Electronegativity Trend Across a Period: IB Chemistry Guide

The electronegativity trend across a period is a small topic with a surprisingly high chance of exam mistakes. Students often know the arrow points left to right, but they do not explain why.

In IB Chemistry, the key idea is that electronegativity usually increases across a period because nuclear charge increases while shielding stays similar, so the nucleus attracts bonding electrons more strongly.

Quick Answer

• Electronegativity is the ability of an atom in a bond to attract the bonding pair of electrons.
• Across a period, electronegativity generally increases from left to right.
• The nuclear charge increases because there are more protons.
• Electrons are added to the same main energy level, so shielding does not increase much.
• The attraction for bonding electrons becomes stronger.
• More electronegative atoms become delta negative in polar bonds.
• Do not confuse electronegativity with electron affinity.

Electronegativity Trend Across a Period

Across a period, electronegativity generally increases from left to right. For Period 3, the trend runs from sodium toward chlorine.

The simple exam statement is:

Electronegativity increases across a period because nuclear charge increases while shielding remains similar, so bonding electrons are attracted more strongly.

That sentence is stronger than just writing "it increases because atoms want electrons more". Keep the answer about attraction, protons and shielding.

What Electronegativity Means

Electronegativity is about atoms in a bond. It describes how strongly an atom attracts the shared pair of electrons in a covalent bond.

This matters because electronegativity differences create polar bonds. If one atom attracts the bonding electrons more strongly, that atom becomes slightly negative, while the other atom becomes slightly positive.

Why Electronegativity Increases Across a Period

The reason is a balance between nuclear charge and shielding.

Across a period:

• proton number increases
• nuclear charge increases
• electrons are added to the same main shell
• shielding stays similar
• atomic radius generally decreases
• attraction for bonding electrons increases

This is why the trend is not explained by "more electrons" alone. The important idea is that the nucleus has a stronger pull on bonding electrons.

Electronegativity and Bond Polarity

When two atoms have different electronegativities, the bonding electrons are pulled closer to the more electronegative atom. This creates a polar bond.

For example, in H-Cl, chlorine is more electronegative than hydrogen. Chlorine attracts the bonding electrons more strongly, so chlorine is delta negative and hydrogen is delta positive.

The dipole arrow points toward the more electronegative atom.

Common Exam Mistakes

These mistakes are easy to fix:

For markscheme wording, always connect the trend to nuclear charge and shielding.

Worked Example

Question: Explain why chlorine is more electronegative than sodium.

Mark-worthy answer: Chlorine has a higher nuclear charge than sodium and its outer electrons are in the same main energy level across Period 3, so shielding does not increase much. Chlorine therefore attracts bonding electrons more strongly.

Why this works: The answer names nuclear charge, shielding and attraction for bonding electrons.

How EduNinja Helps with This Topic

Use IB Chemistry periodic table questions to practise trends such as electronegativity across a period. Then use IB Chemistry covalent model questions to connect electronegativity to bond polarity and electron sharing.

If the electron-shell explanation feels weak, review IB Chemistry electron configuration questions before doing mixed practice.

Related Study Links

• IB Chemistry periodic table questions
• IB Chemistry covalent model questions
• IB Chemistry electron configuration questions
• IB Chemistry SL Question Bank
• IB Chemistry Question Bank

FAQ

What is the electronegativity trend across a period?

Electronegativity generally increases from left to right across a period. Nuclear charge increases while shielding stays similar, so atoms attract bonding electrons more strongly.

Why does electronegativity increase across Period 3?

Across Period 3, proton number increases and electrons are added to the same main shell. Because shielding does not increase much, the nucleus attracts bonding electrons more strongly.

Is electronegativity the same as electron affinity?

No. Electronegativity is attraction for bonding electrons in a covalent bond. Electron affinity is about the energy change when an atom gains an electron.

How does electronegativity affect bond polarity?

The more electronegative atom attracts the bonding electrons more strongly and becomes delta negative. The less electronegative atom becomes delta positive, so the bond is polar.

Final Takeaway

Use this memory line: more protons, similar shielding, stronger pull. That is the cleanest way to explain why electronegativity increases across a period.