Lecture 4 - Covalent and non-covalent bonding Flashcards
Atoms want complete….
outer valence shells
Hydrogen has one electron in it’s outer valence shell. It will either…
- lose it and become a proton
- share the electron to form a covalent bond
Atoms with nearly empty or complete shells tend to…
ionise easily to form salts.
Give an example of a salt and why/how it forms.
NaCl
- Chlorine has seven outer valence electrons. It will gain one electron to get a full shell.
- Sodium has one electron in its outer valence shell. It wants to lose the electron.
- Sodium and chlorine form a salt in this way.
Basic definition of covalent bond
the electrostatic attraction between positively charged nuclei of the bonding atoms and the shared pair of negatively charged electrons.
Explain covalent bonding in hydrogen.
Both hydrogen atoms have an unpaired electron in the 1s orbital. When the two atoms get close together the two 1s orbitals overlap and merge into a single bond orbital that contains both electrons.
Two important points about covalent bonding
Electrons must have opposite spins and each electron is able to exist in any part of the bond orbital and so belongs to both atomic nuclei.
Explain why the formation of a bond is accompanied by excess energy release and therefore what bond dissociation energy is
The molecule formed is at a lower energy than two atoms on their own. Therefore, the formation of a bond is accompanied by excess energy being released. To break the bond, the energy must be put back in- this is the bond dissociation energy (a measure of how strong the bond is)
How does orbital overlap effect strength of bond?
greater overlap of orbitals= stronger bond
What is a sigma bond?
- single bond
- strongest type of covalent bond
- orbitals overlap head on.
Give an example of S-S sigma bonds and the shape you would get.
- The simplest example is in dihydrogen. Two 1s atomic orbitals- one from each hydrogen atom will overlap to form a sigma bond. A single bond.
- The new molecular orbital in H2 (sigma s-s) has a shape that you would get from merging two s orbitals- sausage shaped
see notes
Sigma bonds can also form with an s and a p orbital. Draw this. Explain the shape of the sigma s-p orbital formed. What is the smaller lobe involved in?
The new molecular orbital formed has two lobes- one is much larger than the other. The smaller lobe is involved in substitution reactions.
Sigma bonds can also form with an s and an sp orbital. Draw this and describe the shape of the new sigma s-sp hybrid molecular orbital formed. What is the smaller lobe used for?
The new molecular orbital- sigma s-sp hybrid has two lobes. One lobe is bigger than the other. The smaller lobe is involved in substitution reactions.
What is a Pi bond?
- Covalent bonds formed from the sideways overlap of two p orbital lobes on one atom with two p orbital lobes on another
The node between the lobes in a p orbital have no electron density. What is the effect of this?
In a pi bond when the two p orbitals overlap there is a plane between the molecular orbitals with no electron density.
Pi bonds form with an existing sigma bond. What is the result of this?
double/triple bond is formed
Why is a Pi bond weaker than a sigma bond?
There is less of an overlap between p orbitals in a pi bond because of their parallel orientation, in contrast to sigma bonds where the bond is formed by the head on overlap of orbitals.
Lone pairs of electrons affect the shape of molecules. Explain this concept using ammonia an example (refer to hybridization)
- For nitrogen to form ammonia it is hybridized. Creates four sp3 hybrid orbitals. Three of the orbitals are filled with paired electrons. The fourth is home to two unpaired electrons- a lone pair of electrons. The lone pair behaves like a bond. The ammonia molecule adopts a tetrahedral shape- one arm is occupied by the lone pair. The result of this is a molecule shaped like a pyramid. The lone pair has a stronger repulsion than the sigma bonds which compresses the bond angle to 107. Reduces the bond angle by 2.5
What is a hydrogen bond?
A hydrogen bond is a partially electrostatic attraction between a hydrogen which is bonded to a more electronegative atom (hydrogen bond donor) and another atom with a lone pair of electrons (hydrogen bond acceptor)
What does the strength of the hydrogen bond depend upon?
the environments and geometries of the donor and acceptor atoms.
Hydrogen bonds do have some covalent nature. Explain
- they have direction
- bond distances are shorter than would be expected of van der waals.
In hydrogen bonds, the more electronegative the donor….
the more covalent the nature of the bond.
Hydrogen bonds increase…
- melting points
- boiling points
- solubility
- viscosity
What is the hydrogen bond donator?
The atom covalently bonded to hydrogen