In this section we’re going to focus on the classification of elements in the periodic table.
It might be handy to have a periodic table to look at whilst you’re wrapping your head around this.
The periodic table arranges chemical elements by their atomic number, electron configurations and recurring chemical properties. This ordering allows us to identify periodic trends easily as all elements with similar behaviours are found in the same row.
Within the table there are groups which are in columns and periods which are in rows. Just remember that the period number is the same as the number of electrons in an element’s outer energy shell.
Elements also increase in atomic number across each period or row by one. An example is Li, Be, B, C, N, O, F, Ne (the second period).
The periodic table is set up with each period representing adding an electron to each successive element going across. We split it into s, p, d, and f blocks, corresponding to which is the outer sub-shell.
Most of the right hand side of the p block undergoes reactions that are characterized by the ability to accept electrons. More electronegative p block elements are therefore more reactive, as they accept electrons more easily. For example oxygen is more reactive than sulphur etc.
Within the table the elements are organised into groups which have similar chemical properties. The metals and non-metals are divided by a staircase.
Elements are substances which are made up of only one type of atom – this means there are about 100 different elements. However, only 92 elements occur naturally on Earth. The other elements might exist for only a very short amount of time and they have to be made artificially.
Atoms with different numbers of protons must be different elements. All atoms of the same element have the same number of protons – for example, every oxygen atom has eight protons in its nucleus.
Now you know the basic rules that help build the periodic table, let’s look at some examples!
For instance if you were asked to find out how many protons, neutrons and electrons a tungsten atom has if its mass number is 184 you can use the periodic table to help you.
The atomic number of tungsten is 74, so a tungsten atom has 74 protons. Mass number = number of protons + number of neutrons= 184.
So number of neutrons = relative atomic mass – number of protons = 184 – 74 = 110. Number of electrons = number of protons. So number of electrons = 74.
Another example: Phosphorus is in group five of the periodic table. So how would you work out how many electrons a phosphorus atom has in its outermost energy level?
Well, we know that the way the periodic table is organised means that elements in the same group have the same number of electrons in their outermost energy level.
This number is equal to the group number. Phosphorus is in group five so it has five electrons in its outermost energy level. Atomic number of phosphorus = 15. So phosphorus has 15 protons and 15 electrons. Electronic structure = 2,8,5.