Question

Write the chemical formula for the following crisscrossing valence:
a. Hydrogen chloride
b. Hydrogen sulphide
c. Carbon tetrachloride
d. Magnesium chloride
e. Calcium oxide
f. Sodium nitrate
g. Calcium hydroxide
h. Sodium carbonate
i. Ammonium sulphate
j. Aluminum chloride
k. Sodium sulphate
l. Magnesium hydroxide

Answer 1

Hint:The valence of an element is its ability to combine with other elements or the ability an element to replace or add hydrogen atoms. And a chemical formula represents the chemical properties of atoms that constitute a particular chemical compound or molecule.

Complete answer:
The crisscross valence method of writing the chemical formula is a popular method and a relatively easy one to write the chemical formula of a given compound or molecule.
Let’s begin with a more clear idea of valence, the valence of an element largely depends on the group that it resides, the valence of a particular group remains equal throughout the group. E.g., halogens (group 17) have a valence of -1, meaning they need only one electron to attain the stability of the nearest noble gas.
Since we are using the criss cross valence method, we will just exchange the valence (or charge) of one atom with the other, (1 valence = 1 atom, 2 valence = 2 atoms,….. n valence = n atoms)

i. First off we have hydrogen chloride, the valence of group 1 (for hydrogen) is +1 and the valence of group 17 (for chloride) is -1. And if we criss cross their valences we will be needing 1 atom of chloride and 1 hydrogen atom, and the chemical formula can be written as: ${\text{HCl}}$.

ii. Hydrogen sulphide, we just saw the valence of hydrogen that is +1 and the valency of sulphide (from group 16) is -2. And if we criss cross their valence we will be needing 2 hydrogen atoms for every one Sulphur atom, the chemical formula can be written as: ${{\text{H}}_{\text{2}}}{\text{S}}$.

iii. Carbon tetrachloride, the name of this compound has the clue to the number of chlorine atoms present i.e., 4 (tetra = 4). Now if we look at the valence of Carbon (group 14) is +4 and the valence of chlorine is -1, therefore after crisscrossing their valence we get the chemical formula as: ${\text{CC}}{{\text{l}}_{\text{4}}}$.

iv. Magnesium chloride, we have seen the valence of chloride is -1 and the valence of Magnesium (for group 2) is +2. Therefore, after crisscrossing their valence, we will be needing 2 atoms of chlorine for every one atom of Magnesium, the chemical formula can be written as: ${\text{MgC}}{{\text{l}}_2}$.

v. For calcium oxide, we have seen the valence of group 16 as -2 (for oxygen), and the valence of group 2 as +2 (for calcium). Therefore, after crisscrossing their valence we will be needing 2 atoms of calcium and 2 atoms of oxygen, the chemical formula can be written as:
${\text{C}}{{\text{a}}_2}{{\text{O}}_2}$ or since the number 2 is divisible, we can also write the same chemical formula as ${\text{CaO}}$.

vi. In this case, we have a polyatomic ion i.e., nitrate, the chemical formula for nitrate ion is ${\text{NO}}_{\text{3}}^ - $ . And the valence of Sodium is +1 (group 1), and the nitrate ion has a -1 negative charge on it. Therefore, after crisscrossing we will be needing one sodium atom for every one nitrate ion, the chemical formula can be written as: ${\text{NaN}}{{\text{O}}_3}$.

vii. Hydroxide ion (${\text{O}}{{\text{H}}^ - }$) has a -1 charge on it, and we know the valence of calcium is +2 (group 2). Therefore, after crisscrossing for the chemical formula of calcium hydroxide, we will be needing one atom of calcium and 2 molecules of hydroxide ion, the chemical formula can be written as: ${\text{Ca(OH}}{{\text{)}}_2}$.

viii. Carbonate ion (${\text{CO}}_{\text{3}}^{ - {\text{2}}}$) has a -2 charge on it, and we know the valence of sodium is +1 (group 1). Therefore after crisscrossing we will be needing 2 atoms of sodium and I molecule of the carbonate ion, the chemical formula can be written as: ${\text{N}}{{\text{a}}_2}{\text{C}}{{\text{O}}_3}$.

ix. Ammonium ion (${\text{NH}}_{\text{4}}^{\text{ + }}$) has +1 charge on it, and sulphate ion (${\text{SO}}_{\text{4}}^{{\text{2}} - }$) has -2 charge on it. Therefore, after crisscrossing, we will be needing 2 molecules of ammonium ion and one molecule of sulphate ion, the chemical formula for ammonium sulphate can be written as: ${{\text{(N}}{{\text{H}}_4})_2}{\text{S}}{{\text{O}}_4}$.

x. Aluminum chloride, we know the valence of chlorine i.e., -1 (group 17), and the valence of Aluminum is +3 (group 3). Therefore, after crisscrossing we will be needing 2 atoms of chlorine and 1 atom of Aluminum, the chemical formula for aluminum chloride is written as: ${\text{AlC}}{{\text{l}}_3}$.

xi. We know that sulphate ion has a -2 charge on it, and the valence of sodium is +1 (group 1). Therefore, after crisscrossing we will be needing 2 atoms of sodium for every one molecule of sulphate ion, the chemical formula can be written as: ${\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}$.

xii. We know that hydroxide ion has -1 charge on it and the valence of magnesium is +2 (group 2). Therefore, after crisscrossing we will be needing 2 molecules of hydroxide ion for every one atom of magnesium, the chemical formula can be written as: ${\text{Mg(OH}}{{\text{)}}_{\text{2}}}$.

Additional information:
The positive counterpart of any chemical formula is always written on the left side and the negative part is written on the right side. The same goes for written coordination compounds (complex salts).

Note:
Sometimes one may forget the crisscrossing part and write the chemical formula considering the individual valence of atoms. E.g., instead of writing ${{\text{H}}_{\text{2}}}{\text{S}}$ one may ${\text{H}}{{\text{S}}_2}$ without crisscrossing the valence. Also, there might be some confusion ${\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}$ since 2 and 4 are divisible one may write it as ${\text{NaS}}{{\text{O}}_2}$ which is incorrect because the ‘4’ in ${\text{S}}{{\text{O}}_4}$ only represent the 4 oxygen atoms and not the entire sulphate ion. But incase of ${\text{C}}{{\text{a}}_2}{{\text{O}}_2}$ it can be written as ${\text{CaO}}$ since 2 and 2 both are divisible and they represent the number of oxygen and calcium atom present.