In 1869, Dmitri Mendeleev arranged the known elements of the time into what we call the periodic table. Mendeleev recognized that elements placed in order of increasing atomic mass exhibit a regular, or periodic repetition of their chemical and physical properties. The modern periodic table is a table of the elements ordered by increasing atomic number into rows and columns so that the elements in any one column have similar or regularly changing properties. A period consists of the elements in any one horizontal row of the periodic table. A group or family consists of the elements in any one column of the periodic table.
             Valence electrons are electrons found in the outer most occupied energy level. They are the electrons with highest energy, and they are those electrons that can be gained of lost in a chemical reaction. The electrons found in the inner electron shells are called core electron. Core electrons do not participate in chemical reactions. This is why elements like Cl and Br, elements with different masses, react in the same stoichiometry with elements with such different masses like Na and K. They have the same number of valence electrons.
             Atomic orbitals are grouped in electron shells. The orbital within a shell are further subdivided into subshell, each subshell having orbital of a specific shape and energy. The s subshell has one orbital. All s orbitals are spherical in shape and have spherical symmetry. There's also a p subshell; it has three orbitals. The next subshell is called d. A d subshell has 5 orbitals.
             Groups I and II make up the s-block. They have their valence electron in s-orbitals, so they are in the s-block. Groups III-VII make up the p-block. They have their valence electron in p-orbitals, so they make the p-block. The transitional metals make up the d-block.
             An electron configuration is a particular distribution of electrons among the different subshell
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