Atomic theory

2.1 - The Nuclear atom

Isotope -- atoms with same atomic number but different mass number (ie different numbers of neutrons)

• Atomic number (Z) = number of protons
• mass number (A) - atomic number (Z) = number of neutrons.

Isotopes may differ in physical properties (mass, density, rate of diffusion etc.) and radioactivity but not generally in chemical properties.

Relative atomic mass is the average of the atomic masses of each isotope (isotopic mass) multiplied by the isotope's relative abundance. This results in non-integral atomic masses. ^note*

Relative atomic mass calculation

2.2 - Mass spectrometer

This is an instrument used for measuring the exact masses of particles.

It can be used on elements to determine the isotopic abundances and very accurate mass measurements or it can be used, on molecules to find out the nature of the molecule by looking at the fragmentation pattern of its destructive ionisation.

It has several stages of operation that you must get famiiliar with (i.e. learn!):

1. injection of the sample
2. vaporisation of the sample (if it's not already gaseous)
3. ionisation of the sample
4. acceleration of the ions
5. deflection of the ions
6. detection of the ions

2.3 - Electron Arrangement

Spectral types:

1. Continuous emission
2. Continuous absorption
3. Line emission
4. Line absorption

Continuous spectra show broad bands either of electromagnetic radiation (emission) or shadow (absorption). A line spectrum contains only some discrete lines of electromagnetic radiation (emission), or shadows (black lines superimposed on a continuous spectrum).

Most important - line emission spectra which are produced by excited atoms (heated) promoting electrons to higher energy levels which then emit discrete frequencies of energy when they return to lower states.

The relationship between energy and frequency of electromagnetic radiation is given by E= hv

where:

• E is the energy of the radiation
• h is Plancks constant
• v is the frequency of the radiation

The main electron levels go : 2, 8, 18

After each shell is filled, move to the next...2, 8, 18...(standard level only up to Z = 20 is required)

Example: Sodium (2,8,1) , Potassium (2,8,8,1) etc.

Resources

Line spectra

Useful links

Notes:

1. The atomic mass scale is based on the mass of one atom of the carbon 12 isotope (6 protons and 6 neutrons) being equal to exactly 12 units - all other masses are compared to this. For example the relative atomic mass of helium is 4 which has been calculated from the fact that it is one third as heavy as carbon 12.