Atomic Structure and Chemical Bonding

Q: What is the maximum number of electrons in the M shell according to the 2n² rule? Why does potassium have only 8 electrons in its M shell?

2 × 32 = 18. The outermost shell cannot hold more than 8 electrons, so the 19th electron of potassium goes into the N shell.

ICSE Class 9 · Chemistry · Chapter 4

What this chapter covers

The atom and its constituents
Atomic number and mass number
Rutherford's model: discovery of the nucleus
Bohr's model of the atom
Distribution of electrons in orbits
Isotopes
Electrovalent bond
Covalent bond
Radical formation on breaking a covalent bond

Key points

An atom is the smallest particle of an element that takes part in a chemical reaction; it contains protons and neutrons in a nucleus, with electrons around it.
Atomic number (Z) = number of protons; mass number (A) = protons + neutrons; neutrons = A – Z.
Rutherford's gold foil experiment showed that an atom is mostly empty space with a tiny, dense, positive nucleus; his model failed to explain the stability of the atom.
In Bohr's model, electrons move in fixed orbits (K, L, M, N) of definite energy without losing energy, and jump between orbits by absorbing or giving out energy.
A shell holds at most 2n² electrons, and the outermost shell at most 8.
Atoms are chemically active because their outer shells are incomplete; they react to reach an octet (or a duplet).
Isotopes have the same atomic number but different mass numbers: hydrogen (1, 2, 3), carbon (12, 13, 14), chlorine (35, 37); their chemical properties are the same.
An electrovalent bond forms by transfer of electrons from a metal to a non-metal, giving ions: NaCl, MgCl₂, CaO.
A covalent bond forms by sharing electron pairs: single (H₂, Cl₂, HCl, H₂O, NH₃, CH₄, CCl₄), double (O₂), triple (N₂).
Breaking a covalent bond evenly gives free radicals, neutral and highly reactive species with an unpaired electron.

Questions answered

Define an atom. Name its three main constituent particles and their charges.

An atom is the smallest particle of an element that can take part in a chemical reaction. Its constituents are protons (+1), neutrons (0) and electrons (–1).

Define atomic number and mass number.

Atomic number is the number of protons in the nucleus of an atom. Mass number is the total number of protons and neutrons in the nucleus.

An atom has mass number 27 and atomic number 13. Find its protons, electrons and neutrons and write its configuration.

Protons 13, electrons 13, neutrons 27 - 13 = 14; configuration 2, 8, 3 (aluminium).

State three observations of Rutherford's gold foil experiment and the conclusion drawn from each.

Most alpha particles passed straight through, so the atom is mostly empty space. A few were deflected, so the positive charge is concentrated in a small region. Very few bounced back, so that region is small, dense and holds almost all the mass.

State one demerit of Rutherford's model. How did Bohr's model remove it?

A revolving electron should lose energy and fall into the nucleus, so the atom would be unstable. Bohr proposed fixed orbits in which an electron does not lose energy.

What is the maximum number of electrons in the M shell according to the 2n² rule? Why does potassium have only 8 electrons in its M shell?

2 × 3² = 18. The outermost shell cannot hold more than 8 electrons, so the 19th electron of potassium goes into the N shell.

State the octet rule. Why are noble gases chemically inactive?

Atoms combine by losing, gaining or sharing electrons to get eight electrons in their outermost shell. Noble gases already have a complete outer shell, so they have no tendency to react.

Define isotopes. Name the three isotopes of hydrogen and give the number of neutrons in each.

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Protium (0), deuterium (1), tritium (2).

Past board exam questions

ICSE 2019 sample paper · 5 marks

Fill in the blanks with the correct choice given in brackets. (i) In an ionic compound, the bond is formed due to _____________ of electrons. [sharing / transfer] (ii) Zinc is _____________ reactive than aluminum. [less / more] (iii) The chemical reaction in which heat is absorbed is known as _____________ reaction. [endothermic / exothermic] (iv) The molecular formula of ammonium sulphate is _____________. [NH₄SO₄ / (NH₄)₂SO₄] (v) If an element has two electrons in its outermost shell, then it is likely to be _____________. [Metallic / Non-metallic]

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(i) transfer (ii) less (iii) endothermic (iv) (NH₄)₂SO₄ (v) Metallic

ICSE 2019 sample paper · 4 marks

Draw the orbit structure for each of the following compounds: (i) Methane [H = 1, C = 6] (ii) Magnesium chloride [Mg = 12, Cl = 17]

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(i) Methane (CH₄), a covalent compound: the carbon atom (2, 4) is drawn at the centre with its K shell of 2 electrons and an L shell; each of its 4 valence electrons is paired with the 1 electron of a hydrogen atom, the four shared pairs lying in the overlap of carbon's L shell with the shell of each of the 4 hydrogen atoms. Carbon then has 8 electrons (octet) and each hydrogen 2 (duplet). Four single covalent bonds are formed. (ii) Magnesium chloride (MgCl₂), an electrovalent compound: Mg (2, 8, 2) loses its 2 valence electrons, one to each of two chlorine atoms (2, 8, 7). The orbit structures show Mg²⁺ with shells 2, 8 and two Cl⁻ ions each with shells 2, 8, 8 (the gained electron marked …

ICSE 2019 sample paper · 3 marks

(i) What do you understand by the term isotopes? (ii) Why do isotopes of an element possess identical chemical properties? (iii) Name the isotope of hydrogen which does not have a neutron in it.

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(i) Isotopes are atoms of the same element having the same atomic number but different mass numbers (different numbers of neutrons). (ii) Chemical properties depend on the number of electrons (the electronic configuration); isotopes of an element have the same number of electrons and the same electronic configuration, so their chemical properties are identical. (iii) Protium (¹₁H).

ICSE 2019 sample paper · 3 marks

State the type of covalent bonding in the following molecules: (i) Chlorine (ii) Nitrogen (iii) Oxygen

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(i) Chlorine (Cl₂): single covalent bond (Cl–Cl) (ii) Nitrogen (N₂): triple covalent bond (N≡N) (iii) Oxygen (O₂): double covalent bond (O=O)

ICSE 2019 sample paper · 6 marks

Complete the following table: | Element | Mass Number | Atomic Number | Number of Electrons | Number of Protons | Number of Neutrons | |---|---|---|---|---|---| | Phosphorous | 31 | 15 | | | | | Potassium | | | 19 | | 20 |

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| Element | Mass Number | Atomic Number | Number of Electrons | Number of Protons | Number of Neutrons | |---|---|---|---|---|---| | Phosphorous | 31 | 15 | 15 | 15 | 16 | | Potassium | 39 | 19 | 19 | 19 | 20 | (Electrons = protons = atomic number; neutrons = mass number − atomic number; for potassium, mass number = 19 + 20 = 39.)

ICSE 2019 sample paper · 2 marks

Write the electronic configuration of the following: (i) ⁴⁰₂₀Ca (ii) ³²₁₆S

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(i) Ca (Z = 20): 2, 8, 8, 2 (ii) S (Z = 16): 2, 8, 6

ICSE 2019 sample paper · 2 marks

State the valency of the element having: (i) 6 electrons in the valence shell. (ii) the electronic configuration of : 2, 3

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(i) 2 (it gains 2 electrons to complete its octet) (ii) 3 (it loses its 3 valence electrons)