Chemistry topic 2

• All the chemical elements are arranged in the periodic table in horizontal rows (periods) in order of increasing atomic number and also in vertical columns (groups).
• The strongest oxidising agents are at the bottom of the left-hand column of the elecrochemical series.
• The strongest reducing agents are at the top of the right-hand column of the electrochemical series.
• Elements in the same group have similar reactivities resulting from a common number of outer electrons.
• Within the first 20 elements there are various different types of bonding displayed, moving from metallic to non-metallic across the periodic table.
• Metallic bonding occurs between the atoms of metal elements - Lithium, Beryllium, Sodium, Magnesium, Aluminium and Calcium.
• The outer electrons in metallic bonding are delocalised (free to move).
• This produces an electrostatic force of attraction between the positive metal ions and the negative delocalised electrons.
• Metallic elements are able to conduct electricity due to the delocalised 'sea of electrons'.
• Discrete covalent molecules are small groups of atoms held together by strong covalent bonds inside the molecule and weak intermolcular forces between the molecules.
• The covalent bond itself is a shared pair of electrons electrostatically attracted to the positive nuclei of two non-metal atoms.
• The atoms achieve a stable outer electron arrangement (a noble gas arrangement) by sharing electrons.
• Most of the discrete covalent molecules are diatomic elements: Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2).
• There are also some larger covalent molecular elements: Phosphorous (P4), Sulfur (S8), Fullerenes (C60).
• Covalent networks are large, rigid three-dimensional arrangements of atoms held together by strong covalent bonds.
• Covalent networks have high melting points because they only contain strong bonds.
• Boron, Carbon and Silicon all form covalent networks.
• Examples of covalent networks include carbon in the forms of diamond and graphite.
• They are stable atoms.
• Group 0 elements (the noble gases) including Helium, Neon and Argon, exist as single, unattached particles.
• Group 0 elements do not usually form molecules with other atoms.
• Group 0 elements have low melting and boiling points as they are easily separated by overcoming the weak forces of attraction between the atoms.
• Covalent radius shows different trends if you are moving across a period or down a group.
• The covalent radius (a measure of how large individual atoms are) shows different trends if you are moving across a period or down a group.
• Across a period from left to right, the covalent radius decreases.
• As you move from left to right across the periodic table, atoms have more electrons in their outer energy level and more protons in their nucleus.
• The greater attraction between the increased number of protons (increased nuclear charge) and electrons, pulls the electrons closer together, hence the smaller size.
• As you move down a group in the periodic table, the covalent radius increases.
• Atoms increase in size as you move down a group due to the screening effect of the filled inner electron levels.
• The first ionisation energy is the energy involved in removing one mole of electrons from one mole of atoms in the gaseous state.
• The first ionisation energy of magnesium is 785 kJ mol-1.
• London dispersion forces are caused by an uneven distribution of electrons within an atom.
• London dispersion forces are the electrostatic attractions set up between the slightly positive end of one atom/molecule and the slightly negative end of one atom/molecule.
• The strength of London dispersion forces depends on the size of the molecule or atom.
• Larger atoms and molecules have more electrons, leading to larger dipoles being established.
• London dispersion forces increase the larger the atomic size.
• Permanent dipole-permanent dipole interactions occur between molecules with a permanent dipole, which are polar molecules.
• Hydrogen bonding is the strongest type of intermolecular bond and occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.
• Hydrogen bonding is a specific type of permanent dipole to permanent dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.
• In the diagram below, the hydrogen bonds are shown as the + hydrogen atoms of one molecule are attracted to the - oxygen atoms of another.