AQA AS Chemistry Unit 1 & 2 Notes

UNIT 1

Proton: Relative mass: 1 Relative charge: +1

Neutron: Relative mass: 1 Relative charge: 0

Electron: Relative mass: 1/1840 Relative charge: -1

ATOMIC NUMBER-number of protons in the nucleus of the atom.

MASS NUMBER-total number of protons and neutrons in nucleus of an atom.

Number of neutrons = mass number - atomic number.

Number of electrons = number of protons.

ISOTOPES-atoms of the same element with different mass numbers. Properties: same chemical properties, different physical properties, different rates of diffusion, radioactivity. Different numbers of neutrons but the same number of protons.

Mass spectrometer: sorts atoms according to their mass.

1. IONISATION-a vapourised sample is put into the vacuum chamber. An electron gun bombards the sample with electrons. Electrons are knocked from sample particles, producing ions with different positive charges. Mg(g)-->M+(g) + e-. Some particles are broken into fragments, some fragments are positively ionised.

2. ACCELERATION-negatively charged accelerating plates produce an electric field which accelerates the positive ions towards the electromagnet.

3. DEFLECTION-the electromagnet produces a strong, variable magnetic field that deflects the ions from a straight path. The extent of deflection is determined by the mass and charge on the ions. The greater the mass, the less deflection and greater the charge, the more deflection. The larger the m/z charge ratio (mass: charge) the less deflection.

4. DETECTION-when a positive ion reaches the detector, it produces a tiny current. The signal is fed to a recorder that count the number of signals for each setting of the magnetic field. All the counts together produce a mass spectrum.

Calculating relative atomic mass from its isotopic abundance:

24Mg, 25Mg, 26Mg, 78.60 : 10.11 :  11.29 = 24.31.

RELATIVE ATOMIC MASS-the average mass of an atom of an element compared to 1/12 the mass of an atom of carbon 12.

RELATIVE MOLECULAR MASS-average mass of a molecule compared to 1/12 the mass of an atom of carbon-12.

Describing electrons: electrons are arranged in electron shells around the nucleus. Electrons can be described as being in a particular energy level. Within each energy level there are sub-levels/sub-shells-s,p,d,f.

Hund's rule: electrons organise themselves so that as far as possible they remain unpaired, occupying the maximum number of sub-orbitals possible.

IONISATION ENERGY-energy required to remove 1 mole of electrons from one mole of gaseous atoms to form 1 mole of gaseous +1 cations.

FIRST IONISATION ENERGY-the enthalpy change when 1 mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.

First ionisation energies of Period 2 elements-general increase. Boron shows a dip in ionisation energy because it is the first to enter the p orbital. It is higher in ionisation energy than the 2s orbital, so it is easier to remove. Oxygen then shows a dip because the new electron enters a p orbital with an electron already in it. There is mutual repulsion between the two electrons, so it takes less energy to remove the unpaired electrons. This is similar in other periods.

First ionisation energies of Group 2 elements-decreases…