M7 Question 1 Higher Explain why carbon dioxide is a compound by oxygen is an element Carbon dioxide contains two elements carbon and oxygen. Oxygen contains only one type of atom.
M7 Question 2 Higher A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells (highest occupied energy levels) of the atoms. An atom that shares one or more of its electrons will fill its outer shell. Covalent bonds are strong, and a lot of energy is needed to break them.
M7 Question 3 Higher Intermolecular forces All of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Because the weak intermolecular forces break down easily these substances have low melting and boiling points. This means simple molecular substances are gases, liquids or solids with low melting points, and low boiling points. Melting and boiling points The animation below shows how the weak intermolecular forces between water
M7 Question 4 Higher Ionic bonding When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic
M7 Question 5 Higher Physical properties of ionic compounds The ions in a compound such as sodium chloride are arranged in a lattice structure. This regular arrangement results in the formation of a crystal. The diagram shows part of the crystal lattice of sodium chloride: Positively charged Na ions, negatively charged Cl ions This pattern is repeated in all directions, giving a giant three-dimensional lattice structure in sodium chloride crystals. Because of the strong electrostatic forces between them, it takes a great deal of energy to separate the positive and negative ions in a crystal lattice. This means that ionic compounds have high melting points and boiling points. Solid ionic compounds do not conduct electricity, because the ions are held firmly in place. They cannot move to conduct the electric current. But when an ionic compound melts, the charged ions are free to move. Molten ionic compounds do conduct electricity. When a crystal of an ionic compound dissolves in water, the ions separate. Again, the ions are free to move, so a solution of an ionic compound in water also conducts electricity.
M7 Question 6 Higher You can use the charge on the ions shown in the table to work out the formulae of the ionic compounds. Charges must cancel out to form a neutral compound. Sodium ions each have a single positive charge. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt sodium chloride, they must be in a ration of 1:1. So the formula of sodium chloride is NaCl. Magnesium ions each have two positive charges. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt magnesium chloride, they must be in a ration of 1:2. So the formula of magnesium chloride is MgCl 2.
M7 Question 7 Higher We can explain the properties of metals by taking a more detailed look at their structure. Metal crystals are made up of positive metal ions surrounded by a sea of negative electrons.
M7 Question 8 Higher The strong electrostatic attraction between positive ions and negative electrons means that a lot of energy is needed to separate these particles from the crystal lattice. This means that metals are strong and have high melting and boiling points. Much less energy is needed to slide one layer of positive metal ions over another layer. This explains why some metals are malleable, meaning that they are easy to beat or press into shape.
M7 Question 9 Higher Silicon dioxide Much of the silicon and oxygen in the Earth's crust is present as the compound silicon dioxide also known as silica. Silicon dioxide has a giant covalent structure. Each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms. This means that, overall, the ratio is two oxygen atoms to each silicon atom, giving the formula SiO 2. Silicon dioxide is very hard. It has a very high melting point (1,610 C) and boiling point (2,230 C), is insoluble in water, and does not conduct electricity. These properties result from the very strong covalent bonds that hold the silicon and oxygen atoms in the giant covalent structure