Periodic Classification of Elements.
5. Elements lithium, sodium and potassium make Dobereiner's triad.
6. According to periodic law of Mendeleev, the properties of elements are periodic function of their atomic masses.
7. The lightest element in the modern periodic table is hydrogen.
8. The first element of group 15 in the modern periodic table is nitrogen.
9. Atomic number (Z) of an element is the number of protons present in the nucleus of an atom of that element.
10. Iodine (I2) exists as solid at room temperature. 