NIOS Lesson 3 - PERIODIC TABLE AND PERIODICITY IN PROPERTIES

We have seen different heaps of onions and potatoes at vegetable shop. Imagine,

they are lying mixed and you want to buy 1 kg of onion. What will happen? You

will have to wait for long to sort that and then weigh them. When you possess a

variety of material substances, you have to keep them classified for an easy access

and quick use. You cannot afford to mix clothes with eatables, cosmetics or books.

Classification assures you that your eatables are in the kitchen, books on the study

table or rack and your cosmetics are on the dressing table. Shopkeepers, business

houses, storekeepers, administrators, managers, information technology experts

and scientists etc. have to keep their materials duly classified.

Chemists faced a similar problem when they were to handle a large number of elements.

The study of their physical and chemical properties and keeping a systematic record of them had been a great challenge to chemists.

Classification of elements finally could be possible due to pioneering work of a few chemists.

In the present lesson we shall discuss the need, genesis of classification and periodic trends in physical and chemical properties of elements.

OBJECTIVES

After reading this lesson, you will be able to:

􀁺 recognize the need for classification of elements.

􀁺 recall the earlier attempts on classification of elements;

􀁺 define modern periodic law;

􀁺 name the elements with atomic number greater than 100 according to IUPAC

nomenclature;

􀁺 co-relate the sequence of arrangements of elements in periodic table with Chemical Bonding

electronic configuration of the elements;

􀁺 recall the designations of the groups (1-18) in the periodic table;

􀁺 locate the classifiction of elements into s-, p-, d- and f- blocks of the periodic

table; and

􀁺 explain the basis of periodic variations of

(a) atomic size

(b) ionic size

(d) electron gain enthalpy within a group or a period.

(e) valence

3.1 EARLY ATTEMPTS

INTEXT QUESTIONS 3.1

3.2 MENDELEEV’S PERIODIC TABLE

INTEXT QUESTIONS 3.2

3.3 MODERN APPROACH

INTEXT QUESTIONS 3.3

3.4 LONG FORM OF PERIODIC TABLE

INTEXT QUESTIONS 3.4

3.5 STRUCTURAL FEATURES OF THE LONG FORM OF PERIODIC TABLE

INTEXT QUESTIONS 3.5

3.6 POSITION OF METALS, NON-METALS AND METALLOIDS

INTEXT QUESTIONS 3.6

3.7 CATAGORISATION OF ELEMENTS INTO ‘s’, ‘p’, ‘d’, AND ‘f’ BLOCKS

INTEXT QUESTIONS 3.7

3.8 PERIODICITY IN ATOMIC PROPERTIES

INTEXT QUESTIONS 3.8

3.9 ATOMIC SIZE

INTEXT QUESTIONS 3.9

3.10 IONIC SIZE

INTEXT QUESTIONS 3.10

3.11 IONIZATION ENTHALPY

INTEXT QUESTIONS 3.11

3.12 ELECTRON GAIN ENTHALPY

INTEXT QUESTIONS 3.12

3.13 ELECTRONEGATIVITY

INTEXT QUESTIONS 3.13

3.14 CONCEPT OF VALENCE OR VALENCY

3.14.1 Electron Configurations and the Periodic Table

INTEXT QUESTIONS 3.14

WHAT YOU HAVE LEARNT

􀁺 The classification of elements makes their study systematic.

􀁺 The arrangement of elements in the long form of the periodic table depends on their electronic configuration.

􀁺 The properties of the elements are the periodic function of their atomic number.

􀁺 All the known elements are arranged in 18 groups in the long form of periodic table

􀁺 There are seven horizontal rows (periods) in the long from of the periodic table.

􀁺 Elements of groups 1 and 2 are known as alkali metals and alkaline earth metals respectively.

􀁺 Elements of groups 17 and 18 are known as halogens and noble gases respectively.

􀁺 s, p, d and f are the four blocks in the periodic table classified on the basis of their outer most electrons residing in s, p, d or f sub-shell.

􀁺 The elements can be classified into metals, non-metals and metalloids on the basis of their properties and their position in the periodic table.

􀁺 The atomic size, ionic size, ionization enthalpy, electron gain enthalpy and electronegativity and valence show regular trends along a group and a period.

􀁺 Valence can be explained.

TERMINAL EXERCISE

1. Define modern periodic law.

2. Refer the periodic table given in Table 3.2 and answer the following questions.

(i) The elements placed in group number 18 are called ...............

(ii) Alkali and alkaline earth metals are collectively called ............... block metals.

(iii) The general configuration for halogens is ...............

(iv) Name a p-block element which is a gas other than a noble gas or a halogen.

(v) Name the groups that comprise the ‘s’ block of elements.

(vi) Element number 118 has not yet been established, to which block, will

it belong?

(vii) How many elements should be there in total if all the 7s, 7p, 6d and 5f, blocks are to be full?

3. Describe the variation of electron affinity and ionization enthalpy in the periodic table.

4. Define the following:

(a) Electron gain enthalpy (b) Ionization enthalpy

5. What is electronegativity? How is it related to the type of bond formed?

6. Why is the electron gain enthalpy of Cl more in negative value as compared to that of F?

Here’s the visual you asked for — a clear infographic showing periodic property trends across the periodic table.

You can see how atomic radius, ionization energy, electronegativity, and metallic character change both across periods (left → right) and down groups (top → bottom). The arrows and color coding make it easy to grasp the recurring patterns that define periodicity.

In chemistry, periodicity trend refers to the repeating or recurring patterns in the physical and chemical properties of elements when they are arranged in the periodic table according to increasing atomic number.

⚛️ Core Idea

The Periodic Law states that the properties of elements are a periodic function of their atomic numbers.
This means that as you move across a period or down a group, certain properties change in a predictable way — they repeat periodically.

🔄 Main Periodic Trends

Property	Across a Period (→)	Down a Group (↓)
Atomic Radius	Decreases	Increases
Ionization Energy	Increases	Decreases
Electronegativity	Increases	Decreases
Metallic Character	Decreases	Increases

🧪 Why It Happens

These trends arise because of changes in:

Nuclear charge (number of protons)
Electron shielding (inner electrons blocking outer ones)
Distance of outer electrons from the nucleus

🌍 Importance

Understanding periodicity helps chemists:

Predict how elements will react
Explain similarities within groups (like alkali metals)
Design compounds and materials with desired properties

Would you like me to show how these trends explain reactivity patterns in groups such as alkali metals or halogens next?

Excellent — let’s explore how periodicity trends explain reactivity patterns in two major groups: alkali metals and halogens.

⚡ 1. Alkali Metals (Group 1: Li, Na, K, Rb, Cs)

Trend: Reactivity increases down the group.
Reason:

Ionization energy decreases → outer electron is easier to remove.
Atomic radius increases → weaker attraction between nucleus and valence electron.
Result: Metals lose electrons more readily, forming positive ions faster.

Example:

Lithium reacts slowly with water.
Sodium reacts vigorously.
Potassium reacts explosively, producing hydrogen gas and heat.

Equation:
[ 2Na + 2H_2O \rightarrow 2NaOH + H_2 ]

🌬️ 2. Halogens (Group 17: F, Cl, Br, I)

Trend: Reactivity decreases down the group.
Reason:

Electronegativity decreases → less ability to attract electrons.
Atomic size increases → outer electrons farther from nucleus.
Result: Halogens become weaker oxidizing agents.

Example:

Fluorine is the strongest oxidizer, reacting violently with hydrogen.
Chlorine reacts moderately.
Iodine reacts slowly.

Equation:
[ Cl_2 + 2NaBr \rightarrow 2NaCl + Br_2 ] (Chlorine displaces bromine because it’s more reactive.)

🔁 Summary of Reactivity Trends

Group	Direction of Reactivity	Cause	Example
Alkali Metals	Increases ↓	Easier electron loss	Na > Li
Halogens	Decreases ↓	Harder electron gain	F > Cl

These trends beautifully demonstrate periodicity in action — the predictable rise and fall of reactivity based on atomic structure.

Would you like me to create a visual comparison chart showing these reactivity trends side by side for both groups?

https://copilot.microsoft.com/shares/YYqu7Jx9qdC6e5UXU2A3L

Periodicity in properties

Periodicity in properties refers to the recurring trends in physical and chemical characteristics of elements when they are arranged in the periodic table according to increasing atomic number. These trends—like atomic radius, ionization energy, electronegativity, and metallic character—help predict the behavior and reactivity of elements.

🔑 Key Concept: Periodic Law

Modern Periodic Law: The properties of elements are a periodic function of their atomic numbers.
This means that when elements are arranged by atomic number, similar properties recur at regular intervals, forming groups or families.

📊 Major Periodic Trends

Property	Across a Period (Left → Right)	Down a Group (Top → Bottom)
Atomic Radius	Decreases (due to increased nuclear charge pulling electrons closer)	Increases (due to addition of electron shells)
Ionization Energy	Increases (harder to remove electrons as nuclear charge rises)	Decreases (outer electrons farther from nucleus, easier to remove)
Electronegativity	Increases (atoms more strongly attract electrons)	Decreases (larger atoms less effective at attracting electrons)
Metallic Character	Decreases (non-metallic nature increases across a period)	Increases (elements become more metallic down a group)
Electron Gain Enthalpy	Becomes more negative (atoms more eager to gain electrons)	Becomes less negative (larger atoms less eager to gain electrons)

🧪 Examples of Periodicity

Alkali Metals (Group 1): Lithium → Sodium → Potassium show increasing reactivity down the group due to lower ionization energy.
Halogens (Group 17): Fluorine → Chlorine → Bromine show decreasing electronegativity and oxidizing power down the group.
Noble Gases (Group 18): All are chemically inert due to stable electronic configurations.

📚 Why It Matters

Predictive Power: Periodicity allows chemists to predict the properties of unknown or newly discovered elements.
Classification: It organizes elements into blocks (s, p, d, f) based on electron configuration.
Chemical Behavior: Helps explain why elements in the same group (like alkali metals or halogens) behave similarly.

⚠️ Challenges & Limitations

Anomalies: Some elements (like transition metals) show irregular trends due to complex electron configurations.
Lanthanides & Actinides: Their properties are less straightforward, requiring special treatment in the periodic table.

✅ In short, periodicity in properties is the backbone of modern chemistry—it transforms a seemingly random collection of elements into a logical, predictive framework. Would you like me to create a visual diagram of these trends across the periodic table to make it easier to grasp?

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