Ionic compounds
or an ionic compound with 2 elements only (one metal and one non-metal), the
F
metallic element is named first, and the name of the non-metal is shortened and the
suffix “-ide” is added to its shortened name.
Example: NaCl
odium (Na) is the metal. Chlorine (Cl) is the non-metal, so its name is shortened to
S
chlor- and the –ide is added, i.e. chloride.
So, its name is sodium chloride.
Formation of a cation
or Group 1 & 2 metals, the number of electrons lost is the same as the group
F
number.
Sodium atom
sodium ion + 1 electron
Na
Na+ +
1e-
Group 1 loses 1 electron. Na forms Na+,
●
● Group 2 loses 2 electrons. Mg forms Mg2+
or Group 13 metals, the number of electrons lost is the same as the group number
F
minus 10.
● Group 13 loses 3 electrons. Al forms Al3+
ransition Metals (Group 3-12) have slightly different rules and can lose a varied
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number of electrons. We will not delve into them here.
Formation of an anion
or Group 1 & 2 metals, the number of electrons lost is the same as the group
F
number.
Sodium atom
sodium ion + 1 electron
Na
Na+ +
1e-
Group 1 loses 1 electron. Na forms Na+,
●
● Group 2 loses 2 electrons. Mg forms Mg2+
or Group 13 metals, the number of electrons lost is the same as the group number
F
minus 10.
● Group 13 loses 3 electrons. Al forms Al3+
ransition Metals (Group 3-12) have slightly different rules and can lose a varied
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number of electrons. We will not delve into them here.
or non-metals, the number of electrons gained is 18 minus the group number. The
F
exceptionis hydrogen, which LOSES one electron (hydrogenwithout an electron is
just a proton).
Oxygen atom + 2 electrons
oxygen ion
O
+ 2e-
O2 -
Group 15 gains 3 electrons (18-15 = 3)
●
● Group 16 gains 2 electrons (18-16 = 2)
● Group 17 gains 1 electron (18-17 = 1)
N forms N3-
O forms O2-
Cl forms Cl-
Chemical formulas
1. Look at the metal and work out how many electrons it can lose - this will
tell you the positive charge of the ion
2. Look at the non-metal and work out how many electrons it can gain -
this will tell you the negative charge of the ion
3. Do a balancing act so the positive charges = negative charges
4.
Polyatomic ions
s we conduct experiments in this unit of work you will see some chemical
A
names that include some ions that are not found on the Periodic Table.
ome negative ions are made up of more than one type of atom tightly bonded
S
together. These are called polyatomic ions. Some common polyatomic ions
are listed in the table below.
Polyatomic Formula of polyatomic
ion
ion
Charge of ion
Hydroxide
OH-
1-
Nitrate
NO3-
1-
Carbonate
CO32-
2-
Sulfate
SO42-
2-
hese polyatomic ions can combine with metals to form ionic compounds.
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For example:
sodium hydroxide NaOH
●
● sodium nitrate NaNO3
● calcium carbonate CaCO3
● potassium sulfate K2SO4
t this stage, you just need to be able to identify these common polyatomic
A
ions and their elemental composition.
otassium chloride has the formula KCl, while potassium sulfide has the formula K₂S.
P
The difference in these formulas is due to the charges of the ions that potassium and
the other elements (chlorine and sulfur) form, and the need to balance these charges
to create a neutral compound.
1. Ions and Their Charges
● P
otassium (K) is a metal in Group 1 of the periodic table. It always forms a +1
ion:
→ K⁺
● C
hlorine (Cl) is a nonmetal in Group 17 (halogens). It forms a –1 ion:
→ Cl⁻
● S
ulfur (S) is a nonmetal in Group 16. It forms a –2 ion:
→ S²⁻
2. Balancing Charges in Ionic Compounds
In ionic compounds, the total positive charge must balance the total negative charge
so the compound is electrically neutral.
KCl (Potassium Chloride)
● K⁺ (charge +1)
● Cl⁻ (charge –1)
One K⁺ ion balances one Cl⁻ ion:
✅
→ KCl (1:1 ratio)
K₂S (Potassium Sulfide)
● K⁺ (charge +1)
● S²⁻ (charge –2)
o balance the –2 charge from one sulfide ion, you need two K⁺ ions (2 × +1 = +2):
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→ K₂S (2:1 ratio)
3. What the "2" Means
he "2" in K₂S shows that there are two potassium ions for every one sulfide ion in
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the compound. It is necessary to ensure that the total positive and negative charges
cancel out, making the compound neutral.
Summary
● KCl has a 1:1 ratio because K⁺ and Cl⁻ have equal and opposite charges.
● K₂S has a 2:1 ratio because two K⁺ ions are needed to balance one S²⁻ ion.
● T
he "2" indicates how many potassium ions are needed to balance the –2
charge of sulfur.
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