Transcript Slide 1
Why do atoms bond? They want to have a full outer electron shell. This is why oxygen that we breathe in is O2, chlorine gas is Cl2 etc… What type of bond? METAL NON-METAL METAL NON-METAL metallic ionic ionic covalent What are the properties of the different types of chemical bond? IONIC Type of structure Are ions present? Are delocalised electrons present? How strong is the chemical bond? High/Low melting point? Conductor of electricity? METALLIC COVALENT COVALENT What are the properties of the different types of chemical bond? IONIC METALLIC COVALENT COVALENT Type of structure giant giant Giant Simple molecules Are ions present? yes yes no no Are delocalised electrons present? no yes no (except no very strong in graphite) How strong is the chemical bond? strong strong very strong High/Low melting point? high high low very high Conductor of electricity? when molten/ in solution yes no (except no graphite) What is an ionic bond? Li Li What’s missing? Giant Ionic Structures Oppositely charged ions are attracted to each other. This attraction forms a strong ionic bond. The charge on a ion acts in all directions. The ions arrange themselves into a lattice structure, involving huge numbers of ions, which is why they are named giant structures. When an ionic substance dissolves, the ions break out of their lattice structure and are free to move about and carry a charge. Because of this, an electric current can be passed through the solution. The same is true when ionic substances are molten. When atoms share electrons, they are held together very tightly. This is a covalent bond. Example – Hydrogen: x H H Hydrogen atoms H x H Hydrogen molecule H2 H H C H H You are very likely to get a question about why giant covalent substances (like diamond or silicon dioxide – sand) have very high melting points. Lots (thousands, millions!) of atoms joined together by covalent bonds • very strong bonds • lots of bonds to be broken • need a huge amount of energy/heat to break the bonds • therefore a very high melting point Simple covalent substances (molecules) Molecules: a small number of atoms covalently bonded. They have very different properties to giant covalent substances. Simple molecules all have low melting and boiling points. Why? The covalent bonds are very strong, so the atoms are held together tightly. H H H x H But, the individual molecules are separate from each other. There is a force of attraction between individual molecules (called the intermolecular force). This is weak, and only a small amount of energy is needed to overcome it. Cl Cl STRONG covalent bond WEAK intermolecular force When a molecular substance melts/boils, it is the intermolecular forces that are overcome. The covalent bonds are not broken. Finally, metallic bonds • outer shell electrons delocalise (come away from the atom) and are free to move • forming a ‘sea of electrons’ and leaving positive metal ions • the attraction between negative electrons and the positive ions holds the metal together – the electrons act like glue • the metal ions are held tightly in neat rows that can slide over each other