4.3.5 Electronegativity

This topic explains electronegativity — the ability of an atom to attract electrons toward itself in a chemical bond.

Definition

Electronegativity is the ability of an atom to attract electrons toward itself in a chemical bond.

Simple explanation: Electronegativity tells you how strongly an atom pulls shared electrons toward itself when it forms a bond.

Main Idea

The American chemist Linus Pauling devised a method for calculating the relative electronegativities of elements. This scale is shown in Figure 4.7 and is one of the most widely used ways to compare how strongly different atoms attract electrons.

• Higher electronegativity → atom attracts electrons more strongly.
• Lower electronegativity → atom attracts electrons less strongly.

Simple explanation: Atoms with high electronegativity "pull" shared electrons toward themselves more than atoms with low electronegativity.

Core Concept

Pauling's Electronegativity Scale

The book gives a table of electronegativity values. Some key values:

Trends in Electronegativity

Simple explanation: Electronegativity increases as you go right and decreases as you go down.

Detailed Explanation

Trend 1: Electronegativity Increases Across a Period

As you move from left to right in a period:

• The nuclear charge increases (more protons).
• The atomic size decreases.
• The nucleus holds the bonding electrons more tightly.
• Therefore, electronegativity increases.

Example: In Period 2:

Simple explanation: Fluorine (F) has the highest electronegativity of all elements (4.0).

Trend 2: Electronegativity Decreases Down a Group

As you move from top to bottom in a group:

• The atomic size increases.
• The shielding effect increases.
• The valence electrons are farther from the nucleus.
• The nucleus has a weaker pull on bonding electrons.
• Therefore, electronegativity decreases.

Example: In Group 17 (halogens):

Example: In Group 1 (alkali metals):

Activity 4.2 (from the book)

The book asks students to:

• Move across the second period from left to right and note down the variation in electronegativity values.
• Move across the 3rd period from left to right and note down the variation in electronegativity values.
• Make a generalization about the variation in electronegativities across a period and write the reason.
• Move from top to bottom in Groups IA and IIA and note down the variation in electronegativity values.
• Move from top to bottom in Groups VIA and VIIA and note down the variation in electronegativity values. Make a generalization about the trend in electronegativity values in a group. Give reason.

• Across a period: electronegativity increases (nuclear charge increases, atomic size decreases).
• Down a group: electronegativity decreases (atomic size increases, shielding increases).

Intermediate Level

Why Electronegativity Increases Across a Period

• Protons increase → nuclear charge increases.
• Atomic size decreases → bonding electrons are closer to the nucleus.
• Shielding remains constant.
• So the nucleus attracts bonding electrons more strongly → electronegativity increases.

Why Electronegativity Decreases Down a Group

• New shells are added → bonding electrons are farther from the nucleus.
• Shielding effect increases.
• The nucleus has a weaker pull on bonding electrons → electronegativity decreases.

Electronegativity and Bond Type

Electronegativity differences determine the type of bond formed:

Simple explanation: The bigger the electronegativity difference, the more polar (or ionic) the bond.

Electronegativity vs Electron Affinity

Highest and Lowest Electronegativity

• Highest: Fluorine (F) — 4.0
• Lowest: Cesium (Cs) and Francium (Fr) — 0.7

Simple explanation: Fluorine pulls electrons the hardest; cesium and francium pull them the least.

Important Points

• - Electronegativity: ability of an atom to attract electrons toward itself in a chemical bond.
• - Developed by: Linus Pauling.
• - Across a period: electronegativity increases.
• - Down a group: electronegativity decreases.
• - Most electronegative element: Fluorine (4.0).
• - Least electronegative elements: Cs, Fr (0.7).
• - **Key values:** H = 2.1, C = 2.5, N = 3.0, O = 3.5, F = 4.0, Na = 0.9, Cl = 3.0.
• - Electronegativity differences determine bond type (non-polar covalent, polar covalent, ionic).

Remember

• - Electronegativity = ability to attract electrons in a bond.
• - Pauling developed the electronegativity scale.
• - Across a period → increases.
• - Down a group → decreases.
• - **F** = most electronegative (4.0).
• - **Cs, Fr** = least electronegative (0.7).
• - **Key values:** H = 2.1, C = 2.5, N = 3.0, O = 3.5, F = 4.0, Na = 0.9.
• - Larger electronegativity difference → more polar/ionic bond.

Common Mistakes

• 1. Thinking electronegativity increases down a group. It decreases.
• 2. Thinking electronegativity decreases across a period. It increases.
• 3. Confusing electronegativity with electron affinity. Electronegativity is about attracting electrons in a bond; electron affinity is about energy released when gaining an electron.
• 4. Thinking all atoms have the same electronegativity. Values range from 0.7 to 4.0.
• 5. Forgetting that fluorine is the most electronegative element. F = 4.0, the highest on the Pauling scale.
• 6. Thinking electronegativity has units. It is a relative scale with no units.

Quick Revision

• Electronegativity: ability of an atom to attract electrons toward itself in a chemical bond.
• Developed by: Linus Pauling.
• Across a period: electronegativity increases (nuclear charge increases, atomic size decreases).
• Down a group: electronegativity decreases (atomic size increases, shielding increases).
• Most electronegative element: Fluorine (F = 4.0).
• Least electronegative elements: Cs and Fr (0.7).
• **Key values:** H = 2.1, C = 2.5, N = 3.0, O = 3.5, F = 4.0, Na = 0.9, Cl = 3.0.
• Electronegativity difference determines bond type: non-polar covalent, polar covalent, or ionic.

60-Second Revision

• Electronegativity = ability to attract electrons in a bond.
• Across a period → increases.
• Down a group → decreases.
• **F = 4.0** (highest); **Cs, Fr = 0.7** (lowest).
• **Key values:** H = 2.1, C = 2.5, O = 3.5.
• Warning: electronegativity is not the same as electron affinity.