Summary General Chemistry

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General Chemistry for the Life
Sciences
Lecture 1

Chemical bonding
Valence electrons, at the outer boundaries of both atoms, encounter each other first.
Cation: carries positive charge
Anion: carries negative charge

Bond formation: redistributing valence electrons

- Covalent compound: one or more pairs of electrons are shared equally between the
atoms
o Covalent bond
- Ionic compound: one or more electrons are totally transferred from one atom to
another
o Ionic bond

Chemical identity (type of element) determines whether an atom forms an ionic of covalent
compound.
- Two non-metals  covalent bonds/compound
- Non-metal + metal  ionic bonds/ compound

Metals fall on the left side of the periodic table
 Groups 1 and 2
Non-metals fall on the right side of the periodic table
 Groups 16 and 17




Polyatomic compounds
Polyatomic: an ion that contains more elements
Polyatomic ionic compounds: ionic compounds that have both ionic and covalent bonding.

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- One of the ions in the compound features atoms held together by covalent bonds

Ionic versus covalent bonding
Type of bonding depends on how readily electrons are transferred from one atom to the
other.
- If transferred very readily and large difference in electronegativity  ionic bonding
- If transferred less readily and electronegativity are similar  covalent bonding

If the electrons form an atom are very tightly associated with the atom and cannot interact
with neighbouring atoms, then the atom is unable to form any bonds.
 Chemically inert

The readiness of the transfer can be assessed by considering the electronegativity.
- An element’s electronegativity value indicates how strong an atom of that element
can attract an electron.
o Low value indicates that an electron is weakly attracted
- Electronegativity in the periodic table increases as you move across a period, from
one side of the table to another




Ionic bonds
- Strongest bond
- High boiling point

Polarized bonds: blurring the boundaries
A polar bond is a covalent bond in which the electrons are not evenly shared between the
two joined atoms.

 The distribution of electrons in a polar bond is governed by the electronegativity of
the atoms.

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 The distribution of the electrons that are shared in a polar bond is skewed towards
the most electronegative of the two atoms

The difference in electronegativity of the two atoms predicts how strongly polarized a
particular bond is.
- Large difference in electronegativity  bond is highly polarized
- Small difference in electronegativity  bond will be weakly polarized

 If the electrons in a covalent bond are genuinely equally shared between the two
atoms, the bond is called non-polar
o Atoms pull electrons towards themselves with equal strength
o Electrons are evenly distributed

Molecular interactions
Molecular interactions confer stability upon molecules.
- They strengthen the association between molecules
- Hold molecules in place, instead of drifting apart
- Helping biological systems exhibit specificity (complementary shapes)

Stability enables a biological system to exhibit specifity.
- Shapes not complementary  association not stabilized
- Molecular forces only operate over very small distances
- Contact area determines the strength of interaction

Attractive interactions: opposite charges attract
Repulsive interactions: same charges repel

Chemical bonding versus non-covalent forces

Covalent bonds
- Very strong
- Only interrupted by chemical reaction
- Between atoms

Non-covalent interactions
- Between molecules
- Generally weaker, easier to interrupt
- Only operate over short distances
- Primarily electrostatic in nature

 Many non-covalent interactions may operate between two molecules, such that the
overall effect is large.  Greater stability

Non-covalent interactions are electrostatic interactions, which exploit the forces of
attraction that operate between opposite charges. With the exception of ionic forces, non-
covalent interactions are relatively weak, and so can operate over only short distances: the

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strength of a non-covalent interaction decreases as the distance between two molecules (or
different parts of the same molecule) increases.

Sorts of interactions:
1. Dispersion forces (throughout entire
molecule)
2. Permanent dipolar interactions
3. Steric repulsion
4. Hydrogen bonds (between portions)
5. Ionic interactions (between portions)
6. Hydrophobic forces

All forces, except for steric repulsion, are based on
negative and positive charges.

Bond strength
The covalent bond is stronger than all the
mentioned non-covalent interactions here.
Dispersion forces are the weakest of the non-
covalent interaction. They appear as electrons in a
molecule are not always completely evenly distributed over that molecule. As a result of this
there are parts that carry a very small negative charge and parts with a very small positive
charge. These parts attract each other, but the interaction is very weak (and short lived). The
hydrogen bond is one of the strongest interactions of permanent dipoles.

Intramolecular versus intermolecular
Molecular interactions operate at two levels:

 Intramolecular
o Interactions operate between separate parts of the same molecule
o Within molecules
 Intermolecular
o Interactions operate between different molecules
o Between molecules, formation of complex
o Not as strong as intramolecular forces
o Result of attractions between positively and negatively charged regions of
separate molecules

Electrostatic forces: the foundations of molecular interactions
A vast majority of molecular interactions are electrostatic in nature: they are based on the
notion of opposite charges attracting (and, in one case, repelling) one another.
- Electrons are distributed towards the electronegative atom
- Their polarity means that polar molecules experience relatively extensive non-
covalent interactions, giving them high melting and boiling points.


Polar: chemical entity in which electrons are distributed unequally. Net dipole moment.