krit.club logo

Atomic Structure - Electronic Configuration of Elements 1 to 20

Grade 7CBSE

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

🔑Concepts

•

The arrangement of electrons in different orbits or shells around the nucleus of an atom is called its electronic configuration.

•

Bohr-Bury Scheme: The maximum number of electrons that can be accommodated in a shell is given by the formula 2n22n^2, where nn is the orbit number (n=1,2,3...n = 1, 2, 3...).

•

Orbits are represented by the letters K,L,M,N...K, L, M, N... starting from the nucleus outward.

•

Maximum capacity of shells: KK shell (n=1n=1) can hold 22 electrons, LL shell (n=2n=2) can hold 88 electrons, and MM shell (n=3n=3) can hold up to 1818 electrons.

•

The outermost shell of an atom cannot accommodate more than 88 electrons, even if it has a higher capacity (Octet Rule).

•

Electrons are not accommodated in a given shell unless the inner shells are completely filled (Stepwise filling).

•

Atomic Number (ZZ) represents the number of protons, which is equal to the number of electrons in a neutral atom.

•

Valence electrons are the electrons present in the outermost shell of an atom.

📐Formulae

Maximum number of electrons in nth shell=2n2\text{Maximum number of electrons in } n^{th} \text{ shell} = 2n^2

Atomic Number (Z)=Number of protons=Number of electrons\text{Atomic Number (Z)} = \text{Number of protons} = \text{Number of electrons}

K shell (n=1)  ⟹  2(1)2=2K \text{ shell (n=1)} \implies 2(1)^2 = 2

L shell (n=2)  ⟹  2(2)2=8L \text{ shell (n=2)} \implies 2(2)^2 = 8

M shell (n=3)  ⟹  2(3)2=18M \text{ shell (n=3)} \implies 2(3)^2 = 18

💡Examples

Problem 1:

Write the electronic configuration of Carbon (Z=6Z = 6).

Solution:

K=2,L=4K=2, L=4

Explanation:

Carbon has 66 electrons. The first 22 electrons fill the KK shell. The remaining 6−2=46 - 2 = 4 electrons go into the LL shell.

Problem 2:

Determine the electronic configuration and number of valence electrons for Magnesium (Z=12Z = 12).

Solution:

Configuration: 2,8,22, 8, 2; Valence electrons: 22

Explanation:

Magnesium has 1212 electrons. KK shell takes 22, LL shell takes 88 (total 1010). The remaining 22 electrons enter the MM shell. The outermost shell (MM) has 22 electrons, which are the valence electrons.

Problem 3:

What is the electronic configuration of Potassium (Z=19Z = 19)?

Solution:

2,8,8,12, 8, 8, 1

Explanation:

Potassium has 1919 electrons. KK shell takes 22, LL shell takes 88. Although the MM shell can hold up to 1818, the octet rule states the outermost shell cannot have more than 88. Therefore, after MM shell gets 88 electrons, the 19th19^{th} electron enters the NN shell.

Problem 4:

Calculate the maximum number of electrons the MM shell can hold using the formula.

Solution:

2(3)2=182(3)^2 = 18

Explanation:

For the MM shell, the orbit number n=3n = 3. Applying the formula 2n22n^2: 2×32=2×9=182 \times 3^2 = 2 \times 9 = 18.