5.2 sheet a Bonding, structure and properties Covalent Bonds Form when non-metals share electrons between them. Attraction between each atom and the shared electron pair. Atoms share electrons to complete their outer shells One bond is formed for each electron that was needed. Bonding represented with dot-and-cross diagrams showing only the outer-shell electrons. Forming Ions Cations are positive (cat…pussitive!) ions They are formed when atoms lose electrons. Metals form cations by losing the electrons in their outer shells In the example, aluminium loses its three outer-shell electrons to become Al3+…each lost electrons cause 1 ‘+’ charge. Anions are negative ions They are formed when atoms gain electrons. Non-metals form anions by filling their outer shells. Name ends with ‘-ide’ to show it is a negative ion, In the example, oxygen gains two outer-shell electrons to become O2-, giving it 8 electrons in its outer shell. 3+ Making Ionic Compounds An ionic bond is the attraction between a positive and a negative ion. The overall number of positive and negative charges must cancel out. Ionic bonds form between a metal and a non-metal Ionic compounds do not form molecules Example 1: Magnesium reacting with chlorine. Anion: Cl forms Cl- ions Cation: Mg forms Mg2+ ions Formula = MgCl2 Why: two Cl- gives a 2- charge to balance 2+ from Mg2+. Name: magnesium chloride Example 2: aluminium reacting with oxygen. Anion: O forms O2- ions Cation: Al forms Al3+ ions Formula = Al2O3 Why: Two Al3+ gives a 6+ charge, three O2- gives a 6- charge. Name: aluminium oxide Example 1: Water Each hydrogen needs one more electron to complete it’s outer shell and the oxygen needs two more. Oxygen forms two single bonds: one to each hydrogen. Example 2: Carbon dioxide Carbon needs two more electrons to complete it’s outer shell and each oxygen needs two more. Carbon forms two double bonds: one to each oxygen. H O O C Covalent structures Simple Covalent Molecules Molecule = A particle made of a small group of atoms, covalently bonded together. Low melting and boiling point, due to weak attractive forces between molecules.. Electrical insulator as no electrons free to move. Examples: water, ammonia, oxygen 5.2 sheet a Bonding, structure and properties Ionic Structures A repeating 3D lattice of positive and negative ions. Strong electrostatic bonds between ions. Diamond vs Graphite These are two examples of giant covalent compounds. They are carbon allotropes. Displayed formulas This format shows the covalent bonds as lines. Double bonds are shown as two parallel lines. Properties of Ionic Compounds Melting point: High due to strong bonds between ions. Boiling point: Higher, due to strong bond between ions. Solid: do not conduct electricity Molten (liquid): do conduct electricity Dissolved (aqueous): do conduct electricity Why? Electrical Conductivity Electricity is conducted when there are charged particles that are free to move. Solid: there are charged particles (the ions), but they are not free to move, so they do not conduct. Liquid/Aqueous: the ions are now free to move, so they do conduct High Melting/Boiling Points Ionic bonds (attraction between positive and negative ions) are very strong. Melting and boiling require these bonds to be broken. This takes lots of (heat) energy. Diamond: Very hard, as all carbon atoms joined with 4 strong covalent bonds. Used to make cutting tools Insulator as all electrons locked-tight in bonds, so can’t move. Graphite: 3 covalent bonds per carbon atom Layers of hexagonal carbon mesh that rub away from each other, as there are only weak forces between the layers. Used as a lubricant. Conductor as the electrons between the layers are free to move. This is very rare for a giant covalent structure. Metallic Bonding For alloys, see sheet 5.10a Electrons are delocalised, moving freely between all the atoms creating a ‘sea of electrons’ All atoms have a positive charge as their outer-shell electrons have left them. The bond is the attraction between the positive ions and the sea of electrons. Conduct electricity as electrons are free to move. Malleable (change shape but don’t shatter when hit) because rows of atoms slide past each other when hit

5.2 sheet b Types of matter States of matter State symbols Solids Gas Material comes in three different forms – solid, liquid and gas. These are the three states of matter. Which state a material is in depends on how strong the force of attraction is between the particles (atoms, ions or molecules). The strength of the force is determined by: The material The temperature The pressure We show these states of matter in particle diagrams (see the diagram below). State symbols Solid - s Liquid – l Gas – g Aqueous - aq Solids Strong forces of attraction Regular lattice arrangement Keep a definite shape and volume Do not flow like liquids Particles vibrate around their fixed positions. Vibrate more when the temperature increases. This causes the solid to expand slightly. 5.2 sheet b Types of matter Gas Force of attraction between the particles is very weak Particles are free to move constantly with a random motion Particles travel in straight lines until they collide with other particles or walls Particles far apart Most gas is empty space Don’t keep a definite shape or volume Will fill any container The hotter a gas gets, the faster the particles will move. The more the particles move, the more they collide with each other and the container. This causes the pressure to increase or, if the container isn’t sealed, the volume to increase. Liquids Weak forces of attraction between the particles The particles are randomly arranged and are free to move past each other Particle tend to stick closely together but move around Have a definite volume Do not keep a definite shape Will flow to fill the bottom of a container Nanoparticles These are tiny particles often measured in nanometres (nm) or micrometres (μm). Particles are put into three categories depending on their diameter: Coarse particles – diameters between 250nm and 10,000nm. Also referred to as dust or PM10 (Particulate matter up to 10 micrometres in diameter). Fine particles – diameters between 100nm and 2500nm. Also known as PM2.5. Nanoparticles – diameters between 1nm and 100nm. Nanoparticles contain only a few hundred atoms. Nanoparticles have a huge surface area to volume ratio. Surface area to volume ration can be calculated using the following: Surface areas to volume ratio = surface area  ÷ volume Nanoscience (the study of nanoparticles) may lead to the development of: new catalysts new coatings new computers stronger and lighter building materials sensors that detect individual substances in tiny amounts. Changes of state