The elements of group one of modern periodic table are called alkali metals. The members of group1 are lithium, sodium, potassium, rubidium, caesium and francium. Alkali metals are always found in combined state (in the form of silicates, carbonates, nitrates, oxides and halides) because they are highly reactive. All of them have 1 electron in valence shell and they form monovalent cation. Alkali metals have very low ionization energy because of their larger atomic size. Alkali metals are shiny white solid metals. Alkali metals have low melting and boiling points and their melting and boiling point decreases down the group. The chemical properties of alkali metals include their reaction with water, halogens, hydrogen, oxygen and non-metals. Group 17 of the periodic table is called halogen family or group. The elements present in this group are fluorine, chlorine, bromine, iodine and astatine. They are commonly found in combined state. All the halogens have seven valence electrons and therefore, they show similar properties. They accept an electron and form monovalent anion. Under ordinary conditions, all the halogens form diatomic molecules and both the halogen atoms attain stable configuration. The ionisation energy of halogens is very high as they have very little tendency to lose electron. Halogens are non-metals. They are highly electronegative and form negative ions. Melting and boiling point of halogens increase down the group. Halogens react with metals, non-metals and hydrogen. Halogens are strong oxidising agents as they accept electrons. Atomic number represents the number of protons present in the nucleus of an atom whereas, mass number tells about the total number of protons and neutrons present in the nucleus of one atom of an element. Stability of an element depends on the neutron-proton (n/p)ratio.

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