2. Periodic Classification of Elements

The number of electrons in the outermost shell of alkali metals is (i) 1.

Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in (i) Group 2.

The element (i) Na is present in the same group as X.

The nonmetals are found in the (ii) p-block of the modern periodic table.

Total electrons = 2 + 8 + 2 = 12. So the atomic number is 12. (The element is magnesium, Mg.)

It has 2 valence electrons, so it belongs to group 2.

Its electrons are in 3 shells (K, L, M), so it belongs to period 3.

It would resemble Be (4). The electronic configuration of Be is 2, 2. Like this element, Be has 2 valence electrons, so both are in group 2 and have similar properties.

Si, Na and P belong to period 3, because their electrons are in three shells (K, L and M).

Ca and Be belong to the second group, because both have 2 electrons in their outermost shell.

Oxygen (O) is the most electronegative. N, C, O and B are in period 2, and O lies farthest to the right. Electronegativity increases from left to right in a period. Al is in period 3, below B, so it is less electronegative.

Aluminium (Al) is the most electropositive. Be, C, O and B are in period 2, but Al is in period 3 and has three shells. Its valence electrons are farther from the nucleus, so they are held less firmly and are lost easily. Electropositivity increases down a group.

Sodium (Na) has the largest atoms. All five are in period 3, and atomic radius decreases from left to right in a period. Na is the first element of this period.

Beryllium (Be) has the smallest atomic radius. Li and Be have only two shells, and Be lies to the right of Li in period 2. Na and K have more shells, so they are bigger.

Sodium (Na) has the highest metallic character. All are in period 3, and metallic character decreases from left to right. Na has only 1 valence electron, which it loses most easily.

Fluorine (F) has the highest nonmetallic character. All are in period 2, and nonmetallic character increases from left to right. F has 7 valence electrons and accepts one electron most easily.

1. The Russian scientist Dmitri Mendeleev developed the periodic table during 1869 to 1872.
2. He arranged the 63 elements known at that time in an increasing order of their atomic masses.
3. He studied their physical and chemical properties, like the formulae of their hydrides and oxides, melting points, boiling points and densities.
4. He found that elements with similar properties repeat after a definite interval. On this basis he stated the periodic law: “Properties of elements are a periodic function of their atomic masses.”
5. In his table, the vertical columns are called groups and the horizontal rows are called periods.

1. In the modern periodic table, elements are arranged in an increasing order of their atomic numbers.
2. It has 7 horizontal rows called periods (1 to 7) and 18 vertical columns called groups (1 to 18).
3. Each box is the place for one element. Its atomic number is written in the upper part of the box.
4. Two rows are shown separately at the bottom: the lanthanide series and the actinide series. There are 118 boxes in all.
5. The table is divided into four blocks: s-block (groups 1 and 2), p-block (groups 13 to 18), d-block (groups 3 to 12, transition elements) and f-block (lanthanides and actinides).
6. A zig-zag line in the p-block separates metals and nonmetals. Metals are on its left, nonmetals on its right, and metalloids lie along its border.

In Mendeleev’s periodic table:

1. Isotopes were discovered long after Mendeleev made his periodic table.
2. Isotopes have the same chemical properties but different atomic masses. Example: Cl-35 and Cl-37.
3. Mendeleev’s table was based on atomic mass. So it was a challenge to decide whether isotopes should get different places or the same place. They had no proper position.

In the modern periodic table:

1. The modern table is based on atomic number.
2. Isotopes of an element have the same atomic number. So all the isotopes of an element are placed in the same place.
3. Example: ³⁵₁₇Cl and ³⁷₁₇Cl both have atomic number 17, so both are placed in the box of chlorine (group 17, period 3).

1. From left to right in a period, the atomic number increases one by one. So the positive charge on the nucleus increases by one unit at a time.
2. But the added electron enters the same outermost shell. No new shell is added.
3. The greater nuclear charge pulls the electrons closer to the nucleus.
4. Hence the atomic radius goes on decreasing from left to right in a period.

1. Metallic character is the tendency of an atom to lose its valence electrons.
2. From left to right in a period, the nuclear charge increases and the atomic radius decreases. So the effective nuclear charge increases.
3. The valence electrons are held more strongly, and the tendency to lose them decreases.
4. Hence metallic character goes on decreasing from left to right in a period.

1. While going down a group, a new shell is added at each step.
2. So the distance between the nucleus and the outermost electrons increases.
3. Hence the atomic radius goes on increasing down a group, even though the nuclear charge also increases.

1. The valency of an element depends on the number of valence electrons in its outermost shell.
2. All elements in a group have the same number of valence electrons. Example: Li (2, 1), Na (2, 8, 1) and K (2, 8, 8, 1) all have 1 valence electron.
3. Hence elements belonging to the same group have the same valency.

1. The number of elements in a period depends on the electron capacity of the shells and the law of electron octet.
2. By the octet law, the outermost shell of an atom cannot have more than 8 electrons.
3. In the third period, electrons fill the M shell from 1 (Na: 2, 8, 1) to 8 (Ar: 2, 8, 8).
4. The next electron goes into a new shell, N (K: 2, 8, 8, 1), which starts the fourth period.
5. Hence the third period has only eight elements.

Inert gases (noble gases) are the elements of group 18. Their main applications: