IMAT Chemistry · Chemical Bonding

Ionic, Covalent and Metallic Bonds

The electronegativity difference between two atoms decides whether they share electrons equally, share them unequally, or transfer them to make ions.

All Chemical Bonding notes

Why this matters

The papers ask which pair of elements forms a covalent bond (2012), which atomic numbers fit an ionic formula (2015), and what a pure covalent bond is (2025). All three are answered by electronegativity and ion charges from the group number.

Concept 1 of 3: Electronegativity difference and bond character

Two atoms in a bond pull on the same pair of electrons. If they pull equally, the pair sits in the middle. If one pulls harder, the pair moves towards it and that end becomes slightly negative. If one pulls much harder, it takes the electron completely and the two atoms become ions.

Definition

Electronegativity is the ability of an atom in a bond to attract the shared electrons. On the Pauling scale fluorine is the highest (4.0); it rises across a period and falls down a group, so the most electronegative elements are F, O, N and Cl, and the least are at the bottom left (Cs, Fr).

$\Delta\chi = 0$ (two atoms of the same element): pure (non-polar) covalent bond, electrons shared exactly equally, as in $\mathrm{Cl_2}$ or $\mathrm{H_2}$ .
$\Delta\chi$ below about 0.4: treated as non-polar in practice (the C to H bond, 0.35).
$\Delta\chi$ from about 0.4 up to about 1.7 to 1.9: polar covalent, with partial charges $\delta^+$ and $\delta^-$ .
$\Delta\chi$ above about 1.7 to 1.9: mostly ionic (Italian textbooks use 1.9, many English ones 1.7). The line is a guide, not a wall: bond character changes gradually.
Quick rule: a metal from Group 1 or 2 with a non-metal from Group 16 or 17 gives an ionic compound; two non-metals give a covalent one. Beryllium is the usual exception: its compounds are largely covalent.

Electronegativity difference

\Delta\chi = \left|\chi_A - \chi_B\right|

$\chi_A, \chi_B$ Pauling electronegativities of the two bonded atoms
$\Delta\chi$ the difference, always taken as positive

Worked example

Use the electronegativities H 2.20, C 2.55, O 3.44, Na 0.93 and Cl 3.16 to classify the bonds C to H, O to H, Na to Cl and Cl to Cl. Which end of the O to H bond is

\delta^-

Practice this conceptself-check · 4 quick reps

Pure covalent means identical atoms, not just similar ones

A bond is purely covalent only when

\Delta\chi = 0

, which means two atoms of the same element. Atoms with similar electronegativities (C and H) make a bond that is nearly non-polar, but not pure. An option saying "atoms of quite similar electronegativity" describes a weakly polar bond.

A metal and a non-metal are not always ionic

Beryllium, and aluminium with chlorine, sit close to the boundary and form largely covalent compounds. When an option pairs beryllium with a halogen, expect covalent, not ionic.

Concept 2 of 3: Ionic bonds and the formula of an ionic compound

A metal atom with one, two or three outer electrons loses them easily; a non-metal atom a few electrons short of a full shell gains them. Both end up with a noble gas configuration. The positive and negative ions then attract each other in every direction, so they pack into a huge lattice rather than forming separate molecules.

Definition

An ionic bond is the electrostatic attraction between oppositely charged ions, formed by electron transfer from a metal to a non-metal.

Ion charge from the group: Group 1 $+1$ , Group 2 $+2$ , Group 13 (Al) $+3$ ; Group 15 $-3$ , Group 16 $-2$ , Group 17 $-1$ .
The formula is the simplest ratio that makes the total charge zero. It is a formula unit, not a molecule.
The bond is stronger when the ions have higher charges and are smaller: MgO ( $\mathrm{Mg^{2+}}$ , $\mathrm{O^{2-}}$ ) melts near 2850 °C, NaCl at 801 °C.
To use atomic numbers, write the electron configuration (2, 8, 8 ...) and count the outer electrons to find the group.

Charge balance in an ionic formula

\mathrm{M}_a\mathrm{X}_b: \quad a \times (\text{charge of M}) + b \times (\text{charge of X}) = 0

$a, b$ the smallest whole numbers that balance the charges

Worked example

Element X has atomic number 13 and element Y has atomic number 8. Find the formula of the ionic compound they form.

Practice this conceptself-check · 4 quick reps

An ionic formula is a ratio, not a molecule

\mathrm{NaCl}

does not exist as NaCl molecules. Each ion is surrounded by ions of the opposite charge throughout a lattice, and the formula only gives their ratio. An option calling solid sodium chloride molecular is wrong.

Concept 3 of 3: Metallic bonding and the properties it explains

Metal atoms hold their outer electrons loosely. In the solid each atom gives these electrons up to a shared pool, the sea of delocalised electrons, and becomes a positive ion. The ions are held together by their attraction to this sea, and because the electrons can move, metals conduct.

Definition

A metallic bond is the attraction between a lattice of positive metal ions and a sea of delocalised electrons.

The bond has no direction, so layers of ions can slide without breaking it.
It is stronger when each atom gives more electrons to the sea and when the ions are smaller.
Alloys mix in atoms of a different size, which stops the layers sliding so easily: alloys are harder than the pure metal.

Property	Explanation
Conducts electricity as a solid and as a liquid	Delocalised electrons move through the lattice when a voltage is applied
Good conductor of heat	Free electrons carry energy quickly through the metal
Malleable and ductile	Layers of ions slide over each other and the electron sea still holds them
Usually high melting point	Strong attraction between the ions and the electron sea
Melting point rises from Na to Mg to Al	More delocalised electrons per atom (1, 2, 3) and smaller, more charged ions

Practice this conceptself-check · 3 quick reps

Solid ionic compounds do not conduct; solid metals do

Both contain charged particles, but in a solid ionic compound the ions are locked in place. It conducts only when molten or dissolved. A metal conducts as a solid because its electrons are free to move.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Electronegativity difference and bond character
Electronegativity difference
$\Delta\chi = \left|\chi_A - \chi_B\right|$
Ionic bonds and the formula of an ionic compound
Charge balance in an ionic formula
$\mathrm{M}_a\mathrm{X}_b: \quad a \times (\text{charge of M}) + b \times (\text{charge of X}) = 0$

Reference tables (1)

Metallic bonding and the properties it explains5 rows

Property	Explanation
Conducts electricity as a solid and as a liquid	Delocalised electrons move through the lattice when a voltage is applied
Good conductor of heat	Free electrons carry energy quickly through the metal
Malleable and ductile	Layers of ions slide over each other and the electron sea still holds them
Usually high melting point	Strong attraction between the ions and the electron sea
Melting point rises from Na to Mg to Al	More delocalised electrons per atom (1, 2, 3) and smaller, more charged ions

Watch out for (4)

Pure covalent means identical atoms, not just similar ones
→ Electronegativity difference and bond character
A metal and a non-metal are not always ionic
→ Electronegativity difference and bond character
An ionic formula is a ratio, not a molecule
→ Ionic bonds and the formula of an ionic compound
Solid ionic compounds do not conduct; solid metals do
→ Metallic bonding and the properties it explains

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