Question

The position of eight elements in the periodic table is given below where atomic number of elements are given in parentheses
Period No.
$2$ ${\text{Li}}\left( 3 \right)$ ${\text{Be}}\left( 4 \right)$
$3$ ${\text{Na}}\left( {11} \right)$ ${\text{Mg}}\left( {12} \right)$
$4$ ${\text{K}}\left( {19} \right)$ ${\text{Ca}}\left( {20} \right)$
$5$ ${\text{Rb}}\left( {37} \right)$ ${\text{Sr}}\left( {38} \right)$
i).Write the electronic configuration of ${\text{Ca}}$?
ii).Predict the number of valence electrons in ${\text{Rb}}$?
iii).What is the number of shells in ${\text{Sr}}$?
iv).Predict whether${\text{K}}$is a metal or a non metal?
v).Which one of these elements has the largest atoms in size?
vi).Arrange ${\text{Be,Ca,Mg,}}$and ${\text{Rb}}$in the increasing order of the size of their respective atoms?

Answer 1

As we know that from the group number and period number we can easily determine the valency and the number of valence electrons.

Complete answer:
We know that the group number tells the valence electrons and elements in the same group have similar properties.
Metals are present in the left side of the periodic table and non metals are present in the right side of the periodic table. Periodic table divided into four blocks that is s block, p block, d block and f block.
The electronic configuration of elements can be written with the help of atomic number. Atomic number of elements is equal to the number of protons. Number of protons is equal to the number of electrons. Number of neutrons is equal to the atomic number subtracted from the mass number.
The electrons present in the outermost shell are called valence electrons.
The combining capacity of elements is called valency.
As we move across the period, electrons are added to the same shell thus effective nuclear charge decreases and atomic radius decreases.
As we move down the group, electrons are added to new shells and thus effective nuclear charge decreases and atomic radius increases.
i).Electronic configuration of calcium is shown below:
${\text{Ca: }}1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}4{{\text{s}}^2}$
ii).Rubidium belongs to the first group of periodic tables. Thus, valence electrons in Rubidium is $1.$
iii).The atomic number of strontium is $38.$ Strontium belongs to fifth period of the periodic table, thus its number of shells are $5.$
iv).Potassium belongs to the first group of the periodic table and this group is also called alkali metals. Thus, potassium is a metal.
v).Rubidium has the largest atomic radius.
vi).Increasing order of atomic size of the atoms is shown below:
${\text{Be < Mg < Ca < Rb}}$

Note:
Students might get confused with valence electrons and valency.
Elements of first group elements are called alkali metals. Elements of second group elements are called alkaline earth metals.