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Structure of the Atom - Combining Capacity of an Atom: Valency

Grade 9CBSE

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

🔑Concepts

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Valence electrons are the electrons present in the outermost shell of an atom. These electrons determine the chemical properties and reactivity of the element.

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The combining capacity of an atom is called its valency. Atoms react with other atoms to achieve a stable electronic configuration, specifically a completely filled outermost shell (octet).

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According to the Octet Rule, atoms tend to gain, lose, or share electrons to have 88 electrons in their outermost shell. Helium is an exception, as it is stable with 22 electrons (duplet) in its only shell (KK shell).

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If an atom has 1,2,3, or 41, 2, 3, \text{ or } 4 valence electrons, its valency is equal to the number of valence electrons because it is easier to lose these electrons to achieve stability.

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If an atom has 5,6,7, or 85, 6, 7, \text{ or } 8 valence electrons, its valency is calculated by subtracting the number of valence electrons from 88, as it is easier to gain electrons to complete the octet.

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Noble gases like Neon (Ne) and Argon (Ar) have 88 electrons in their outermost shell, making them chemically inert with a valency of 00.

📐Formulae

Valency=Number of valence electrons (if ≤4)\text{Valency} = \text{Number of valence electrons (if } \le 4)

Valency=8−Number of valence electrons (if >4)\text{Valency} = 8 - \text{Number of valence electrons (if } > 4)

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

💡Examples

Problem 1:

Calculate the valency of Magnesium (MgMg) given its atomic number is 1212.

Solution:

The electronic configuration of Magnesium is (2,8,2)(2, 8, 2). The number of valence electrons is 22. Since this is less than 44, the valency is 22.

Explanation:

Magnesium has 22 electrons in its outermost MM shell. It is energetically easier for the atom to lose 22 electrons than to gain 66 to complete its octet. Thus, its combining capacity (valency) is 22.

Problem 2:

Determine the valency of Oxygen (OO) with atomic number 88.

Solution:

The electronic configuration of Oxygen is (2,6)(2, 6). The number of valence electrons is 66. Using the formula: Valency=8−6=2\text{Valency} = 8 - 6 = 2.

Explanation:

Oxygen has 66 valence electrons. To reach a stable octet of 88, it needs to gain 22 more electrons. Therefore, its valency is 22.

Problem 3:

What is the valency of an element with atomic number 1818?

Solution:

The electronic configuration is (2,8,8)(2, 8, 8). The number of valence electrons is 88. Valency=8−8=0\text{Valency} = 8 - 8 = 0.

Explanation:

This element is Argon (ArAr). Since it already has a complete octet in its outermost shell, it has no tendency to react or combine with other atoms, resulting in a valency of zero.