Chapter 2 - Atoms, Ions and Compounds.

• Every atom of the same element has the same number of protons, so the same atomic number. The number of protons defines the element.
• Different elements in the periodic table have a different number of protons, so a different atomic number.
• Isotopes have the same number of protons, so the same atomic number, but different numbers of neutrons and therefore different masses.
• Isotopes of the same element will have the same number of electrons in the outer shell. It is the valence shell electrons that determine the reactivity of an element.
• The number of neutrons has no effect on the reactivity, so different isotopes of the same element react in the same way.
• Isotopes of the same element may have small differences in physical properties. Heavier isotopes will be more dense. Melting and boiling temperatures may also be higher for higher mass isotopes
• Cations: Positively charged with fewer electrons than protons
• Anions: Negatively charged with more electrons than protons
• The mass of any atom is relative to the mass of the carbon-12 isotope, which has a mass of 12 atomic mass units (12u)
• Hydrogen has a mass one twelfth of carbon, so it is 1u.

  1u is approximately the mass of a proton or neutron.
• Relative atomic mass: The weighted mean mass of an atom relative to one-twelfth of the mass of an atom of carbon 12
• Determination of relative atomic mass

  1. A sample is placed in the mass spectrometer
  2. The sample is vaporised and then ionised to form positive ions
  3. The ions are accelerated – heavier ions move more slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated
  4. The ions are detected on a mass spectrum as a mass to charge ration m/z
  5. Each ion reaching the detector adds to the signal so the greater the abundance, the larger the signal
• Atoms gain or lose electrons to achieve the same electron structure as the nearest noble gas.
• A binary compound has 2 elements chemically joined together.
• Polyatomic ions: Ions that consist of more than one atom that are chemically bonded together
• Ionic Formula: Ions that consist of more than one atom that are chemically bonded together.
• In a correct formula:

  • The overall charge is zero
  • The sum of the positive charges = the sum of the negative charges