The modern Periodic Table
Using the relative atomic mass of elements was the only option available to Mendeleev at that time. His periodic table produced patterns which were recognised and accepted, but it had its limitations.
If we put the elements in order of the mass of their atoms, most elements end up in a group which behave in a similar way. But not all elements do this.
For example, Argon atoms have a greater atomic mass than Potassium atoms. This would mean that Argon (a noble gas) would get grouped with extremely reactive metals such as Sodium and Lithium. And Potassium (an extremely reactive metal) would be put with the noble gases. So Argon would go in front of Potassium in the Periodic Table, even though its atoms are heavier.
The modern Periodic Table 2
Once scientists begain to find out more about atoms at the start of the 20th centurym they could solve the problem described above. The structure of the atom was key to developing the modern Periodic Table.
We now arrange the elements in order of their atomic (proton) number. This puts them all in exactly the right place in the Periodic Table. Their patterns of physical and chemical properties show this.
The Periodic Table is now a reliable model. It arranges the elements in groups with similar properties. It also provides us with an important summary of the structure of the atoms of all the elements.
The modern Periodic Table 3
Elements in the same group of the periodic table have similar properties. Thats because their atoms have the same number of electrons in the highest occupied energy level.
Within a group, the properties of the elements are affected by the number of lower energy levels underneath the outer level. As we go down a group the number of occupied energy levels increases, the atoms get bigger. This means that the outer electrons are further from the positive nucleus. This has 2 effects:
- The larger atoms lose electrons more easily
- The larger atoms gain electrons less easily
In both cases, this happens because the outer electrons are further away from the nucleus. Not only that, he inner energy levels 'screen' or 'shield' the outer electrons from the positive charge in the nucleus. We can see this effect with the alkali metals and halogens.