Molecules, structures, melting & boiling points.

Andrew LawsonChemistry, UncategorizedLeave a Comment

This is a brief summary of the different types of structures you encounter in chemistry, from molecules to all the types of crystal lattices. It gives a description of the forces of attraction holding them together – either chemical bonds or intermolecular forces, and how this affects their melting points and boiling points. A description of the bonding or intermolecular forces is also provided.

Atom – single particle of an element, neutral in terms of overall charge, proton number determines the element. Different neutrons lead to different isotopes.
Always gaseous as they are individual atoms, so no melting/boiling points.

Molecule – made up of a finite number of atoms. Atoms are covalently bonded to each other. Overall neutral charge.
Intermolecular forces hold them together. Overcome these for the melting/boiling points.

Ion – charged atom (ion), or charged molecule (molecular ion or polyatomic ion). Negative ions tend to be non metals (Cl -, O 2-), or polyatomic ions (non metal atoms covalently bonded to each other like NO3 -, CO3 2-, SO4 2-, HCO3 – etc. or organic ones like CH3COO-). Positive ions are metals (Na +, Mg 2+) or can occasionally be polyatomic ions (NH4 +).
Individual ions will be gaseous as they are on their own, so no melting/boiling points.

Ionic compounds are made from any positive ion and negative ion, ionically bonded. They form a massive crystal lattice of positive and negative ions, so not a molecule as they aren’t of a fixed size, but a lattice which can be small or MASSIVE. Ionic compounds are also referred to as salts. A salt is a compound consisting of positive and negative ions, i.e. an ionic compound.
Large melting/boiling points as you have to overcome the strong ionic bonds, i.e. the strong electrostatic forces of attraction.

Molecules that form large scale solids form a crystal lattice where the individual molecules are held together by intermolecular forces. When the temperature gets too big, they form a liquid, still held together by intermolecular forces. An even higher temperature will overcome these forces to form a gas, where they are now free molecules, with the intermolecular forces pretty much zero.
Intermolecular forces hold them together. Overcome these for the melting/boiling points. These intermolecular forces tend to be lower
– van der waals (or induced dipoles) the weakest
– permanent dipole dipoles (between two atoms with different electronegativities that form a polar bond, i.e. one with two poles, one d+ and one d-)
– and then the strongest H-bond (between the positive bit [a H atom attached to N, O or F] and the negative bit [a lone pair of electrons] )

Some non metal atoms (like carbon) and combination of non metal atoms (like silicon and oxygen in SiO2) can form giant covalent lattices where every atom is bonded to other atoms in a huge structure. Like SiO2, or graphite, graphene or diamond. These are held together by covalent bonds, not intermolecular forces.
Covalent bonds are very strong (the strongest of the 3 types – ionic, metallic and covalent) and as there are many strong covalent bonds to each atom, and all must be overcome they have high melting/boiling points.

All metals can form giant lattice structures, known as a metallic structure. Here every positive metal ion is held together by a huge sea of delocalised electrons.
Overcoming the attraction between the positive ions and sea of delocalised electrons requires a lot of energy, hence high melting/boiling points.

Polymers are where individual molecules chain together to form large chains. The forces keeping the individual chains in one piece are covalent bonds. The chains are held next to other chains by intermolecular forces – van der waals between C-C and C-H, dipole dipole if there are polar bonds (such as C-Cl or C=O), or the all-conquering hydrogen bonds (between the positive bit [a H atom attached to N, O or F] and the negative bit [a lone pair of electrons].
To overcome the forces between the chains, you need to overcome the intermolecular forces only. Breaking individual chains up requires a lot more energy and hence MUCH higher temperatures.

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