Chapter 2, Problem 2.51AP

Interpretation Introduction

Interpretation:

The reason should be explained for the fourth period in the periodic table contains eighteen elements.

Concept introduction:

Electronic configuration:

The electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals.

1. (1) Electrons occupy in the lowest energy orbitals. The increasing order of orbital energy is $\text{s, p, d and f}$. The lowest energy orbital is $\text{1s}$. The order of energy of the orbital for the for the first three period is given below,

   $\text{1s, 2s, 2p, 3s,}\text{and 3p}$.

2. (2) The orbital which is closer to the nucleus has lower energy, therefore the 2s is lower in energy than 3s. Accordingly $2s$ is lower in energy than $\text{2p and 4s}$ is lower in energy than $\text{3d}$.
3. (3) In general, the orbitals can hold maximum of two electrons, the two electrons must have opposite spin.

The subshell ordering by aufbau principle is given below, $\text{1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s, }\mathrm{...}$

Aufbau principle:

Aufbau principle is states that in the ground state of an atom, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. For example, the 1s shell is filled before the 2s subshell is occupied.

The subshell ordering by this rule is

$\text{1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s, }\mathrm{...}$

Number of electrons in the shells is given below,

Each shell contains only a fixed number of electrons. The first shell contains two electrons, the second shell contains eight (2 + 6) electrons, the third shell contains 18 (2 + 6 + 10), fourth shell contains (2 + 6 + 10 + 14 ) and so on.