Making and Breaking Bonds

It takes energy to break chemical bonds. Therefore, this process is endothermic.

Conversely, when bonds form, energy is released. Bond formation is exothermic.

We can apply this to our understanding of melting (and boiling) points. We can also use this to understand what is happening at a particle level in an endothermic or exothermic chemical reaction.

Energy Changes

Some reactions feel hot; some feel cold. This means there are energy differences between the reactants and the products. We call this an enthalpy change.

If something feels hot, it is releasing energy. We call this exothermic.

If something feels cold, it is absorbing energy from our skin. We call this endothermic.

Metals

Metals have some characteristic properties:

• malleability
• ductility
• electrical conductivity
• thermal conductivity
• relatively high melting/boiling points

This is due to the particles (cations in a sea of valence electrons) and bonding (strong, non-directional metallic bonds).

Covalent Networks

Not all non-metals form molecules. Some create complex 3D networks, with the atoms bound together by multiple covalent bonds.

Properties of Molecular Substances

Now we understand the nature of molecular substances, we need to be able to use these to explain a couple of characteristic properties:

1. Melting/Boiling Point
2. Solubility

Molecular Polarity

We can explain a lot of properties about molecules once we understand if they are polar or not. How do we work out if they are polar?

Polar molecules have a dipole due to asymmetric charge distribution. One side of the molecule is slightly negative and the other side is slightly positive. Polar molecules have higher melting/boiling points, and can dissolve ionic substances.

NB. This only works if there are more than two atoms in the molecule.

Shapes of Molecules

Once we have the Lewis Diagram for a molecule, we can infer its shape.

Regions of Electron Density: We talk about "regions of electron density when explaining shapes. These are:

1. Lone pairs of electrons around the central atom
2. Bonds between the central atom and other atoms.

There are some shapes and bond angles to remember. We used toothpicks to represent regions of electron density, and marshmallows to represent bonded atoms.

For two regions of electron density (eg carbons dioxide), there is only one shape (linear).

There are two factors that affect shape:

1. Regions of electron density (lone pairs and/or bonds) around the central atom
2. Number of atoms bound to the central atom

Lewis Diagrams

Lewis Dot Diagrams show us the bonding electrons in atoms and molecules. We use our knowledge of the Periodic Table to work out how many bonding (valence) electrons an atoms has.

Once we work out how to draw Lewis Diagrams for atoms, we try to put them together to represent the bonding in molecules.

Some molecules do not seem to work. This is when we need to move lone pairs of electrons between atoms to create double (or triple) bonds.

Molecular Substances - What are Molecules?

In this lesson, we looked at the make up of molecules, and how the atoms are bound together. We explored the covalent bond, and how the sharing of electrons is not always "fair". Electronegativity can be used to predict whether a covalent bond is polar (uneven sharing of bonding electrons) or non-polar (even sharing of bonding electrons).