Notes

Ions & Ionic Bonding

The story of give and take, in terms of electrons

1. Give me the magic phrase: the electronic configuration of a noble gas

“Funny” chemistry “comic” of two lonely helium atoms, happy with their number of valence electrons

To understand ionic bonding, we must first explore why some atoms form ions, while others are so stable that they do not.

Noble gases like helium are the stable ones. They are monatomic gases, existing as individual atoms without any bond holding them together.

To think about this in terms of electrons, it means that the electronic configuration of a noble gas is stable. They do not lose or gain any valence electron easily.


2. Three’s a crowd, eight’s an octet

Noble GasElectronic Configuration
Helium2
Neon2.8
Argon2.8.8
Krypton2.8.8.18.8

With the exception of helium, the electronic configuration of all other noble gases entails 8 valence electrons in the outermost shell, which we call an octet.


3. Metallic elements: lose to gain

Atoms of metallic elements lose their valence electrons to gain the electronic configuration of a noble gas.

Red dots represent electrons, blue protons, and grey neutrons. Note that in this positively charged ion, there are fewer electrons than protons.

In the process of losing electrons, they become positively charged as they now have fewer electrons than protons. For example, when lithium atom loses its only valence electron, it forms a charged particle with just 2 electrons, fewer than the 3 protons it has. This charged particle is called an ion. To be exact, a positively charged ion is called a cation (pronounced cat-eye-yawn).

All metallic elements tend to lose electrons to form metallic cations.


4. Picking up the crumbs: non-metallic elements gain electrons

If metal loses electrons, where do these electrons go to?

Besides sharing electrons in a covalent bond, non-metallic elements can complete their octet by gaining electrons.

In fluoride ions, there are more electrons represented by red balls than protons represented by blue balls

For example, fluorine atom can gain an extra electron to complete its octet. The atom becomes a negatively charged ion, as it has one more electron than there are protons. A negatively charged ion is also called an anion (pronounced an-eye-yawn).


5. Ionic bonding: opposites attract

In this diagram, an ion pair is simplified as two points on a plane. The charges carried by the ions create an electric field, represented by invisible lines that emanate from the two points.

An ion can be thought of as a point in space with a charge. The point charge emanates an invisible electric field. When an oppositely charged ion comes deep into the electrostatic field, an attractive force results, bringing the oppositely charged ions together. This very force is the basis of ionic bonding.

Ionic bonding is the electrostatic forces of attraction between positively-charged ions and negatively-charged ions.

Usually, ionic compounds held by ionic bonds are formed between metals and non-metals, like the lithium fluoride shown above.


6. Extensive ionic bonding: from ion pairs to giant ionic lattice structure

From the OG pair of lithium and fluoride ions represented by balls in solid colour, more ions represented by shaded balls are attracted as the electric field emanates in all directions.

Ionic bonding does not stop at two oppositely charged ions.

The electric field due to the charge of an ion emanates in all directions. Therefore, a cation can attract multiple anions in all directions, and vice versa. In other words, there is no requirement for directionality in the cation-anion interaction.

Giant ionic lattice structure, with the cations and anions represented by spheres of different colours

The contagious attraction causes an ion pair to attract more ions, hence forming an huge arrangement of ions. We call this arrangement a giant ionic lattice structure.


7. Predicting the charges of common ions in an ionic compound

Main-group elements in the first four periods are those highlighted. Amongst them, metallic ones are in blue, while non-metallic ones are in red.

Given that metallic ions lose electrons while non-metallic ions gain electrons to achieve the noble gas electronic configuration, we can use this as a rule of thumb to predict the charges of main-group elements.

Main-group elements are those in groups I to VII, and group 0 (noble gases). They do not include the transition metals between group II and III.

Metallic Elements
GroupMetallic ElementCharge
ILithium (2.1)
Sodium (2.8.1)
Potassium (2.8.8.1)
1+
IIBeryllium (2.2)
Magnesium (2.8.2)
Calcium (2.8.8.2)
2+
IIIAluminium (2.8.3)3+
Metallic elements lose their valence electrons (as bolded in the table) to achieve the noble gas configuration, hence forming positively charged ions.
Non-metallic Elements
GroupMetallic ElementCharge
VNitrogen (2.5)3-
VIOxygen (2.6)
Sulfur (2.8.6)
2-
VIIFluorine (2.7)
Chlorine (2.8.7)
Bromine (2.8.8.7)
1-
Non-metallic elements gain electrons to complete their octet, hence forming negatively charged ions. The number of electrons gained is the difference between an octet and the number of valence electrons.

8. We charge ahead as one: polyatomic ions

Metallic and non-metallic ions are monatomic ions: charged particles formed from just one atom each.

Dot-and-cross diagram of ammonium, a polyatomic cation. There is an overall positive charge, as the central nitrogen is short of one electron, making it isoelectronic as carbon.

There are also polyatomic ions: charged particles formed from multiple atoms covalently bonded to each other. Each polyatomic ion is seen as one unit. Here is a list of common polyatomic ions and their names:

ChargePolyatomic IonChemical Formula
1+AmmoniumNH4+
1-Hydroxide
Bicarbonate
Nitrate
OH
HCO3
NO3
2-Carbonate
Sulfate
CO32-
SO42-
3-PhosphatePO43-
Besides hydroxide, polyatomic anions containing oxygen end with the suffix -ate.

9. Naming ions and ionic compounds

Cations formed from metallic elements have the same name as the metal, like sodium ion, calcium ion, and aluminium ion.

Transition MetalCation NameChemical FormulaCharge
IronIron(II)
Iron(III)
Fe2+
Fe3+
2+
3+
CopperCopper(I)
Copper(II)
Cu+
Cu2+
1+
2+
If a transition metal can form different cations with different charges, the positive charge is indicated by a Roman numeral in brackets that come after the name of the metal.

Monatomic anions formed from non-metallic elements are named by replacing the ending syllable of the element name with -ide. For example, nitrogen forms nitride, oxygen forms oxide, and fluorine forms fluoride.

Polyatomic anions containing oxygen end with -ate. For example, nitrate refers to NO3 while nitride refers to N3-. Likewise, sulfate refers to SO42- while sulfide refers to S2-.

Ionic compounds are named by placing the cation name before the anion name, like lithium fluoride and sodium chloride.


10. Test yourself