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Mendeleev arranged the elements known at that time according to atomic mass. The elements were arranged in rows called periods, in increasing order. Each row had eight elements. He also arranged them accordingly in columns, where the elements in each column, called groups, had similar properties. 

He left some blanks on his table because he predicted that the big gaps between some atomic mass' of elements indicated that there were other elements yet to be discovered that would fill in that gap. He was able to predict the atomic weights of some of the elements in between. 
CintiaSalazar

By definition, Mendeleev's periodic table was rearranged according to atomic number, hence there are 7 periods (rows) and 18 groups (columns)

Mendeleev proposed to arrange the elements in lines and columns (also called "periods" and "groups") within a rectangle, with their atomic weights in ascending order from left to right within the same line down to the second and so on.

In other words, the elements are ordered by atomic number, which is defined as the number of protons found in the nucleus.

Also, Mendeleev showed that elements with similar chemical properties appeared periodically, giving rise to eight groups.

In this way he reorganized the periodic table leaving holes where an element did not exist, so that it can be discovered.

Finally, in summary, Mendeleev's periodic table was rearranged according to atomic number, hence there are 7 periods (rows) and 18 groups (columns).

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