Topic 2 - Bonding and structure

• A metallic bond is a strong electrostatic attraction between the nuclei of the cations (positive metal ions) and the sea of delocalised electrons that surround them. These forces of attraction are oppositely charged, so form extremely strong bonds.
• Metals are structured as a lattice of positively charged metal ions (cations) surrounded by a sea of delocalised electrons. A greater charge on the positive ions increases the attractive force, since more electrons are delocalised in the sea. Ions that have a greater atomic radius produce weaker attractive forces.
• Delocalised electrons are electrons that aren't contained within a single atom or covalent bond.
• Metals have four key properties : high melting point, good electrical conductivity, good thermal conductivity and good malleability/ductility.
• Metals have a high melting point, so are typically solid at room temperature. They have this property due to the strong electrostatic forces of attraction between positive ions and delocalised electrons in giant lattice structures, which require lots of energy to overcome.
• If a metal has more delocalised electrons, the electrostatic forces of attraction will be stronger, resulting in a higher melting point. If a metal ion is smaller (i.e. has a smaller atomic radius), then the outer shell electrons are closer to the nucleus. This creates stronger electrostatic forces of attraction, again creating a higher melting point.
• Metals have good electrical conductivity when a potential difference is applied. This is because when a potential difference is applied, delocalised electrons move to the positive terminal of the cell. The electrons move and carry their charge as they do this, creating an electrical current.
• Metals have good thermal conductivity. This is because free moving delocalised electrons allow metals to pass kinetic energy. Also, the cations are closely packed together, so pass kinetic energy between them.
• Metals have good malleability and ductility. This is because the layers of cations can slide over each other when stress is applied, making them malleable. However, the layers are held together by the sea of delocalised electrons, making metals both ductile and malleable.
• Malleable means the metal can be shaped. Ductile means the metal can be drawn into a wire.
• An ionic bond is a strong electrostatic attraction between two oppositely charged ions (often a metal and non-metal).
• In an ionic bond, electrons are transferred from the metal to the non-metal to form full outer shells.
• The strength of an ionic bond depends on the relative sizes and charges of the ions involved.
• Ionic bonding occurs within an ionic lattice, with strong electrostatic attraction between oppositely charged ions.
• In an ionic lattice, the ions are arranged so that the electrostatic attraction between oppositely charged ions outweighs the electrostatic repulsions between ions of the same charge.
• The electrostatic interaction between ions in an ionic lattice is not directional.
• Electrolysis is able to provide evidence for the existence of ions. For example, ionic compounds can conduct electricity and undergo electrolysis when in a molten or aqueous state.
• When an ionic compound undergoes electrolysis, the positive metal ions (cations) will migrate towards the negative electrode (cathode) and gain electrons to become metal atoms.
• When an ionic compound undergoes electrolysis, the negative non-metal ions (anions) will migrate towards the positive electrode (anode) and lose electrons to become non-metal atoms.
• Ions in an ionic compound can migrate to their respective electrodes in electrolysis because the ions are separate and are no longer held in a lattice structure, so the ions are free to move and carry their charge as an electrical current.
• You can measure the strength of an ionic bond by calculating the amount of energy required in one mole of solid to separate the ions to infinity (i.e. into a gaseous state). The units for this is kJ/mol^-1.
• As the charge on the ion increases, the ionic bond strength also increases. This is because ions with a greater charge will have a greater attraction to the other ions, resulting in stronger forces of attraction and hence stronger ionic bonding.
• As the ionic radius increases, the ionic bond strength decreases. This is because ions with a greater ionic radius (i.e. bigger ions) will have weaker forces of attraction to oppositely charged ions because the attractive forces have to act over a greater distance.
• As you go down each group in the periodic table, the ions have more electron shells, so the ions get larger.
• As you go across a period (left to right) the number of protons in the atoms of an element increases.
• Increasing protons increases the positive charge of the nucleus, so the electrons are attracted more strongly and move closer to the nucleus.
• As the number of protons (nuclear charge) increases, atomic radius decreases.
• Some of the physical properties of ionic materials include : brittleness, high melting temperatures, good solubility and good electrical conductivity in a molten/aqueous state.
• Ionic substances are very brittle. This is because when stress is applied to a crystal of an ionic solid, the layers of ions may slide over each other. When these layers of alternating charge are distorted, like charges repel and cause the lattice to break apart into fragments.
• Ionic substances have very high melting temperatures because ionic solids are made of a giant lattice network of oppositely charged ions, and the sheer quantity of ions produces a huge combined electrostatic force of attraction. As a result, a large amount of energy is required to overcome the forces of attraction for the ions to break free.
• Most ionic compounds are water soluble because the energy required to break apart the lattice structure can sometimes be supplied by the hydration of the separate ions produced.
• Ionic compounds are mostly water soluble because the positive and negative ions in the lattice are attracted to the water molecules due to their polarity. As a result, the oxygen end of the water molecules is attracted to the positive ions, and the hydrogen end of the water molecules is attracted to the negative ions.
• Ionic compounds tend to be very good electrical conductors when in a molten or aqueous state. This is because the ions are mobile and able to migrate to the oppositely charged electrodes when a potential difference is applied.
• Generally, solid ionic compounds can't conduct electricity because there are no delocalised electrons and ions can't move freely under an applied potential difference.
• Aqueous solutions of ionic compounds can also conduct electricity and undergo electrolysis, since the lattice is broken down when the compound dissolves.
• A covalent bond is formed between two atoms when two atomic orbitals overlap and share electrons between them. It is a strong electrostatic attraction between two nuclei and the shared pair of electrons between them.
• Covalent bonding occurs between two non-metals.
• There are two different types of covalent bond : sigma bonds and pi bonds.
• In a sigma bond, the ends of two s-orbitals or p-orbitals overlap, and a single covalent bond is formed between two atoms.
• Another type of sigma bond is when an overlap between two p-orbitals occurs.