Inside the Atom - Standard 8 - General Science - Balbharati

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 5. Inside the Atom 

 Exercise 

Q. 1 Answer the following.

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

Thomson’s Model Rutherford’s Model
Atom is a uniform sphere of positive charge. Atom has a tiny, dense, positively charged nucleus.
Electrons are embedded in the positive sphere like plums in pudding. Electrons revolve around the nucleus.
No concept of empty space inside the atom. 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?

Ans. Valency of an element is the combining capacity (number of chemical bonds it can form). If the number of valence electrons x ≤ 4, valency = x. If x ≥ 4, valency = 8 − x (the number of electrons needed to complete the octet).

(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) is the total number of protons and neutrons in the nucleus. Carbon has atomic number 6 because it has 6 protons; its mass number is 12 because proton number (6) + neutron number (6) = 12.

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

Ans. Subatomic particles are the particles that make up an atom (protons, neutrons, electrons). Proton (p): charge +1e, mass ≈ 1 u, located in nucleus. Neutron (n): charge 0, mass ≈ 1 u, located in nucleus. Electron (e⁻): charge −1e, mass ≈ 1/1800 u (negligible), located in shells (extranuclear region) around nucleus.

2. Give scientific reasons.

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

Ans. Protons and neutrons (nucleons) have almost all the mass of the atom and they are confined to the nucleus; electrons have negligible mass. Therefore nearly the entire mass of an atom is in its nucleus.

(b) Atom is electrically neutral.

Ans. In a neutral atom the number of electrons (negative charges) equals the number of protons (positive charges) in the nucleus, so the total positive and negative charges balance.

(c) Atomic mass number is a whole number.

Ans. Mass number counts discrete particles (protons + neutrons) in the nucleus; since these are whole particles, their total is necessarily a whole (integer) number.

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

Ans. Classical theory predicted radiation loss for revolving charges, but Bohr’s model (and later quantum mechanics) showed that electrons occupy stable quantized orbits (energy levels) where they do not radiate energy, so atoms are stable.

3. Define the following terms.

(a) Atom: The smallest particle of an element that retains its chemical identity in chemical reactions.
(b) Isotope: Atoms of the same element (same atomic number Z) having different mass numbers (different neutron numbers). Example: ¹²C, ¹³C, ¹⁴C.
(c) Atomic number (Z): Number of protons in the nucleus of an atom (equals number of electrons in a neutral atom).
(d) Atomic mass number (A): Total number of protons and neutrons in the nucleus (A = Z + N).
(e) Moderator in nuclear reactor: A substance (e.g., graphite, heavy water) used to slow down fast neutrons so they can induce fission efficiently in the nuclear fuel.

4. Draw a neat labelled diagram.

(a) Rutherford’s scattering experiment: 



(b) Thomson’s atomic model: 



(c) Electronic configuration of Magnesium (Z = 12): 


(d) Electronic configuration of Argon (Z = 18):


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 nearest to the nucleus is the K shell.

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

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

Q. 6 Match the pairs.

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

Q. 7 Deduce from the datum provided.

Datum What to find Answer
²³Na₁₁ Neutron number 12
¹⁴C₆ Mass number 14
³⁷Cl₁₇ Proton number 17

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