How do you write the formula for ionic compounds?

How do you write the formula for ionic compounds?

Method to deduce the formulae of ionic compounds:

Metals from Groups 1, 2 and 13 combine with non-metals from Groups 15, 16 and 17 to form ionic compounds.

Table shows how the metal atoms of Groups 1, 2 and 13 form positive ions with different charges.

GroupNumber of valence electronsNumber of electrons donated to achieve a stable noble gas electron arrangementCharge of positive ionExample of ions
111+ 1Li+, Na+, K+
222+2Mg2+, Ca2+, Ba2+
333+3Al3+

Table shows how the non-metal atoms of Groups 15, 16 and 17 form negative ions with different charges.

GroupNumber of valence electronsNumber of electrons accepted to achieve a stable noble gas electron arrangementCharge of negative ionExample of ions
1553-3N3-
1662-2O2-, S2-
1771-1F, Cl, Br, I

Table shows the formulae of ionic compounds obtained when a metal from Group 1, 2 or 13 combines with a non-metal from Group 15, 16 or 17.

Elements that combinedFormula of ionic compoundExample
Metal atom R fromNon-metal atom T from
Group 1Group 17RTPotassium chloride, KCl
Group 1.Group 16R2TSodium oxide, Na2O
Group 1Group 15R3TLithium nitride, Li3N
Group 2Group 17RT2Calcium fluoride, CaF2
Group 2Group 16RTMagnesium sulphide, MgS
Group 2Group 15R3T2Calcium nitride, Ca3N2
Group 13Group 17RT3Aluminium chloride, AICI3
Group 13Group 16R2T3Aluminium oxide, Al2O3
Group 13Group 15RTAluminium nitride, AlN

The information in above Tables can be used to determine the formulae of ionic compounds as shown in the following examples.

People also ask

Writing formulas for ionic compounds examples

1. An atom of element D has 20 electrons. An atom of element E has 18 neutrons and a nucleon number of 35. Deduce the formula of the compound formed between elements D and E.
Solution:
How do you write the formula for ionic compounds 1
1 atom of element D combines with 2 atoms of element E to form a compound.
∴ Formula of the compound = DE2

2. Element X has a proton number of 13. An ion of element Y has 16 protons and 18 electrons. Deduce the formula of the compound formed between elements X and Y.
Solution:
Electron arrangement of atom X = 2.8.3
Element X has 3 valence electrons.
During bond formation, atom X loses 3 electrons to form a X3+ ion in order to achieve a stable octet electron arrangement.
Charge of a Y ion = Charge of 18 electrons + Charge of 16 protons
= (-18) + (+16)
= -2
How do you write the formula for ionic compounds 2
2 atoms of element X combine with 3 atoms of element Y to form a compound.
∴ Formula of the compound = X2Y3

3. The following table shows the number of neutrons and the nucleon numbers of atoms of elements Q and R. Q and R are not the actual symbols of the elements.
How do you write the formula for ionic compounds 3
Element Q reacts with element R to form a compound. What is the molar mass of the compound formed?
Solution:
How do you write the formula for ionic compounds 4