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Chemistry - Atomic Structure and Chemical Bonding (Subatomic Particles, Isotopes, Octet Rule, Electrovalent/Covalent Bonds)

Grade 9ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Atoms are composed of three primary subatomic particles: Protons (p+p^+), Neutrons (n0n^0), and Electrons (e−e^-). The nucleus contains protons and neutrons, while electrons revolve in discrete shells.

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Atomic Number (ZZ) is the number of protons in the nucleus of an atom. In a neutral atom, Z=number of electronsZ = \text{number of electrons}.

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Mass Number (AA) is the total number of protons and neutrons in the nucleus (A=p+nA = p + n).

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Isotopes are atoms of the same element having the same atomic number (ZZ) but different mass numbers (AA). They have identical chemical properties due to the same number of valence electrons but different physical properties.

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The Octet Rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their valence shell, similar to noble gases. Elements like Hydrogen and Helium follow the Duplet Rule (2 electrons).

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Electrovalent (Ionic) Bonding occurs when there is a complete transfer of one or more electrons from a metallic atom to a non-metallic atom, resulting in the formation of cations (M+M^+) and anions (X−X^-) held by electrostatic forces.

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Covalent Bonding is formed by the mutual sharing of one or more pairs of electrons between two non-metallic atoms to achieve stability. This can result in single bonds (sharing 11 pair), double bonds (sharing 22 pairs), or triple bonds (sharing 33 pairs).

📐Formulae

A=Z+nA = Z + n

Number of Neutrons(n)=A−Z\text{Number of Neutrons} (n) = A - Z

Maximum electrons in a shell=2n2\text{Maximum electrons in a shell} = 2n^2

Electrovalent bond formation (e.g., NaCl):Na→Na++e−, Cl+e−→Cl−, Na++Cl−→NaCl\text{Electrovalent bond formation (e.g., NaCl)}: Na \rightarrow Na^+ + e^-, \text{ } Cl + e^- \rightarrow Cl^-, \text{ } Na^+ + Cl^- \rightarrow NaCl

Covalent bond formation (e.g., H2):H⋅+⋅H→H:H or H−H\text{Covalent bond formation (e.g., } H_2): H \cdot + \cdot H \rightarrow H : H \text{ or } H-H

💡Examples

Problem 1:

An element XX has an atomic number 1212 and mass number 2424. Calculate the number of electrons, protons, and neutrons. Also, identify its valency and the type of ion it will form.

Solution:

Protons = 1212, Electrons = 1212, Neutrons = 24−12=1224 - 12 = 12. Electronic configuration is (2,8,2)(2, 8, 2). Valency = +2+2. It forms a cation X2+X^{2+}.

Explanation:

Since Z=12Z = 12, the number of protons and electrons in a neutral atom is 1212. Neutrons are calculated using A−ZA - Z. The electronic configuration (2,8,2)(2, 8, 2) shows 22 valence electrons; the atom loses these 22 electrons to satisfy the Octet Rule, forming a divalent cation.

Problem 2:

Explain the formation of Magnesium Chloride (MgCl2MgCl_2) using electron dot symbols.

Solution:

Mg⋅⋅+2⋅Cl¨:→[Mg]2+2[:Cl¨:]−→MgCl2Mg \cdot \cdot + 2 \cdot \ddot{\text{Cl}} : \rightarrow [Mg]^{2+} 2[: \ddot{\text{Cl}} :]^- \rightarrow MgCl_2

Explanation:

Magnesium (Z=12Z=12) has a configuration of (2,8,2)(2, 8, 2) and needs to lose 2e−2e^- to be stable. Chlorine (Z=17Z=17) has a configuration of (2,8,7)(2, 8, 7) and needs to gain 1e−1e^-. Therefore, one Magnesium atom transfers one electron to each of the two Chlorine atoms, forming an electrovalent bond.

Problem 3:

Represent the formation of a Nitrogen molecule (N2N_2) using Lewis structure.

Solution:

:N⋮N: or N≡N:N \vdots N: \text{ or } N \equiv N

Explanation:

Nitrogen (Z=7Z=7) has an electronic configuration of (2,5)(2, 5). It requires 33 more electrons to complete its octet. Two nitrogen atoms share 33 pairs of electrons (a triple covalent bond) to achieve the stable configuration of (2,8)(2, 8).

Atomic Structure and Chemical Bonding (Subatomic Particles, Isotopes, Octet Rule…