Periodic Table and Periodicity – NDA Chemistry Notes

You have seen a chart on the wall of your science classroom. It has all the known elements arranged in rows and columns. That chart is the Periodic Table. The periodic table is one of the most powerful tools in chemistry. It tells you the properties of an element just from its position.

The chapter rewards students who know the structure of the periodic table, understand valency rules, and have memorised key facts about groups 1 and 2.

1.  The Periodic Table – Structure and Modern Periodic Law

Think of a large grid. The horizontal rows are called periods. The vertical columns are called groups. The modern periodic table has 7 periods and 18 groups.

The period number tells you how many electron shells an atom of that element has. An element in Period 2 has 2 electron shells. An element in Period 3 has 3 shells.

Elements in the same group have the same number of electrons in their outermost shell. This is why elements in the same group have similar chemical properties.

Modern Periodic Law

The modern periodic law states: the properties of elements are a periodic function of their atomic numbers.

This law was established by Henry Moseley. Moseley showed that atomic number, not atomic mass, is the most fundamental property of an element.

Atomic number is the most characteristic property of an element.  [NDA 2020-I]

Before Moseley, Mendeleev arranged elements by atomic mass. Mendeleev’s periodic law stated: properties of elements are a periodic function of their atomic masses. Mendeleev’s table had gaps and some wrong placements that were fixed when atomic number became the basis.

2.  Blocks of the Periodic Table

The periodic table is divided into four blocks based on which subshell is being filled.

d-block elements are called transition metals. They have partially filled d-orbitals.  [NDA 2010-I]

Manganese (Mn) is a transition metal. Aluminium (Al) is a p-block metal, not a transition metal. Magnesium (Mg) and Calcium (Ca) are s-block elements (Group 2, alkaline earth metals). Only Manganese is a transition metal.

[NDA 2010-I]

f-block elements are separated at the bottom of the table. They are called inner transition metals. Lanthanides are also called rare earth elements. Actinides include uranium and other radioactive elements.

3.  Classification of Elements – Metals, Non-Metals, Metalloids

Metals are good conductors of heat and electricity. They are malleable and ductile. They have lustre. They are mostly solid at room temperature.

Non-metals are poor conductors. They are brittle. They lack metallic lustre. They may be solid, liquid, or gas at room temperature.

Metalloids (also called semi-metals) have properties of both metals and non-metals. They are also called semiconductors.  [NDA 2013-II]

Metalloids include: Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb), Tellurium (Te), Bismuth (Bi).  Germanium is a metalloid. Coke is a form of carbon (non-metal). Iodine is a non-metal. Sugar is a compound, not an element.

4.  Elements Liquid at Room Temperature

Most elements are solid at room temperature. Only two elements are liquid at standard room temperature (25°C) and normal pressure.

Mercury (Hg) is a metal. It is liquid at room temperature. Its melting point is −38.8°C.

Bromine (Br₂) is a non-metal. It is a reddish-brown liquid at room temperature. Its melting point is −7.2°C.

These are the ONLY two elements liquid at 25°C and normal pressure.  [NDA 2021-II, NDA 2009-II]

5.  Valency

Valency is the combining capacity of an atom. It tells how many bonds an atom can form with other atoms. Valency depends on the number of electrons in the outermost shell of the atom.  [NDA 2017-I]

How Valency Is Determined

For elements in Groups 1 to 4: Valency = number of electrons in outermost shell.

For elements in Groups 5 to 7: Valency = 8 minus number of outermost electrons.

For Group 18 (noble gases): Valency = 0 (outermost shell is completely filled).

Zero valency: Noble gases (He, Ne, Ar, Kr, Xe, Rn) have zero valency. Their outermost shells are completely filled. They do not need to gain, lose, or share electrons.  [NDA 2016-II]

Nitrogen has a common valency of 3 (in NH₃). It also has a valency of 5 (in HNO₃).  [NDA 2024-II]

The nitride ion (N³⁻) has a charge of −3. So its valency is 3. Nitrate (NO₃⁻) has valency 1. Sulphide (S²⁻) has valency 2. Sulphate (SO₄²⁻) has valency 2.  [NDA 2024-II]

Valency order: Ne (0) < Mg (2) < N (3) < Si (4).  [NDA 2022-I]

Same valency in a group: Elements in the same group always have the same valency. Mg, Ca, Ba are all in Group 2. They all have valency 2.  [NDA 2021-I]

Identifying element from formula: XCl₂ means element X has valency 2. A solid XCl₂ with a high melting point suggests an ionic compound, typical of Group 2 metals. Mg forms MgCl₂ in the same way. So X is in the same group as Mg.  [NDA 2025-I]

Gallium and Oxygen compound: In Ga₂O₃, oxygen has valency 2. Three O atoms give total valency 3 × 2 = 6. Two Ga atoms share this. So valency of Ga = 6 ÷ 2 = 3.  [NDA 2009-I]

Highest Valency of Selected Elements

Inert Pair Effect – Group 14

All elements in Group 14 (carbon family: C, Si, Ge, Sn, Pb) have a common valency of 4.

However, the heavier elements in the group (Germanium, Tin, Lead) can also show a valency of 2. This is because the two s-electrons in the outermost shell become reluctant to participate in bonding as you go down the group. This is called the inert pair effect.

Carbon and Silicon only show valency 4.  Germanium, Tin, and Lead can show both valency 4 and valency 2.  [NDA 2011-II]

6.  Periodic Trends

A periodic property is a property that changes in a regular pattern as you move across a period or down a group in the periodic table.

Atomic Size

Atomic size decreases across a period (left to right). The nuclear charge increases but electron shells stay the same, so electrons are pulled in more tightly.

Atomic size increases down a group. Each new period adds a new electron shell.

Electronegativity

Electronegativity increases across a period and decreases down a group. Fluorine is the most electronegative element.

Properties That Are Periodic vs. Not Periodic

Properties that ARE periodic (show trends): atomic size, valency, ionisation energy, electron affinity, and electronegativity.

Property that is NOT periodic: Radioactivity does not follow a regular trend across periods or down groups.

7.  Alkali Metals – Group 1

The alkali metals are: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), Francium (Fr). They are all in Group 1. They all have valency 1. They are soft metals that react vigorously with water.

Melting Points

Melting points decrease going down Group 1: Li > Na > K > Rb > Cs.

Lithium has the highest melting point among the alkali metals.  [NDA 2015-II]

Caesium has the lowest melting point among Na, K, Rb, Cs. Its melting point is about 28.5°C.  [NDA 2018-I]

Uses of Alkali Metals

Match list: Li = batteries, Na = nerve impulses, K = blood water, Cs = atomic clocks.  [NDA 2015-II]

Equivalent weight and valency: Equivalent weight of an element = Atomic weight ÷ Valency. When valency changes, equivalent weight also changes. Iron has variable valency (2 and 3), so its equivalent weight varies.