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Structure of the Atom - How Are Electrons Distributed in Different

Grade 9CBSE

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

🔑Concepts

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The distribution of electrons into different orbits (shells) of an atom was suggested by Bohr and Bury.

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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 or energy level index.

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The maximum number of electrons in the first shell (KK shell, n=1n=1) is 2×12=22 \times 1^2 = 2.

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The maximum number of electrons in the second shell (LL shell, n=2n=2) is 2×22=82 \times 2^2 = 8.

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The maximum number of electrons in the third shell (MM shell, n=3n=3) is 2×32=182 \times 3^2 = 18.

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The outermost shell of an atom can accommodate a maximum of 88 electrons, even if it has the capacity to hold more (except for the KK shell, which holds 22).

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Electrons are filled in a stepwise manner; shells are occupied in increasing order of energy, meaning inner shells must be filled before outer shells.

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The electrons present in the outermost shell of an atom are known as valence electrons.

📐Formulae

Maximum number of electrons=2n2Maximum\ number\ of\ electrons = 2n^2

n=orbit number (1, 2, 3...)n = \text{orbit number (1, 2, 3...)}

💡Examples

Problem 1:

Write the electronic configuration of an atom of Aluminum, which has an atomic number Z=13Z = 13.

Solution:

The electronic configuration is 2,8,32, 8, 3.

Explanation:

Aluminum has 1313 electrons. Following the Bohr-Bury rules: 1. The first shell (KK) takes 22 electrons. 2. The second shell (LL) takes 88 electrons. 3. The remaining electrons are 13−(2+8)=313 - (2 + 8) = 3, which are placed in the third shell (MM). Calculations: 13−103\begin{array}{r} 13 \\ - 10 \\ \hline 3 \end{array}

Problem 2:

Determine the distribution of electrons for Chlorine (Z=17Z = 17).

Solution:

The distribution is K=2,L=8,M=7K=2, L=8, M=7.

Explanation:

For Z=17Z=17: KK shell is filled with 22 electrons, LL shell is filled with 88 electrons. The remaining 17−10=717 - 10 = 7 electrons go into the MM shell. Since 7≤87 \le 8, it satisfies the octet rule for the outermost shell.

Problem 3:

If KK and LL shells of an atom are full, then what would be the total number of electrons in the atom?

Solution:

Total electrons = 1010.

Explanation:

A full KK shell contains 22 electrons (2×12=22 \times 1^2 = 2). A full LL shell contains 88 electrons (2×22=82 \times 2^2 = 8). Total electrons = 2+8=102 + 8 = 10.