Summary
Chemistry Unit 2 Summary Notes Part 2
- Course
- Institution
The second and final document of concise summary notes for chemistry unit 2.
[Show more]
Seller
Follow
jasonsenoo
Content preview
Topic 2.3 – 2.4: Shapes of molecules, Intermediate bondingSummary of covalent bonding
Covalent bonds are formed by the sharing of electrons between atoms. This occurs due to the overlap of
atomic orbitals on neighbouring atoms which leads to the formation of a molecular orbital. Just like atomic
orbitals, molecular orbitals can accommodate two electrons.
When one electron comes from each atom, a ‘normal’ covalent bond is formed. When both electrons come
from the same atom, we call it a dative covalent bond. This is only of relevance for the purposes of ‘counting
electrons’ as there is no physical difference between a dative and a normal covalent bond. Thus ammonia
+
(NH3) and the ammonium ion (NH4 ) may be represented by the dot-and-cross diagrams below. Despite the
fact that one of the N-H bonds in the ammonium ion is represented as a dative bond, all four bonds are in
fact equivalent.
+
H H
H N H N H
dative
H H bond
When molecular orbitals are formed by overlap of atomic orbitals in the region directly between the two
atomic nuclei, the bond which is formed is known as a sigma (σ) bond. When the overlap between atomic
orbitals takes place in a region either side of this vector, the bond is known as a pi (π) bond.
C2pz C2pz
Osp3
H1s
Sigma orbital Pi orbital
e.g. O-H in water C=C in ethene
The tendency to form π-bonds decreases down a group. Thus oxygen forms O2 molecules (O=O), but
sulphur forms S8 molecules (all sigma bonds); nitrogen forms N2 molecules (N≡N), but phosphorus forms P4
molecules (all single bonds); carbon forms graphite (delocalised π-bonds) and diamond structure (all single
bonds) but silicon forms only a diamond-type structure. For any given atom pair, bond length decreases in
the order single > double > triple and bond strength increases in the order single < double < triple.
Covalent bonding may give rise to giant covalent or molecular structures. Giant covalent structures have
high melting points because strong covalent bonds must be broken on melting. Molecular structures have
low melting points because only the weak intermolecular forces (not the strong intramolecular covalent
bonds) need to be overcome on melting.
The electron density in a covalent bond is only shared equally between the two atoms when the groups at
either end of the bond are identical. In all other cases the electrons are drawn more towards one end of the
bond than the other and the bond is said to be polarised. This is due to the relative attraction for the bonding
electrons by the different nuclei and depends on the atomic radius and effective nuclear charge. The
attraction of an atom for electrons in a covalent bond is represented by its electronegativity which is
measured on the Pauling scale from 0.7 (Cs) to 4.0 (F). The effective nuclear charge increases across the
periodic table due to the ineffective shielding of the outer electrons. The attraction of this charge for the
bonding electrons decreases down the table as the nuclei become more distant from the bonding electrons.
Polar bonds are represented by δ+ and δ- charges on the atoms and lead to bond dipoles. Where the bond
dipoles do not cancel out this leads to a molecular dipole.
δ−
molecular dipole O bond dipole
δ− δ+ δ−
O C O
δ+ δ+
H H
molecular dipole = vector sum of bond (and lone pair) dipoles
N.B. lp dipoles ignored here
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller jasonsenoo. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $3.87. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
67096 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now