Define electronegativity
The ability of an atom to attract the bonding electrons in a covalent bond; an interpretation of Pauling electronegativity values
Explain the electronegativity trend across the periodic table
Electronegativity rises across and up the periodic table:
- Nuclear charge increases across a period; electronegativity increases
- Atomic radius and shielding effect increase down a group; electronegativity decreases
When is a covalent bond polar?
When a (significant) electronegativity difference is present between the atoms
(below 1.8 - above is ionic)
There is an uneven distribution of charge and atoms gain partial charge.
When is a molecule polar?
When a molecule is asymmetrical/dipoles do not cancel out, making the molecule polar overall. There is a separation of charge, and a permanent dipole.
Where do intermolecular forces occur?
Between discrete, simple molecular molecules. Comparatively weak.
What is permanent dipole-dipole bonding, and which molecules does this take place between?
Opposite dipoles in polar molecules attract.
This bonding takes place between polar molecules
What is instantaneous-induced dipole-dipole bonding, and which molecules does this take place between? What else is this called?
- A temporary instantaneous dipole is created in a molecule due to fluctuations in electron density
- This induces a dipole in a neighbouring molecule
- This results in attraction between the two dipoles
- This bonding place between all molecules
- London forces
What does strength of instantaneous-induced bonding depend on?
- Number of electrons: more electrons = stronger forces
- Shape of the molecule: the larger (and less branched) the molecules, stronger the force
Define hydrogen bonding
Intermolecular bonding between molecule containing:
- N, O or F attached to a hydrogen
- With a lone pair on the N, O or F
Why is water more dense than ice?
-When solid, water molecules are held apart by hydrogen bonds in a an open lattice, and there is more space between molecules.
-When ice melts, the hydrogen bonds break and water molecules come closer together, meaning they are less dense than ice.
Why is water liquid at room temperature?
Strong hydrogen bonding to two lone pairs (forming four hydrogen bonds between each molecule) results in an anomalously high melting and boiling point
What is a simple molecular lattice?
Covalent bonded molecules attracted by intermolecular forces (i.e. ice)
