2. Atoms, Ions and Compounds

• Atom - consists of a nucleus with protons and neutrons, and shells surrounding the nucleus containing electrons, overall charge is zero
• Proton - mass similar to neutron and positively charged, contained within nucleus
• Electron - mass of 1/1836 of the mass of a proton, negatively charged and contained within shells surrounding the nucleus
• Neutron - mass slightly greater than proton, no charge, contained within nucleus
• neutrons hold nucleus together despite repulsion from protons within; as the nucleus gets larger, more neutrons are required
• atomic number - the number of protons in the nucleus of the element
• isotope - atom with the same number of protons (atomic number) but different number of neutrons (mass number)
• mass number (A) = number of protons + number of neutrons
• atomic number (z) = number of protons
• different isotopes react in the same way as neutrons have no effect on the reactions, physical properties may differ slightly but not chemical properties
• ion - charged atom where number of electrons differs from number of protons

  • cations are positive as they have fewer electrons than protons
  • anions are negative as they have more electrons than protons
• relative mass - the mass of an atom compared to Carbon-12, measured in atomic mass units (u)
• relative isotopic mass - the mass of an isotope relative to 1/12th of the mass of an atom of carbon-12
• relative atomic mass - the weighted mean mass of an atom relative to 1/12th of the mass of an atom of C-12
• Mass spectrometry is used to find the percentage abundance of isotopes in a sample of an element
• Mass spectrometer:

  1. sample placed in MS
  2. sample is vaporised and ionised to form positive ions
  3. ions are accelerated; heavier ions move more slowly and are more difficult to deflect than lighter ions
  4. ions are detected on a mass spectrum as a mass-to-charge ratio (m/z)
• Atoms on the left of the periodic table lose electrons to form cations
• atoms on the right of the periodic table gain electrons to form anions
• binary compound - contains two elements only
• Polyatomic ions