Ions, Bonds, and Macromolecules: Chemistry Essentials
(a) Ions and Ionic Bonds
Ion: An ion is a charged atom (i.e., the number of protons does not equal the number of electrons). Ions form by losing an electron (oxidation), which increases the charge by 1, or by gaining one (reduction), which decreases the charge by 1. Remember OIL RIG: oxidation is loss, reduction is gain. A positively charged atom is called a cation, and it is usually a metal. A negatively charged atom is called an anion, and it is a non-metal.
Ionic bond: An ionic bond is the bond formed between ions of opposite charge.
Group 1 metals react with halogens. In the reaction, the metal gives one electron to the halogen, as shown below (NOTE the way the ions are represented using brackets and + and – signs).
In a reaction, a metal gives a non-metal its valency electrons.
An ionic compound (in solid state) has a regular arrangement (lattice) of alternating positive and negative ions.
(b) Molecules and Covalent Bonds
Single covalent bond: A bond where 2 non-metals share a pair of electrons to get full-outer shells, as seen in H2, Cl2, H2O, CH4, and HCl, represented by a line connecting the two symbols, e.g., H-H.
Important things to note: Covalent bonds are weak, and ionic bonds are strong. When you melt an ionic solid, you break up the compound into ions, so it takes more energy; therefore, ionic compounds have higher melting points. When you melt a covalent solid, the molecules are broken up from each other but are still bonded (e.g., when you melt ice, you get H2O molecules but in a liquid, but melting NaCl gives you a molten mixture of Na+ and Cl– ions).
(c) Macromolecules
Diamond: Diamond has four bonds, a high melting point, does not conduct electricity, and is very hard. It is used for cutting because it is the hardest known substance (2 left pictures below).
Graphite: Graphite has three bonds and is made of flat sheets that are held together by weak forces, so it is soft and slippery, and is used as a lubricant. It can conduct electricity because it has one free electron. It is slippery because the sheets have weak bonds between them (middle picture).
Silicon (IV) oxide/silicon dioxide/silica: Silicon dioxide makes up most of sand. Each Si atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. As a result, it has a high melting point and is hard, like diamond (right picture).
Silicon IV oxide and diamond both have high melting points and are very hard substances.
General Properties of Giant Molecules
MELTING POINT – Very high since the structure is made up of a large number of covalent bonds, all of which need to be broken if atoms are to be separated.
ELECTRICAL – Does not conduct electricity – has no mobile ions or electrons, BUT… Graphite conducts electricity.
STRENGTH – Hard – exists in a rigid tetrahedral structure, e.g., Diamond and silica (SiO2)… but Graphite is soft.
(d) Metallic Bonding
Metallic bonding: a lattice of tightly packed positive ions in a sea of electrons, resulting in crystals, therefore:
- Metals are malleable and ductile – the layers of ions can slide over each other.
- Metals are good conductors – free electrons take energy.