Question

An atom of an element has one electron in the valence shell. It can be represented as
A. $_6{A^{12}}$
B. $_{19}{A^{39}}$
C. $_{13}{A^{27}}$
D. $_{12}{A^{24}}$

Answer 1

TThe last or outermost shell is called a valence shell and the electrons present in it are called valence electrons. Electronic configuration is the representation arrangement of electrons in the shells.

Complete step by step answer:
An atom consists of a nucleus in the centre. It has protons and neutrons. Almost all the mass of an atom is concentrated in the nucleus. Protons are positively charged species and neutrons are neutral in nature. Electrons are negatively charged and are present in the shells around the nucleus. These shells are named as K,L ,M,N… respectively. Each shell can occupy a ceratin number of electron and that can be calculated by the formula $2{n^2}$ where $n$ is the number of shell. Thus we can fill two electrons in K shell, eight electrons in L shell, eighteen electrons in M shell and so on. According to the octet rule, each shell tries to complete its octet to achieve the nearest noble gas configuration. Let us understand some terminologies
Valence shell: The outermost or last shell of an atom is called valence shell.
Electronic configuration: The representation of electrons in an atom is called the electronic configuration. It is represented as the number of electrons in the shells. For example sodium has $11$ electrons. So the electronic configuration is $2,8,1$ .
Atomic number: The number of protons or electrons in an atom is the atomic number of that element. For example, a sodium atom has $11$ electrons so the atomic number of sodium is $11$ .
Atomic mass number: The sum of the number of protons and neutrons in an atom is called the atomic mass number of that element. For example, sodium has $11$ protons and $12$ neutrons. So the atomic mass number of sodium is $23$ .
The convention used to show the atomic symbol, atomic number and atomic mass number of an element is $_Z^XA$ where $A$ denotes the atomic symbol of the element, $Z$ denotes the atomic number and $X$denotes the atomic mass number of that element.
Now, let us discuss the given options. In option A), the atom has six electrons as its atomic number is six. So the electronic configuration is $2,4$ . It indicates that there are four electrons in the outermost shell.
In option B), the atom has $19$ electrons. So the electronic configuration is $2,8,8,1$ . So there is one electron in the outermost shell.
In option C) the atom has $13$ electrons. So the electronic configuration is $2,8,3$. So there are three electrons in the outermost shell.
In option D) the atom has $12$ electrons. So the electronic configuration is $2,8,2$ . It indicates that there are two electrons in the outermost shell.

Hence correct answer is optionB.

Note:
In the conventional symbol, the atomic number is written at the bottom and atomic mass number is written on the top of the symbol.
Every atom tries to attain the nearest noble gas configuration by either sharing of electrons or donation or acceptance of electrons.
Valency is determined by the number of electrons present in the outermost or valence shell.