Mendeleev arranged the (approx) 60 known elements in order of "atomic weight" in groups with similar properties and left gaps for undiscovered elements. When protons were discovered in the 1900s the reason for the pattern became clear.
Across each period from metals on the left of the staircase to non-metals on the right, there is a repeating trend in properties such as electron configuration, ionisation energy, structure and melting points called periodicity.The chemical properties of an element depend on the number of electrons in the outer shell and the number of shells. Elements with the same number of outer shell electrons are in the same group. Elements with the same number of shells are in the same period.
The modern periodic table has 118 elements arranged in order of atomic number in vertical columns called groups and rows called periods.
Across each period from metals on the left of the staircase to non-metals on the right, there is a repeating trend in properties such as electron configuration, ionisation energy, structure and melting points called periodicity.The chemical properties of an element depend on the number of electrons in the outer shell and the number of shells. Elements with the same number of outer shell electrons are in the same group. Elements with the same number of shells are in the same period.