The vertical columns in the periodic table are called groups. All the elements present in a particular group have similar properties, this is due to the presence of same number of valence electrons.

The horizontal rows of the periodic table is known as periods. Period depicts the number of shells or the orbits in an element. Across a period the physical and chemical properties vary because the number of valence electrons of elements varies from left to right across a period.

A series of chemical elements comprising the 15 metallic chemical elements with atomic numbers 57–71. That is, the elements from lanthanum to lutetium are known as lanthanides. The 14 elements after lanthanum have similar properties to lanthanum hence, they are named as lanthanides. NOTE: These elements have incompletely filled f-orbitals.

A series of elements comprising the 15 metallic chemical elements with atomic numbers from 89 to 103. That is, the set of elements from actinium to lawrencium is known as actinides. The elements from 90-103 have similar properties to actinium hence, they are named as actinides.