A noble gas is any gas with a full outer shell of electrons (full valence shell), these gases are ‘noble’ since they will not readily undergo chemical bonding as their electron configuration makes them highly stable. The first noble gas is helium (He), which has two electrons in its outer shell. The next is neon (Ne) which has eight electrons in its outer shell. Since its got an atomic number of 10, we know it has 10 protons – since we also know that Ne is neutrally charged the number of protons must equal the number of electrons. Therefore, Ne has 10 electrons – there are 2 in the first shell and 8 in the second shell.
Atoms are stable when they have the full valence shell of electrons. Often chemical bonds are trying to establish this full valence shell of electrons.
There are two types of bonds we are going to start looking at. These are covalent bonds which occur between atoms when they ‘share’ electrons. For example, H2 has two hydrogen atoms – both of these hydrogen atoms have one electron, however, they both achieve the desired two electrons in their outer shell by sharing each of their electrons with the other. This would be like if I had a pen and you had a pen and we both decided to share our pens – we would both have two pens then. Ionic bonds occur between metals and non metals, this is when one gives up electron/s inorder to achieve a full valence shell and the other accepts an electron/s inorder to achieve its full valence shell. By giving up an electron, the number of protons exceed the number of electrons thereby making the element have a net positive charge – this will occur for the metal on the left hand side of the periodic table. Vice versa for the atom that accepts electrons – this occurs for the non metal and results in a net negative charge.
When an atom has a charge, we refer to it as an ion. We can classify ions as anions or cations, depending on whether they have a net positive charge or a net negative charge.
Negatively charged ion = anion
Positively charge ion = cation
To determine the net ionic charge of monatomic ions, count how many blocks to the closest nobel gas if it is a metal you will be counting backwards (cation so losing electrons) and if it is a non metal vice versa. This method is counting how many electrons it must lose or gain to gain a nobel gas configuration.
For example Oxygen is two blocks away from Neon – since it is a non metal it must gain these two electrons to achieve its most stable electronic configuration.
O + 2e– → O2-
So far we have looked at atoms which are ions, however, we can also have multiple atoms behaving like ions. These are known as polyatomic ions.
Ethanoate aka acetate CH3COO–
Hydrogen carbonate aka bicarbonate HCO3–
Hydrogen sulfate HSO4–
Dihydrogen phosphate H2PO4–
Hydrogen phosphate HPO42-