Chemistry

Naturally Occurring Elements :
Elements upto atomic number 92 occur in nature except Technetium, Tc (Z = 43) and Promethium, Pm (Z = 61) which are formed from radioactive elements where ‘Z’ represents atomic number.
Synthetic Elements : Elements beyond atomic number 92 are man-made elements. They are also called synthetic elements
Groups :
(1) Elements in a group have same number of valence electrons.
(2) The chemical properties of valence electrons, e.g., all the group 1 elements have 1 valence electron. They form positively charged ions by losing one electron, when required amount of energy is supplied to them i.e., Li⁺, Na⁺, K⁺.
Group 1 elements are called alkali metals. Group 2 elements are called alkaline earth metals.
Group 2 elements when 2 valence electrons in the outermost shells. They can lose both the valence electrons to form dipositive cations, i.e., Be²⁺, Mg₂₊, Ca₂₊, etc. Positively charged ions are called cations.
Group 13 elements belong to boron family, 14 to carbon family, 15 to Nitrogen family, 16 to Oxygen family.
Group 16 elements contain 6 valence electrons in their outermost shells, i.e., two electrons less than the maximum number of electrons that can be present in the outermost shell. They can gain 2 electrons more easily rather than lose 6 electrons. They change into dinegative ions such as O^2–, S^2– Group 17 elements called Halogens contain 7 valence electrons. They can gain one electron to acquire stable electronic configuration, i.e., 8 electrons in the outermost shell and form uni-negative (single negative) ions such as (F^–, Cl^–, Br^–, I^–)
Negatively charged ions are called anions.
Group 18 elements called noble gases, have their outermost, shell completely filled. The elements of this group have no tendency to lose or gain electrons. Thus, the elements of this group have zero valency and are almost unreactive. Hence they are called Noble gases. However, nowadays, compounds of Kr, Xe and Rn have been prepared.
In any particular group, the number of shells increase but the number of valence electrons remains the same.