A modern periodic table arranges the elements in increasing order of their atomic numbers and groups atoms with similar properties in the same vertical column. Each box represents an element and contains its atomic number, symbol, average atomic mass, and (sometimes) name. Atomic number is the number of protons in an atom of an element, and this number is unique for each element. Atomic number increases from left to right across each row and from top to bottom within each column of the periodic table.     Rows of the modern table are called periods. From left to right across a period, each... Show more

A modern periodic table arranges the elements in increasing order of their atomic numbers and groups atoms with similar properties in the same vertical column. Each box represents an element and contains its atomic number, symbol, average atomic mass, and (sometimes) name.

Atomic number is the number of protons in an atom of an element, and this number is unique for each element. Atomic number increases from left to right across each row and from top to bottom within each column of the periodic table.    
Rows of the modern table are called periods. From left to right across a period, each element has one more proton than the element before it. Within each period, elements change from metals on the left side of the table, to metalloids, and then to nonmetals on the right. Some periods in the modern periodic table are longer than others. For example, period 1 contains only two elements. Periods 6 and 7, in contrast, are so long that some of their elements are placed below the main part of the table. These elements are called the lanthanides (period 6) and actinides (period    
).    
Columns of the periodic table are called groups, or families. There are a total of 18 groups in the modern table.    
Elements in the same group have similar properties. For example, all elements in group 18 are colorless, odorless gases.    

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