Structure and Properties of Matter
How atoms combine, and what a molecule weighs
Use the laws of conservation of mass and of constant proportions, tell a covalent bond from an ionic one, write a formula by crossing over valencies, and add up a molecular or formula unit mass.
Two laws sit under the whole of this chapter. Lavoisier weighed a reaction before and after and found the mass unchanged: matter is neither created nor destroyed. Proust analyzed water from rivers, borewells and the sea and found hydrogen and oxygen in the same 1 to 8 ratio by mass each time: a compound's elements combine in fixed proportions, whatever its source.
Two ways for atoms to bond
An atom short of a full outer shell can share electrons or hand them over. Sharing a pair makes a covalent bond, as in a hydrogen molecule, and gives you molecules. Handing electrons over makes an ionic bond: the giver becomes a positive cation, the taker a negative anion, and what you get is a crystal rather than molecules — which is why an ionic compound is described by a formula unit and not a molecule.
Have a play
A sodium atom beside a chlorine atom, with each shell named.
Tap any part of the picture.
Worked example
9 g of purified water is decomposed completely. Hydrogen and oxygen come off in the mass ratio 1 to 8. How much of each is produced, and would water from a different river give anything else?
Add the parts of the ratio: 1 + 8 = 9 parts altogether, and the sample happens to be 9 g, so one part is 1 g.
A mass ratio splits a total into that many parts. Finding what one part weighs is the whole method.
Try it together
Now weigh a molecule by adding up its atoms.
Water is H2O. Take hydrogen's atomic mass as 1 u and oxygen's as 16 u. Answer with a bare number each time.
1.There are 2 hydrogen atoms, each of atomic mass 1 u. What do they contribute, in u?
Have a go
Have a go on your own. Magnesium hydroxide is Mg(OH)2. Taking Mg as 24 u, O as 16 u and H as 1 u, what is its formula unit mass, in u? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.