1.16 calculate relative formula masses (Mr ) from relative atomic masses (Ar)

Relative formula masses

Relative formula mass is given the symbol, Mr.

To calculate the Mr, of a substance, all you have to do is add up the relative atomic masses of all the atoms present in the formula.

Water- H₂0 
Atoms present:(2 x H) + (1 x O) 
Mr= (2 x 1) + 16= 18

There is 1 electron per hydrogen atom, and 2 hydrogen atoms, so that is 2 electrons. There is also 1 oxygen atom, which has 16 electrons, 2 + 16= 18

Atomic Structure 1.12 calculate the relative atomic mass of an element from the relative abundances of its isotopes

Calculating the relative atomic mass

Example: 

Chlorine has two isotopes: chlorine-35 and chlorine-37 

A typical sample of chlorine will be 75% chlorine-35 atoms and 25% chlorine-37 atoms.

Total mass of 100 atoms= (75 x 35) + 5 x 37)= 3550

Mean mass of 1 atom= (3550 ÷ 100)= 35.5

Ar of chlorine is 35.5

Atomic Structure 1.9 understand that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells

Protons, neutrons and electrons


As you can see from the diagram, in the centre of the atom is the nucleus, which contains protons (positive charge) and neutrons (no charge). Orbiting the nucleus is a series of shells, which hold electrons (negative charge). The electrons are much smaller than the protons and neutrons. 
The first shell outside the nucleus can hold 2 electrons, the second can hold 8 and the third can hold 8.

Atoms 1.4 describe and explain experiments to investigate the small size of particles and their movement including: i dilution of coloured solutions ii diffusion experiments

Dilution of coloured solutions

When potassium manganate (VII) crystals are dissolved in water, a purple solution is formed. A very few tiny crystals can produce a highly intense colour. 
When this solution is diluted several times, the colour fades, but does not disappear until a lot of dilutions are made.
This indicated that there are a large number of particles of potassium manganate (VII) in a very small amount of solid. If this is true, then the particles of potassium manganate (VII) must be very tiny.

Diffusion

Particles will move to fill the space available to them. They can do this in both liquids and gases. 
An example is the diffusion of bromine from one flask to another. After five minutes the bromine gas has diffused into the left-hand flask. This happens because both air and bromine particles are moving randomly and there are large gaps between the particles. The particles can therefore easily mix together.