How to use the periodic table
In this post we are going to discuss on how to use the Periodic Table. To a random person, the Periodic Table might look like just an orderly arrangement of atoms but to a chemist, the Periodic Table is the summary of chemistry. The Period Table shows us:
- names the elements we use in chemistry and in daily life
- families to which the elements belong
- how the element are related to each other in terms of their chemical properties
- the numbers of protons, electrons and neutrons of each element
- the number of electron shells each atom has
- the molar mass, relative atomic mass and isotopic mass of each element
All the detail it contains is due to the fact that it took hundreds of years of careful and thorough hard work by hundreds of chemists to develop it. The arrangement and use of the periodic table is based on the following principles:
- the number of electrons equals the number of protons in any atom
- the electrons are arranged in shells
- the outer-shell electrons dictate reactions. So elements with the same number of outer-shell electrons generally react in the same way.
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Different Periodic Table Notations
To illustrate how the Periodic Table looks like we are going to use 2 different copies of the periodic table from 2 different examination boards. The first copy shown below is from a Zimsec Combined Science past exam paper and the second one is from a Cambridge Combined Science past exam paper. Though they use slightly different notations, the arrangement of the atoms remain the same.
Zimsec and Cambridge notations on their periodic tables differ as follows:
- On the Zimsec copy, the relative atomic mass is on top of the chemical symbol and proton number at the bottom.
- On the Cambridge copy, the relative atomic mass is on bottom of the chemical symbol and proton number at the top.
Whatever notation is used on the periodic table, the bigger number is the relative atomic mass and the smaller one is the proton number.
Periodic Table Groups and Periods
Elements with the same number of electrons in the outer shell are arrange in the same columns known as groups. There are 8 groups on the periodic table and each group number is equal to the number of electrons in the outer shell. The number of outer shell electrons determine the chemical properties of an element. Therefore elements with similar chemical properties are found in the same groups.
The horizontal rows are called periods and they show the number of shells an atom has. If you look at the properties of the elements across a period of the Periodic Table you will notice certain trends such as:
- a gradual change from metals on the left hand side to nonmetals on the right hand side.
- an increase in the number of electrons in the outer shell of the elements.
Let us look at a simplified version of the periodic table below:
|Gr 1||Gr 2||Gr 3||Gr 4||Gr 5||Gr 6||Gr 7||Gr 8|
The simplified Periodic Table above shows the first 20 elements except for hydrogen. We will talk later about the position of hydrogen on the periodic table.
Here is some of the information we can get from the periodic table:
- The period number is equal to the number of shells
- The group number is equal to the number of electrons in the outer shell.
For example let us look at the element Calcium (Ca)
|Gr 1||Gr 2 ↓||Gr 3||Gr 4||Gr 5||Gr 6||Gr 7||Gr 8|
|Period 4 →||K||Ca|
Calcium is in period 4 group 2 which means it has:
- 4 shells
- 2 electrons in the outer shell
From that we can infer that the electronic configuration of calcium is 18.104.22.168.
The position of hydrogen on the periodic table
Hydrogen has one shell that contains 1 electron. This shell has a maximum capacity of 2 electrons, which means hydrogen behaves in two ways:
- as a group 1 element because it has 1 electron in the outer
- as a group 7 element because it has a deficiency of 1 electron in the outer shell like all group 7 elements.
For that, hydrogen is not usually given a group on most Periodic Tables. However on some periodic tables it is placed in both group 1 and group 7 as shown below:
|Gr 1||Gr 2||Gr 3||Gr 4||Gr 5||Gr 6||Gr 7||Gr 8|
The table below shows the comparisons of hydrogen against group 1 and group 7 elements:
|Lithium (Group 1)||Hydrogen||Fluorine (Group 7)|
|Forms positive ions||Forms either positive or negative ions||Forms negative ions|
|1 electron in outer shell||1 electron in outer shell and also 1 electron short of a full outer shell||1 electron short of a full outer shell|
Transition metals on the Periodic Table
As we said earlier, the rows on a periodic table are called periods. Period 4 and 5 each contain 18 elements, 8 of which are in group 1 to 8. The other 10 are called transition metals and they have some unique properties such as:
- variable valency
- high densities
- high boiling and melting points
- tendency to form coloured complex ions