Going into a chemistry laboratory just about anywhere in the world will reveal a brightly colored poster of the periodic table on the wall. Most chemistry textbooks and just about every chemistry website have a copy of this table.
It has a long column of seven elements at each end with a series of 16 shorter columns of either four or six elements linking them. In addition, there are two separate rows of elements normally linked by arrows to elements in the sixth and seventh rows. the columns are referred to as groups and are numbered 1 to 18 from left to right. The rows are called periods and are numbered 1-6 from top to bottom.
Why does the periodic table have this form instead of a series of columns all containing the same number of elements? It is all due to electrons and the way they orbit the nucleus of an atom.
Electrons carry a negative charge the protons within the atom’s nucleus are positively charged. While the protons are constrained within the nucleus, held together by neutrons, the electrons orbit the nucleus. The electrons orbit within orbital shells each shell leads to another row or period within the periodic table. Electrons remain in the shells as this allows them to orbit without approaching another electron as the two electros having the same charge repel each other.
Elements in the extreme right column of the table have a full complement of electron in their outer shell. This is a very stable type of atom and such elements form the fairly inert noble gas group.
As the atomic radius of the atom increases, the atom can hold more electrons in its outer shell without them coming too close to another electron. The electron orbital shell is divided into sub-shells.
The two elements in the first period of the table have only one electron shell costing of a single s-sub-shell capable of containing two electrons. The next period builds on the first with an additional s-sub-shell congaing two electrons and a p- sub-shell, which can contain up to six electrons. The third period also adds a p- and an s-sub=shell to the atoms. The noble gases at the end of periods 2 and 3 each contain eight electrons in their outer shells.
The next two periods each have an additional d-sub-shell containing up to 10 electrons. This is in addition to the s- and d- sub-shells.
The final two periods in addition to all the other sub shells also have f-sub-shells containing up to 14 electrons.
The presence of electron sub-shells allows the grouping of the elements into blocks as the sub-shells are filled. The s-block consists of elements in groups 1 and 2. The p-block consists of the elements in groups 13-18. Groups 3-12 contain the d-block elements. The two rows of elements separate from the main table contain the f-block elements.
Each element in the table has an atomic number. This number is the same as the number of electrons or protons contained within in the atom in its non-ionic state. Therefore, element 1 (hydrogen has one electron, element 30 (zinc) has 30 electrons and element 118 (ununoctium) has 118 electrons.
