Question

An element has its electronic configuration as 2,8,2.
i. What is the atomic number of this element?
ii. To which group and period this element belongs?
iii. What is the name of the element? State its symbol.

Answer 1

Elements in the periodic table are arranged in increasing order of the number of protons. The electrons are the same as the number of protons which is the atomic number of the element.

Complete step by step answer:
The electronic configuration of an element is the electron arrangement of an element. An element consists of various types of orbitals. They are named as $s,{\text{ }}p,{\text{ }}d,{\text{ }}f$ etc.
The different orbitals have different shapes and accommodation for the number of electrons to be filled up. For example, the $s$-orbital has a maximum of two electrons, a $p$-orbital can have six electrons, a $d$-orbital has ten electrons and an $f$-orbital can have fourteen electrons.
The number of electrons depends on the number of degenerate orbitals that each orbital is composed of. Since $s$-orbital has one degenerate orbital it can accommodate two electrons with opposite spin according to Hund’s rule. Similarly a $p$-orbital has three orbitals, $d$-orbital has five orbitals, $f$-orbital has seven orbitals.
The electrons are filled up in the orbitals according to increasing energy of the orbitals as $1s,{\text{ }}2s,{\text{ }}2p,{\text{ }}3s,{\text{ }}3p,{\text{ }}4s,{\text{ }}3d,{\text{ }}4p,{\text{ }}5s,{\text{ }}4d,{\text{ }}5p,{\text{ }}6s$ and so on. Thus shell $1$ has $2$ electrons, shell $2$ has $8$ electrons (sum of $2s$ and $2p$ orbitals), shell $3$ has $18$ electrons (sum of $3s$ , $3p$ and $3d$ orbitals).
The given electronic configuration is $2,8,2$ , thus its electronic configuration is $1{s^2}2{s^2}2{p^6}3{s^2}$ .
-The atomic number of this element is $12$ which is equal to the sum of $2 + 8 + 2$ electrons.
-The valence and the number of valence electrons indicate that the group is two and the period is three for $3{s^2}$ .
-The atomic number $12$ belongs to the element magnesium. Its symbol is $Mg$.

Note:
The atomic number is indicative of the period and group of an element. The mass number has nothing to do with the period or group of an element.