Trends in the Periodic Table. The Periodic Table consists of seven rows called periods going across and 18 columns called groups going down. The elements in the Periodic Table are arranged according to their atomic number, which is the number of protons in their nucleus. So, What do the period and the group show?
Well, the period shows the element's electron shell that is being filled. For example, sodium, Na, in period 3, group 1, has the electronic configuration 2, 8, 1, whereas chlorine, Cl, also in period 3 but in group 17, has the electronic configuration 2, 8, 7. In both cases, it is the third shell that is being filled. Can you see a connection between the group an element is in and the number of electrons in its outer shell?
Note that if the group has two numbers, we take the second number. Pause the video while you think. The group an element is in gives the number of electrons in the outer shell of an atom of that element.
Now, Pause the video again and write the electronic configuration for carbon, atomic number 6, period 2, and group 14. The electronic configuration for carbon is 2, 4. Did you get it right? Chemical reactions are to do with the movement of electrons. So... As all elements in a particular group have the same number of outer shell electrons, they have similar chemical properties.
Let's take a look at the trends across the periods. Periodicity is the trend in properties across each period. Let's go from left to right across a period.
What do you notice? First of all, elements change from metals to non-metals. Also note that there is a decrease in atomic radius. This is because more protons are in the nucleus which pull the electrons closer in.
There is also an increase in first ionisation energy, which is the energy needed to remove the outermost electron, and an increase in electronegativity, which means the attraction of a bonded atom for the pair of electrons in a covalent bond. Now let's look at the trends as we go down a group. Notice that the elements become more metallic.
This is clearly seen in group 14, where carbon in period 2 is a non-metal, but lead in period 6 is a metal. By the way, do you remember that metals are elements that react by losing electrons, and non-metals are elements reacting by gaining electrons? Also going down the group there is an increase in atomic radius.
This means that an extra shell of electrons is added for each successive element. However, there is a decrease in first ionisation energy, which, as noted before, is the energy needed to remove the outermost electron. There are three groups of the periodic table that are usually studied.
Group 1, the alkali metals. Group 2, the alkali metals. 17 called the halogens and group 18 called the noble gases. As we go down group 1 the metals become more reactive.
This is because metals react by losing electrons and it is easier to lose the outer shell electron the further it is from the positive nucleus. As we go down group 17 the nonmetals become less reactive. because nonmetals react by gaining electrons, and the fewer shells the greater attraction for the incoming electron. And in group 18, elements have a full outer shell of electrons and so are very unreactive.
Their densities and boiling points increase on going down the group.