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Electronegativity
- Electronegativity depends on
(1) Nuclear charge
(2) The distance between the nucleus and the outer shell electrons
(3) The shielding of the nuclear charge by electrons in the inner shells - The smaller the atom, the closer outer electrons are to the nucleus.
- The most electronegative atoms are found at the top right of the periodic table.
- The most electronegative atoms are: fluorine, oxygen and nitrogen followed by chlorine.
Bond polarity in covalent bonds
Electron density is often used to describe the way in which the negative charge is distributed in a molecule.
The Pauling scale is used to measure electronegativity > running from 0 to 4.
Noble gases have no number because they form covalent bonds.
Polarity is the unequal sharing of the electrons between atoms that are bonded covalently.
- If a covalent bond with two atoms of different electronegativity, the electrons will not be shared equally.
- E.g. Hydrogen has an electronegativity of 2.1 and fluorine of 4.0.
Do you still remember the intermolecular forces acting between molecules in AS/A-level Chemistry?
Forces acting between molecules
Intermolecular forces
- Dipole - dipole forces: Acting only between certain types of molecules
- Van der Waals forces (strongest): Acting between all atoms and molecules
- Hydrogen bonding (weakest): Acting only between certain types of molecules
1. Dipole - dipole forces
- Carbon dioxide is linear and the dipoles cancel out.
- The way in which some dipole moments cancel out is through the shape of the molecule.
2. Van der Waals forces
- Weak electrostatic attractions between all atoms and molecules.
- Electrons are constantly moving therefore the distribution of the charge is changing at every instant.
- The dipole may be in a different direction.
- Distribution effects nearby atoms.
- They are in addition to other intermolecular forces.
- Size of the van der Waals forces increases with the number of electrons present.
- The boiling points of noble gases increase with atomic mass/ numbers.
- The boiling points of hydrocarbons increase with increased chain length.
3. Hydrogen bonding
- Intermolecular force with characteristics of a dipole – dipole attraction (d-d attractions) and some of a covalent bond.
- A hydrogen atom is in between two electronegative atoms.
- A very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom.
- Hydrogen atoms are highly electron deficient – oxygen is very electronegative and attracts shared electrons towards itself.
- Exposed protons have a very strong electric field due to their small size.
- Stronger than d-d attractions but weaker than covalent bonds.
- The only atoms that can form hydrogen bonds are oxygen, nitrogen and fluorine.
- Noble gases show a gradual increase in boiling point because the only forces acting between the atoms are the van der Waals forces and these increase with the number of electrons present.
- Boiling point of H20, HF and ammonia are higher than the hydrides because of the other elements in their group.
References:
1. https://www.chemistrylearner.com/
That's the end of the topic!
Drafted by Bonnie (Chemistry)