AS-Level Chemistry: Unit 1:The Atom

AS-Level Chemistry: Unit 1:The Atom

1. The Structure of an Atom

   • atoms form everything in our universe.
     • They consist of three particles: the proton, neutron and electron.
       • electrons have a -1 charge.
         • electrons 'whizz' around the nucleus of the atom in 'orbitals'.
         • their relative mass is: 1/2000
       • protons have a +1 charge
         • Relative mass: 1
       • neutrons don't have a charge!
         • Relative mass: 1

2. Nuclear Symbols

   • you can work out the no. of protons, neutrons and electrons from the nuclear symbol
   • mass number: the number of protons and neutrons in the atom's nucleus
   • atomic (proton) number: number of protons in the atom.
     • all atoms of the same element have the same proton number!

3. Atoms and Ions

   • neutral atoms (no charge) have the same electron and proton number.
   • the neutron number is just the mass no. minus the atomic no., i.e. "the top minus the bottom" (Nuclear Symbols)
   • Ions
     • Ions have different no. of protons and electrons.
     • Negative ions (e.g. Br-) have more electrons than protons.
     • Positive ions (e.g. Mg2+) have more protons than electrons.
     • Exam Tip: Ions
       • Ions are easy to spot: they always have a plus or minus charge (+ or -) next to them! If the charge is a plus, it means an electron has been lost, e.g. Mg2+ means that the Magnesium ion has lost two electrons. If there is a minus sign, it means electrons have been gained. For example, 3- means 3 electrons have been lost!

4. Isotopes

   • isotope isn't just a scary science word, nope! Isotopes of an element are atoms with the same proton no., but different neutron number.
     • basically atoms with the same no. of protons but different neutron no. are isotopes.
     • different isotopes of the same element react (chemically) the same way

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