Periodic properties · Chapter 1
Electronegativity
Attraction for shared electrons inside a molecule.
By the end, you can
- define electronegativity and state why it has no unit;
- use the Pauling scale and identify fluorine as highest;
- explain the effects of atomic size and nuclear charge;
- reason through trends across a period and down a group;
- distinguish electronegativity from electron affinity;
- recognise diagonal pairs: Li–Mg, Be–Al and B–Si.
A tug-of-war for a shared pair
When two atoms share electrons in a molecule, they may not pull the pair equally.
Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
The word molecule matters: this property describes an atom while it is bonded and sharing electrons. It does not mean that one atom completely takes the pair away.
What does a more electronegative atom attract more strongly?
No unit: the Pauling scale
Electronegativity is a dimensionless property: it has no unit. Its values are comparative numbers, commonly given on the scale devised by Linus Pauling in 1932.
| Fact | What to write |
|---|---|
| Scale | Pauling scale |
| Highest | Fluorine, 4.0 |
| Lowest value given | Caesium, 0.7 |
| Unit | None |
kJ mol⁻¹ is not a unit of electronegativity. That energy unit belongs to electron affinity, not to this relative tendency.
Why is 4.0 kJ mol⁻¹ incorrect for fluorine’s electronegativity?
Period 2: a steady rise
The table in the chapter shows a clear pattern across Period 2.
| Element | Li | Be | B | C | N | O | F |
|---|---|---|---|---|---|---|---|
| Electronegativity | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 |
Shared-electron pull explorer
Choose an atom. The further right the marker sits, the stronger the attraction for a shared pair.
Which is more electronegative: carbon or oxygen?
What controls the pull?
Two linked factors matter most: atomic size and nuclear charge.
A small atom keeps the shared pair closer to the positive nucleus. A greater nuclear charge also attracts that negative pair more strongly—provided other conditions are comparable.
Which change generally raises electronegativity?
Across a period: increases
Electronegativity increases from left to right across a period. Nuclear charge increases while atomic size decreases, so attraction for a shared pair becomes stronger.
In Period 3: Na < Mg < Al < Si < P < S < Cl.
Why does electronegativity increase across a period?
Down a group: decreases
Electronegativity decreases down a group. Extra shells increase atomic size. The shared electrons are farther from the nucleus, and this size effect overcomes the increase in nuclear charge.
| Group 1 element | Li | Na | K | Rb | Cs |
|---|---|---|---|---|---|
| Electronegativity | 1.0 | 0.9 | 0.8 | 0.8 | 0.7 |
Which is more electronegative: chlorine or iodine?
Compare the ideas precisely
| Electron affinity | Electronegativity |
|---|---|
| An electron is added to a neutral isolated gaseous atom. | An atom in a molecule attracts shared electrons. |
| Energy is released. | It is a tendency, not an energy. |
| Has units such as kJ mol⁻¹. | Has no unit. |
| A negative ion forms. | The shared pair is pulled towards one atom. |
Affinity = an added electron. Negativity = shared electrons pulled in a molecule.
Metals, non-metals and noble gases
| Metals | Non-metals |
|---|---|
| Generally low electronegativity | Generally high electronegativity |
| Electropositive | Electronegative |
| Tend to lose electrons | Strongly attract electrons |
Noble gases have complete outer electronic arrangements, so they do not normally show a tendency to attract shared electrons; ordinary table values are not assigned in the same way. Greater attraction for electrons links with greater electronegativity, non-metallic character and oxidising nature.
Why do metals generally have low electronegativity?
Second-period elements and diagonal pairs
Second-period elements can differ from later members of their groups because of their small atomic size and high electronegativity. The chapter also describes a diagonal relationship.
Certain Period 2 elements resemble elements diagonally below them in Period 3. In this chapter, learn the pairs and the stated reason: a very small difference in electronegativity.
| Period 2 | Diagonally related Period 3 element |
|---|---|
| Lithium | Magnesium |
| Beryllium | Aluminium |
| Boron | Silicon |
Name the Period 2 element diagonally related to aluminium.
Catch the common mix-ups
Worked examples and practice
Practice questions
Open the answer key for all practice questions
ICSE-style questions
- Define electronegativity. State whether it has a unit.
- State its variation across a period and down a group; give a reason for each.
- Choose the more electronegative: Li/F, Na/Cl, F/Br, Li/K, C/O.
- Arrange
Na, Al, S, Clin increasing electronegativity. - Distinguish between electron affinity and electronegativity.
- Name the diagonal partner of lithium, beryllium and boron.
Open ICSE-style model answers
1. Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It is dimensionless and has no unit.
2. It increases across a period because nuclear charge increases and atomic size decreases. It decreases down a group because additional shells increase atomic size and weaken attraction.
3. F, Cl, F, Li, O.
4. Na < Al < S < Cl.
5. Electron affinity concerns addition of an electron to an isolated gaseous atom and energy release; electronegativity concerns attraction for shared electrons in a molecule and has no unit.
6. Mg, Al, Si.
Concept check
Answer these without looking back, then open the answer key.
- Define electronegativity.
- Does electronegativity have a unit?
- Name the scale discussed in the chapter.
- Who devised this scale?
- Which element has the highest electronegativity?
- What value is assigned to fluorine?
- What value is given for caesium?
- State the two factors affecting electronegativity.
- What happens to electronegativity across a period?
- What happens to electronegativity down a group?
- Why are non-metals generally more electronegative than metals?
- Name the three diagonally related pairs.
Open concept-check answers
- The tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
- No. It is dimensionless.
- The Pauling scale.
- Linus Pauling.
- Fluorine.
- 4.0.
- 0.7.
- Atomic size and nuclear charge.
- It increases from left to right.
- It decreases from top to bottom.
- Non-metals attract electrons more strongly, whereas metals generally tend to lose electrons.
- Li–Mg, Be–Al and B–Si.