Summary

The quantum theory is necessary to describes electrons. It predicts discrete allowed levels  and wavefunctions, which give probably distributions tor electrons. Wavefunctions for elec trons in atomic orbitals.

Atomic orbitals are labeled by 3 quantum numbers n ,i and m , Orbitals are called s,p,d or f according the value of i ; there are respectively 1 ,3,5 and 7 different possible m values for these orbitals.

Electrons have an intrinsic rotation called spin, which may point in only 2 possible directions, specified by a quantum number s just as a value of + 1 /2 or - 1/2. Two electrons in the same orbital with opposite spin are paired.

The radial distribution function shows how far from the nucleus an electron is likely to be found. The major features depend on n but there is some dependence on 1 . The allowed energy in hydrogen depend on n only. And the angular distribution function shows how it varies with the direction in space.

When the spin quantum number s is included, no two electrons in an atom may have the same set of quantum numbers.Thus the maximum number of electrons in any orbital is two.

The electronic repulsion between electrons weakens their binding in an atom. This is known as screening or shielding. An orbital is screened more effectively if its radial distribution does not penetrate those of other the lowest energy state is formed by putting electrons as far as electrons，When filling orbitals with I>0, the lowest energy state is formed by putting electrons as far as possible in orbitals with different m values, and with parallel spin , which according to the 3 principles of minimum energy principle,and and Hund's rule.

The periodic table shows elements arranged horizontally in periods and vertically in groups according to their chemical similarity .Elements in the same group have the same configuration of outer electrons. The table is divided naturally into s,p,d (ds) and f blocks according to the outer electron configurations. Electronegativity (electron-withdrawing or attraction power ) of the elements vary in a regular manner according to atomic number .

Many elements (macroelements and trace elements) are essential for biology; include their ability to display variable oxidation states and catalytic activity. And some of elements in the table are non-essential elements and even poisonous elements for biology.