Question

An atom of an element has the atomic number 13 and the mass number 27. What is the number of valence electrons present in the valence shell of the atom?
a.) 1
b.) 2
c.) 3
d.) 5

Answer 1

Hint: The element in question belongs to group 3A of the periodic table which is also known as the Boron group. With this information in mind, try and figure out the electronic configuration of the given atom.

Step-by-Step Solution:
Let us first try and understand the concept of valence electrons.
In chemistry and physics, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.
Now, let us analyse this atom in particular.
Since this is a neutral atom,
Therefore, we can say:
number of protons = number of electrons = 13 (because its atomic number is 13)

Hence, there will be 13 electrons.

Therefore, its electronic configuration per shell would be:

2,8,3

To find its electronic configuration per orbital, we must obey the Aufbau principle which states that:
The electrons are added one by one to the various orbitals in order of their increasing energy starting with the orbital of lowest energy.
The increasing order of energy of various orbital is
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s , 4d , 5p , 6s , 4f , 5d , 6p , 5f , 6d , 7p……
Therefore, by the Aufbau principle, the electronic configuration of this element per orbital is:
$1{{s}^{2}}\text{ }2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{1}}$
Now observing its valence shell, we observe 3 electrons present in it. Therefore, the number of valence electrons in an atom of this element is 3.

Note: The element in question is Aluminium, which is the second member of the Boron family. It is a silvery-white metal in appearance and its ductility and malleability are used for many different purposes across different industries.