Bonding & structure flashcards

Key points

• Electrons transfer: the metal atom loses its outer electrons and the non-metal atom gains them.
• The metal forms a positive ion and the non-metal a negative ion, each with a full outer shell.
• An ionic bond is the strong electrostatic attraction between oppositely charged ions.
• Dot-and-cross diagrams show the transfer: the ions in square brackets, the charge outside.

Sodium and chlorine

Key points

• Group 1 metals form 1+ ions, Group 2 form 2+, Group 6 non-metals form 2− and Group 7 form 1−.
• Use enough of each ion for the charges to cancel out to zero.
• Write the metal first, with the number of each ion as a small number after it.

Magnesium chloride

Key points

• It is a giant lattice: a regular, repeating arrangement of positive and negative ions.
• Strong electrostatic forces act between the ions in all directions.
• Melting means overcoming a great many of these forces, which takes a lot of energy, so the melting point is high.
• There are no molecules in sodium chloride, only ions.

Key points

• A covalent bond is a shared pair of electrons between two non-metal atoms.
• Examples of simple molecules: H₂, Cl₂, O₂, HCl, H₂O, NH₃ and CH₄.
• The bonds inside each molecule are strong, but the intermolecular forces between molecules are weak.
• Melting and boiling only overcome those weak forces, so it takes little energy. Bigger molecules have stronger forces.
• They do not conduct: the molecules have no overall charge.

Key points

• Both are giant covalent structures of carbon: allotropes, with very high melting points.
• Diamond: each carbon forms four strong covalent bonds, so it is very hard.
• Graphite: each carbon forms three bonds, in layers of hexagons with no covalent bonds between the layers.
• The layers slide over each other, so graphite is soft and slippery.
• Silicon dioxide (sand) is another giant covalent structure.

Key points

• Graphene is a single layer of graphite, one atom thick. It is very strong and conducts electricity.
• It is useful in electronics and in composite materials.
• Fullerenes are carbon molecules with hollow shapes, built from rings of carbon atoms. The first found was C₆₀.
• Carbon nanotubes are cylindrical fullerenes: very long and thin, strong, and good conductors.

Key points

• A polymer is a very large molecule made of many small repeating units, called monomers.
• The atoms are joined by strong covalent bonds.
• The molecules are so long that the forces between them are fairly strong.
• So most polymers are solids at room temperature, unlike small simple molecules.

Key points

• A metal is a giant structure of positive ions in a regular arrangement.
• Their outer electrons are delocalised: a sea of electrons free to move through the whole structure.
• Metallic bonding is the strong attraction between the positive ions and the delocalised electrons, so most metals have high melting points.
• The layers of ions can slide over each other, so pure metals can be bent and shaped.

Key points

• An alloy is a mixture of a metal with other elements, such as steel (iron with carbon).
• Pure metals are often too soft to use.
• Atoms of a different size distort the layers in the structure.
• So the layers cannot slide over each other as easily, and the alloy is harder.

Key points

• Solid: close together in a regular pattern, vibrating about fixed positions.
• Liquid: close together but random, moving past each other.
• Gas: far apart and random, moving quickly in all directions.
• State symbols: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water.

Key points

• Heating gives the particles energy to overcome the forces between them: melting, then boiling.
• Cooling reverses it: condensing, then freezing.
• The stronger the forces between the particles, the higher the melting and boiling points.
• Below the melting point it is solid, between the two it is liquid, above the boiling point it is gas.

Melts at −7 °C, boils at 59 °C

Key points

• A current needs charged particles that are free to move.
• Metals conduct as solids and liquids: delocalised electrons carry the charge. They conduct heat this way too.
• Graphite conducts: each carbon has one delocalised electron. Diamond does not: all four are used in bonds.
• Ionic compounds conduct only when molten or dissolved, when the ions are free to move.
• Simple molecules do not conduct: they have no charged particles.

Key points

• Low melting point, no conduction: simple molecules.
• High melting point, conducts only when molten or dissolved: ionic.
• Very high melting point, does not conduct: giant covalent, such as diamond or silicon dioxide.
• High melting point, conducts as a solid, can be bent: metallic.

Key points

• Nanoparticles are 1 to 100 nanometres (nm) across, only a few hundred atoms. 1 nm = 1 × 10⁻⁹ m.
• They have a very high surface area to volume ratio.
• When the side of a cube is divided by 10, its surface area to volume ratio is multiplied by 10.
• So they can behave differently from the same material in bulk.

Cubes with sides of 10 nm and 1 nm

Key points

• Catalysts: a large surface area for a small mass, so less material is needed.
• Sun creams: better protection from ultraviolet light and better coverage of the skin.
• Medicine, electronics, cosmetics and deodorants use them too.
• Their effects on health and the environment are not fully known: they could get into the body and harm cells.