Question

Using the periodic table, predict the formulas of compounds which might be formed by the following pairs of elements:
(a) Silicon and bromine
(b) Aluminium and sulphur

Answer 1

Hint: To answer this question we should know the atomic number of silicon, bromine, aluminium and sulphur. Understanding the concept of valency may help us to answer this question within no time.

Complete answer:
The number electrons in an atom of a particular element is known as the atomic number of an element.
Valence electrons are the electrons present in the outermost shell of an atom. The valence electrons can easily react with the electrons in the outermost shell of another atom. The valency is the capacity of an atom to make bonds. Capacity to make bond depends on the number of valence electrons.

Here, the formula will be ${{X}_{2}}{{Y}_{1}}$.
(a) Silicon and bromine
-Silicon
Atomic number of silicon is 14.
The electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{2}}$.
Therefore, the valence electron is 4.
The silicon atom prefers to gain or lose 4 electrons to make stable compounds. Thus, it is tetravalent.
-Bromine
Atomic number of bromine is 35.
The electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{10}}4{{p}^{5}}$.
Therefore, the valence electron is 5.
The bromine atom prefers to gain an electron rather than losing 5 electrons to make a stable compound. . Thus, it is monovalent.
Hence, the compound forms will be $SiB{{r}_{4}}$.
(b) Aluminium and sulphur
-Aluminium
Atomic number of silicon is 13.
The electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{3}}$.
Therefore, the valence electron is 3.
The aluminium atom prefers to lose 3 electrons rather than losing 3 electrons to make a stable compound. Thus, it is trivalent.
-Sulphur
Atomic number of bromine is 16.
The electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{4}}$.
Therefore, the valence electron is 5.
The sulphur atom prefers to gain 2 electrons to make a stable compound as it is easy to gain 2 electrons rather than losing four. Thus, it is monovalent.

Thus, the compound forms $A{{l}_{2}}{{S}_{3}}$.

Note: Depending upon the valency of an element forms bond with another element. The elements would like to attain stable configuration either by losing or gaining electrons. In this case it prefers to spend less energy in order to attain the stable configuration. Therefore, valency is important to understand.