Question

Given below are some elements of the modern periodic table:
$_4{\text{Be,}}{{\text{ }}_{26}}{\text{Fe,}}{{\text{ }}_{14}}{\text{Si,}}{{\text{ }}_{19}}{\text{K,}}{{\text{ }}_{20}}{\text{Ca}}$
(i) Select the element that has one electron in the outermost shell and write its electronic
configuration.
(ii) Select two elements that belong to the same group. Give reason for your answer.
(iii) Select two elements that belong to the same period. Which one of the two has bigger
atomic size?

Answer 1

From the element symbol, determine the atomic number and the number of electrons. Then write the electronic configuration and the valence shell electronic configuration. After that we will be able to identify the valence electrons and the related chemical properties.

Complete Step by step answer: Summarize the element name, atomic number and electronic configuration in the table as shown below:

ElementSymbol	Elementname	Atomic Number	Number ofelectrons	Electronic configuration	Valence shellElectronic configuration
$_4{\text{Be}}$	Beryllium	4	4	${\text{1}}{{\text{s}}^2}2{s^2}$	$2{s^2}$
$_{26}{\text{Fe}}$	Iron	26	26	$\left[ {{\text{Ar}}} \right]{\text{3}}{{\text{d}}^6}4{s^2}$	${\text{3}}{{\text{d}}^6}4{s^2}$
$_{14}{\text{Si}}$	Silicon	14	14	$\left[ {{\text{Ne}}} \right]{\text{3}}{{\text{s}}^2}3{{\text{p}}^2}$	${\text{3}}{{\text{s}}^2}3{{\text{p}}^2}$
$_{19}{\text{K}}$	Potassium	19	19	$\left[ {{\text{Ar}}} \right]4{s^1}$	$4{s^1}$
$_{20}{\text{Ca}}$	Calcium	20	20	$\left[ {{\text{Ar}}} \right]4{s^2}$	$4{s^2}$

(i) The element that has one electron in the outermost shell is potassium having symbol $_{19}{\text{K}}$
And atomic number 19. It has 19 electrons out of which one electron is present in the outermost shell (the valence shell). Its electronic configuration ${\text{1}}{{\text{s}}^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$ or $\left[ {{\text{Ar}}} \right]4{s^1}$. Its valence shell electronic configuration is $4{s^1}$ .

(ii) The two elements that belong to the same group are beryllium and calcium. They have similar valence shell electronic configuration of $n{s^2}$ .
The element beryllium has symbol $_4{\text{Be}}$ and atomic number 4. It has 4 electrons out of which, two electrons are present in the outermost shell (the valence shell). Its electronic configuration ${\text{1}}{{\text{s}}^2}2{s^2}$. Its valence shell electronic configuration is $2{s^2}$ .

The element calcium has symbol $_{20}{\text{Ca}}$ and atomic number 20. It has 20 electrons out of which, two electrons are present in the outermost shell (the valence shell). Its electronic configuration $\left[ {{\text{Ar}}} \right]4{s^2}$. Its valence shell electronic configuration is $4{s^2}$ .

(iii) The elements iron, potassium and calcium belong to the same period. They belong to the fourth period. When moving from left to right in the period, there is a gradual decrease in the atomic radius. Due to this, potassium has bigger atomic size than calcium and iron.

Note: In the modern periodic table, elements are arranged in the increasing order of their atomic numbers. The physical and chemical properties of the elements are the periodic functions of the atomic numbers.