Periodic Table - elements are arranged acc. to increasing order of atomic number. 8 vertical columns: groups, 7 horizontal columns: periods. left side elements are metal and top right elements are non-metals.

Electric configuration - Arrangement of electrons in the shells of atoms. Total of four shells.
1st Shell: 2 electrons
2nd shell: 8 electrons
3rd Shell: 8 electrons or remaining
4th Shell: 2 electrons or remaining

The relation between the outermost shell electrons and group number - The outermost shell electrons of an element gives its group number

The relation between the number of shells and group number - The number of shells in an element gives its period number

Valency - Capacity of an atom to give , accept or share electrons to achieve stable electronic configuration; number of electrons in the outermost shell or valence shell determines the valency of an atom.

Ions - Charged species formed by losing and gaining of electrons by an atom. If an atom loses electrons, it will form positively charged ions. If an atom gains electrons it will form negatively charged ions.

Relative Atomic Mass - Average mass of an atom, taking account each isotope present in a naturally occurring sample of an element. calculate w mass number and relative abundances of all isotopes
(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B)/100