According to the modern periodic table the Physical and chemical properties of the elements are periodic functions of their atomic numbers. It means, when the elements are arranged in the order of their increasing atomic numbers or electronic configuration.
That means the element having same valance electronic configuration has similar properties and on going down the group electronic configuration is same. Therefore elements in a group possess similar properties.
But the valance electron configuration of different element in a same period is different, so the properties of different element in a same period are different.