A1 - Electronegativity Flashcards
Describe the trend in electronegativity on the periodic table:
Electronegativity increases as the groups go along but as the periods decrease.
Which element has the highest electronegativity?
Helium.
What are non-polar bonds?
When the bonded electron pair is shared equally.
Why does HCl form polar molecules? (2)
-Cl is more electronegative than H, this means the electron cloud is distributed unevenly
-HCl is an asymmetrical molecule so the dipoles do not cancel
What is a polar bond?
A bond in which there is an uneven distribution of electrons.
Why can a molecule with polar bonds end up being a non-polar molecule?
If the molecule is symmetrical then the dipoles will cancel each other out and there will be no overall permanent dipole.
How do you know if a molecule is symmetrical?
All the atoms around the central atom are the same and there are no lone pairs on the central atom.
How do you know if a molecule is asymmetrical?
The central atoms has two or more different atoms or the central atom has one or more lone pairs.
What are the 3 types of intermolecular forces?
- Hydrogen bonding
- Permanent dipole-dipole forces
- London forces
What do London forces occur between ?
Non-polar molecules.
What are London forces?
They are temporary, induced dipole-dipole forces.
Do London forces act for a long or short time?
Why?
A short time, as the electron density is constantly changing.
What are the weakest intermolecular forces?
London forces.
What are the strongest intermolecular forces?
Hydrogen bonds.
What are the most electronegative atoms?
Nitrogen, oxygen, florine and chlorine.
What are the least electronegative atoms?
The group 1 metals.
What causes the bond to become ionic rather than covalent?
If the electronegativity difference is large, the more electronegative atom will have gained control of the electrons and the bond will now be ionic rather than covalent.
What does the electronegativity difference need to be greater than for a covalent bond to become ionic?
1.8
What is a permanent dipole?
It’s an intermolecular force caused by the separation of opposite charges due to an atom with a larger electronegativity and another with a smaller electronegativity.
When will permanent dipoles cancel out?
If the dipoles act in opposite directions, creating a non-polar molecule.
What do London forces exist between?
All molecules, whether polar or not.
How are London forces formed?
Due to movement of electrons that produce a changing dipole in a molecule. The instantaneous dipole induces a dipole on neighbouring molecules, which then attract one another.
Are London forces temporary or permanent?
Only temporary.
Explain the changes in increasing strength of London forces:
- The more electrons in each molecule
- The larger than instantaneous and induced dipoles
- The greater the induced dipole-dipole interactions
- The stronger the attractive forces between molecules
What do permanent dipole-dipole interactions act between?
Permanent dipoles in different polar molecules.
Hydrogen bonds are a special type of permanent dipole-dipole interaction found between molecules containing what?
- An electronegative atom with a lone pair of electrons eg O, N or F
- A hydrogen atom attached to an electronegative atoms
What do hydrogen bonds act between?
A lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule.
Why is solid water (ice) less dense than liquid water?
- Each water molecule can form 4 hydrogen bonds, with 2 lone pairs and 2 hydrogen atoms
- These hydrogen bonds extend outwards, holding water molecules slightly apart and forming an open tetrahedral lattice full of holes
- When ice melts, the ice lattice collapses and the molecules move closer together, so liquid water is denser than solid ice.
Does water have a high or low melting point?
Relatively high.
