6 protons + 6 neutrons = 12.

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

Ans.

Particles inside atom are called subatomic particles.

• Proton → + charge, mass ≈ 1u, in nucleus.

• Neutron → no charge, mass ≈ 1u, in nucleus.

• Electron → – charge, negligible mass, revolve around nucleus.

Q.2 Give scientific reasons.

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

Ans.

Because protons and neutrons are in nucleus and their mass is very high compared to electrons.

b) Atom is electrically neutral.

Ans.

Number of protons (+ charge) = number of electrons (– charge), so charges balance.

c) Atomic mass number is a whole number.

Ans.

Because it is the sum of protons and neutrons, which are always whole numbers.

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

Ans.

Bohr explained that electrons revolve in fixed energy shells, so atoms remain stable.

Q.3 Define the following terms.

a) Atom –

Ans.

The smallest particle of matter that retains chemical properties.

b) Isotope –

Ans.

Atoms of same element with same atomic number but different mass numbers.

c) Atomic number –

Ans.

Number of protons in the nucleus of an atom.

d) Atomic mass number –

Ans.

Total number of protons + neutrons in an atom.

e) Moderator in nuclear reactor –

Ans.

A substance (like heavy water/graphite) used to slow down neutrons.

Q.5 Fill 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 shell.

e) The valency of hydrogen is 'one' as per the molecular formula H₂O. Therefore valency of 'Fe' turns out to be three (3) as per the formula Fe₂O₃.

Q.6 Match the pairs.

a. Proton → Positively Charged

b. Electron → Negatively Charged

c. Neutron → Neutral

Q.7 Deduce from the datum provided.

• Na → Neutrons = 23 – 11 = 12

• C → Mass number = 14

• Cl → Protons = 17

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E X T R A

1. Answer the following.

1. Who discovered the nucleus of an atom?

Ans.

Ernest Rutherford discovered the nucleus through his gold foil experiment.

2. What is the charge on a proton?

Ans.

A proton has a positive charge (+1).

3. What is the charge on an electron?

Ans.

An electron has a negative charge (–1).

4. Which particle inside the atom has no charge?

Ans.

Neutron has no charge (neutral).

5. Why are isotopes of an element chemically similar?

Ans.

Because they have the same number of protons and electrons, so they show the same chemical properties.

6. Why is helium chemically inert?

Ans.

Helium has a completely filled K shell (2 electrons), so its valency is zero.

7. Write the electronic configuration of oxygen (Z=8).

Ans.

Oxygen: 2, 6

8. Write the electronic configuration of sodium (Z=11).

Ans.

Sodium: 2, 8, 1

9. Why is the atomic number important?

Ans.

Atomic number tells the number of protons in the nucleus and decides the identity of the element.

10. What is the use of C-14 isotope?

Ans.

C-14 isotope is used to determine the age of archaeological objects (carbon dating).

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