Question

Which of the following electrons has only one valence electron?
A. Oxygen.
B. Lithium.
C. Bromine.
D. Argon.
E. Arsenic.

Answer 1

Hint: In order to answer this problem, we must note that a valence electron is an external shell electron associated with an atom that may participate in the formation of a chemical bond if the external shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron to form a shared pair.

Complete step-by-step answer:
An element's electron configuration describes how the electrons in its atomic orbitals are distributed. Electron configurations of an atom obey a standard notation in which all electron-containing atomic subshells are put in a series (with the number of electrons that they possess written in superscript).
We know that valence electrons are the last electron in the electronic configuration of the element associated with an atom.
The presence of valence electrons will decide the chemical properties of the element, such as its valence — whether or not it can bind with other elements, and if so, how readily and how many.
Let's take one by one, all the possibilities.
1. Oxygen:
The atomic number of oxygen is 8.
Electronic configuration: $1{s^2}2{s^2}2{p^4}$.
The outermost shell contains 4 electrons i.e. it has 4 valence electrons.
2. Lithium:
Lithium has an atomic number of 3.
Electronic configuration: $1{s^2}2{s^1}$.
In the outermost shell there is 1 electron, i.e. it only has 1 valence electron.
3. Bromine.
The bromine's atomic number is at 35.
Electronic configuration: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^5}$.
The outermost shell comprises 5 electrons i.e. it has 5 valence electrons.
4. Argon.
The argon's atomic Number is 18.
Electronic configuration: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}$.
The outermost shell comprises 6 electrons, and it is also fully filled with an orbital. The argon, then, is a noble gas.
5. Arsenic.
Arsenic atomic number is 33.
Electronic configuration: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^3}$.
The outermost shell comprises 3 electrons i.e. it has 3 valence electrons.
Therefore we can say lithium only has one valence electron.
So choice (B) is the correct one.

Note: We will use the definition of electronic configuration of an element if we ask these kinds of questions. We will first find out the electronic configurations of all the elements given and then we will test the valence electrons of those elements. Then we can easily figure out which element has which electrons for valence. We will get the answer through that.