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Modem Periodic Law


In 1913, the English Physicist Henry Moseley studied the X-ray spectra of many elements. He observed that a plot of JV (where vis the frequency of the X-rays emitted) against atomic number (Z) gave a straight line and not the plot of JV against atomic weight. He proposed that atomic number  is a more fundamental property of an element than its atomic weight. Therefore, the physical and chemical properties of the elements are determined by their atomic numbers instead of their atomic weights. This observation led to the development of modem periodic law. The modern periodic law states that:

The physical and chemical properties of the elements are the periodic function of their atomic numbers. It means that if the elements are arranged in order of increasing atomic numbers, the elements with similar properties recur after regular intervals.


Electronic configuration, physical properties and chemical properties are used to classify elements into various groups. The elements in a vertical column have largely similar properties which show regular gradation with increase in atomic number. The elements in a horizontal row show relatively sharp change in physical and chemical properties.

For example, some physical and chemical properties of alkali metals (Na, K), alkaline earth metals (Mg, Ca) and halogens (F2, Cl2, Br2 and lz) are listed below:

• Sodium and potassium are soft metals. Their hardness – decreases with-increase in atomic number.

• They have low melting points which decreases with increase in atomic number.

• They react with water liberating hydrogen gas and form soluble metal hydroxide

2Na (s) + H2O(l) à7 2NaOH (aq) + H2 (g)

2K (s) + H2O (l) à7 2KOH (aq) + H2 (g)

Table 6.1. Comparison of the Properties of Eka-aluminium and Eka-silicon  as Predicted by Mendeleev with those Observed Experimentally for Gallium and Germanium

Reactivity increases with increase in atomic number. The reaction of water with potassium is more vigorous than with sodium.

Caution: Reaction of alkali metals with water is very violent.

• Both Na and K have one electron in their outermost shell.

• They exhibit oxidation state of + 1

• They are strong reducing agents.

• Magnesium and Calcium have two electrons in their outermost shells.

• They are harder than alkali metals and have higher melting points.

• They are less reactive with water than alkali metals. Magnesium reacts only with boiling water.

• Mg and Ca have two electrons in the outermost shell.

• They exhibit oxidation state of +2

• Chlorine and bromine are non-metals. They have very low melting and boiling points.

• Chlorine is a gas whereas bromine is a liquid.

• They react with water slowly in the presence of sunlight.

2Cl2 + 2H2O —-7 4HCl + O2

2Br2 + 2H2 O —7 4HBr + O2

Cl2 is more reactive than Br2.

• They have seven electrons in their outermost shell.

• They exhibit oxidation state of -1

• They are strong oxidizing agents.

By studying physical and chemical properties of different elements, the elements have been classified into various groups or families.

For example, alkali metals, alkaline earth metals; halogens, noble gases, etc.

Many new forms of periodic table have been proposed in recent times with modem periodic law as guiding principle, but the general plan of the table remained the same as proposed by Mendeleev. The most commonly known periodic table is the long form of the periodic table. Before discussing the general plan of the long form periodic table, let us look into the basic cause of periodic repetition of properties.