Group

The vertical columns are called groups. Basically there are eight groups (1 to VIII) but each group is further sub-divided into "A" and "B",

Sub-groups.

The elements of sub-group 'A' are called 'Main' or Representative Elements, as the properties of these elements are represented by valency

electrons.     

The element of sub-group B' are called Transition Elements, because the properties of these elements show a gradual change or transition between the two sets of representative elements, on either side of them.

Elements of a group have similar valency shell configuration hence have similar properties.

The group number indicates the total number of electrons in valency shell of that element.

Group IA (The Alkali Metals) or (Lithium Family)

This group includes Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr). Their valence shell contains one electron only, and on reaction they lose this electron and form univalent positive ions (MI+), they are highly reactive metals with low melting points. Fr is radioactive.

Their atomic radii, atomic volumes, ionic radii increase from Li to Cs due to the addition of extra shell to each element and due to same reason, the melting and boiling points decrease downward. They are called Alkali Metals because they form water soluble base such as NaOH and KOH.

Group IIA (The Alkaline Earth Metals); (Beryllium Family)

It includes Beryllium (Be) Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra).

Their valence shell contains two electrons. On reaction they lose these two electrons and form divalent positive ions (M2+). Ra is radioactive.

These elements are a bit harder, having higher melting and boiling points than the alkali metals, but they have smaller atomic, ionic radii and atomic volumes.

Down the group they do not show a regular trend in melting, boiling points and densities.

Group III A (The Boron Family):

It includes Boron (B); Aluminum (Al); Gallium (Ga); Indium (In) and Thallium (Tl). Their valency shell contains three electrons. They exhibit a valency of 3+ and form M3+ ions. Except boron they are highly electropositive, elements i.e. having metallic character which increases down the group; due to increase in atomic volume. Boron is Metalloid. A metalloid is an element which has some properties of metals and some properties of non-metals.

Group IVA (Carbon Family):

It includes Carbon (C); Silicon (Si); Germanium (Ge), Tin (Sn) and Lead (Pb).

Their valence shell contains four electrons. C, Si and Ge form covalent compounds whereas Sn and Pb exhibit a variable valence of 2 and 4.

Of these elements C is non-metal; Si and Ge are metalloids, Sn and Pb are metals.

Down the group atomic radii and volumes increase due to addition of a new shell and for the same reason metallic character increases down the

Group. C and Sn exist in different allotropic forms.

Group V (Nitrogen Family):

It includes Nitrogen (N), Phosphorus (P) Arsenic (As), Antimony (Sb) and Bismuth (Bi).

Of these elements N and P are non-metals, as and Sb are metalloids and Bi is a metal. Their valence shells contain five electrons. There is a large variation of properties as we go down the group.

Nitrogen exists as diatomic molecules (N2) and forms a number of oxides as NO, N2O, NO2, NO4 and N2O5 Due to small atomic size and large ionization potential, nitrogen has a tendency to accept three electrons to form nitride ion (N3). Phosphorus exists as P4 molecule.

Except Nitrogen all exist in more than one allotropic forms.

Group VI (Oxygen Family):

It includes oxygen (O); Sulphur (S); Selenium (Se), Tellurium (Te) and Polonium (Po).

Of these oxygen and sulphur are non-metals, selenium, tellurium are metalloids and polonium is metal.

All of the elements exhibit the property of allotropy. For example

Allotropic forms of oxygen are oxygen (O2) and ozone (O3).

Oxygen and sulphur form divalent negative ions O2- and S2- Their valence shell contains six electrons.

Group VIIA (The Halogens):

It includes Fluorine (F); Chlorine (CI); Bromine (Br); Iodine (I) and Astatine (-At).

Except Astatine (which is a metalloid) all others are non-metals and exist as diatomic molecules. At room temperature F2 and Cl2 are gases; bromine is a liquid and iodine is a solid. Their valence shell contains seven electrons. They have high ionization energies and large negative electron affinities hence they easily accept an electron to form halide ions (x)1- i.e. (F1-Cl1-, Br1-", I1-")

Group VIIIA (Inert or Noble Gases):

It includes Helium (He); Neon (Ne); Argon (Ar), Krypton (Kr); Xenon (Xe) and Radon (Rn).

Their valence shell contains eight electrons, except helium which has two electrons. With the exception of krypton and xenon (which have large atomic volumes so slightly reactive under drastic conditions) the rest of these elements are totally inert chemically. The reason is that these have completely filled outer shells, a condition that represents greater stability.

INTERESTING TO NOTE
Discovery of Noble Gases

None of the noble gases was known when Mendeleev proposed his periodic table. In 1892. The English scientist Ramsay became interested in the discovery that nitrogen obtained from the air had a slightly higher density than that prepared by chemical reactions. After careful investigation, he concluded that higher density must be due to the presence of unknown gas. When he separated this gas from the air, he found that it was completely uncreative. He called it argon, the 'idle or lazy" gas in Greek in the same year Ramsay isolated helium (He) the lightest of all noble gases, from uranium ores. During 1898 Ramsay and Rayleigh isolated three additional noble gases from air, neon (Ne) krypton (Kr) and xenon (Xe).

Group IB To VIIIB (Transition Elements):

These are metals. In these elements, besides the valence shell penultimate shell is also incomplete. In chemical reactions they show more than one valencies. These elements in compounds having characteristic colours.

The total classification can be summarized in the following scheme:

 

 

 
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