In the world of chemistry, electronegativity is a fundamental concept that unlocks the secrets behind the behaviour of atoms and molecules. For both GCSE and A-level students, understanding electronegativity is like holding the key to a treasure chest of knowledge. So, this blog will teach you everything that you need to know about the concept!
Electronegativity: The Elemental Tug of War
At its core, electronegativity is an element’s ability to attract electrons in a chemical bond. The more electronegative an atom is, the stronger its pull-on electrons.
If the difference in electronegativity between the atoms is very high (>1.7), the electrons will almost completely move to the atom with the higher electronegativity, making it an ionic bond. On the other hand, if the electronegativity difference is relatively small (<1.7), then the electrons will only slightly move the more electronegative atom, resulting in a partial negative charge on that atom, and a partial positive charge on the other atom – a polar covalent bond. If the electronegativity difference is 0, i.e., they are the same atom, then the bond is non-polar covalent.
Hence, the difference in electronegativity between bonded atoms determines the very nature of the chemical bond.
Factors Affecting Electronegativity
Electronegativity values aren’t arbitrary; they are influenced by specific factors:
- Charge density (charge of atom/size of atom): Electronegativity increases with the charge density of the atom. We can understand this by considering the charge and size of the atom separately.
- Atomic Number: Electronegativity increases with increasing atomic number because the number of protons in the nucleus increases, leading to a stronger positive charge that attracts electrons more strongly.
- Atomic Size: Electronegativity tends to decrease with size (number of atomic shells) as the outermost electrons will be farther from the nucleus and experience weaker attraction.
- Effective Nuclear Charge: A greater effective nuclear charge leads to higher electronegativity. Effective Nuclear Charge is the effective charge felt by outer electrons, accounting for the shielding effects of the inner electrons on the nucleus.
- Shielding or Screening Effect: The inner electrons “shield” outer electrons from experiencing the full positive attraction of the nucleus, by providing a repulsive effect with their negative charge. Naturally, as the number of inner electron shells increases, the shielding effect becomes more significant, reducing electronegativity. So, we can imagine the average effect of the inner electrons to be equivalent to a negative charge, which reduces the net charge effectively felt by the outer electrons.
- Bonding Environment: The chemical environment in which an atom is involved can affect its electronegativity. Atoms can exhibit different electronegativities in various compounds and molecular environments.
Trends of Electronegativity in the Periodic Table
Since atomic number and atomic size follows trends in the periodic table, naturally electronegativity which is strongly affected by these two, also follows similar trends in the periodic table:
- Electronegativity generally increases from left to right across a period. This is due to the increasing effective nuclear charge (atomic number increases and atomic size decreases across a group), which intensifies the pull-on electrons.
- Electronegativity generally decreases as you move down a group. The added energy levels in lower rows result in greater atomic size (so less charge density and more shielding), leading to weaker attraction for electrons.
Electronegativity serves as a guide for predicting chemical behaviour. It explains why some bonds are strong and others weak, why certain compounds are polar, and others nonpolar, making it an indispensable concept for students and chemists alike.
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