In 1869 Dmitri Mendeleev arranged the elements known at the time in order of atomic mass, placing elements with similar properties in the same group.
He left gaps for elements that had not yet been discovered and predicted their properties. The gap below silicon was filled in 1886 by germanium, whose properties closely matched his prediction.
Today the elements are arranged in order of atomic number instead. This explains the few places where Mendeleev had to break the order of atomic mass, such as tellurium and iodine.
The rows of the periodic table are called periods and the columns are called groups. There are 7 periods and 18 groups.
The atoms of elements in the same period have the same number of electron shells. Among the main-group elements, those in the same group have the same number of outer electrons and similar properties.
Groups 1, 2 and 13 to 18 hold the main-group elements, and groups 3 to 12 hold the transition elements. Some groups have their own names: the alkali metals in group 1 (apart from hydrogen), the halogens in group 17 and the noble gases in group 18.
More than three quarters of the elements are metals. They fill the left side and the middle of the table, while the nonmetals are gathered at the upper right, with hydrogen as the exception at the upper left.
Along the border between them lie elements such as boron, silicon, germanium, arsenic, antimony and tellurium. Their properties lie between those of metals and nonmetals, and they are often called metalloids.
Metallic character grows stronger towards the lower left of the table, and nonmetallic character grows stronger towards the upper right, apart from the noble gases.
When the elements are lined up in order of atomic number, similar properties keep coming back at regular intervals. This pattern is called the periodic law.
Its cause is the outer electrons. Across periods 2 and 3, the number of outer electrons rises from 1 to 8, and at the start of the next period it drops back to 1.
Elements with the same number of outer electrons, such as lithium, sodium and potassium, therefore behave in similar ways.
Across a period from left to right, atoms get smaller. The number of protons increases, so the nucleus pulls the electrons in the same outer shell more strongly.
Down a group, atoms get larger, because each step down adds another electron shell.
The largest atoms are therefore found at the lower left of the table and the smallest at the upper right, leaving aside the noble gases, whose sizes are measured in a different way.