Classification of elements Note

Science & Technology · Grade 10

Classification
of Elements

Chapter 14 · Periodic Table · Atomic Structure

14

Introduction

There are pure and impure matters around us — elements, compounds, and mixtures. Till now 118 elements have been discovered: 92 natural and 26 artificial.

Elements with similar properties are kept in one group; elements with different properties go into different groups.

📖 Periodic Table A scientific table for the study of elements by keeping elements of similar properties in the same group and different properties in different groups.

✏️ Activity 14.1 — Metals vs Non-metals

Classify: oxygen, iron, silver, sodium, chlorine, aluminium, carbon, sulphur, nitrogen

⚙️ Metals	🌿 Non-Metals
Iron (Fe), Silver (Ag), Sodium (Na), Aluminium (Al)	Oxygen (O), Chlorine (Cl), Carbon (C), Sulphur (S), Nitrogen (N)

Mendeleev's Periodic Table

Russian scientist Dmitri Mendeleev created the first periodic table based on atomic weight.

⚖️ Mendeleev's Periodic Law
"The physical and chemical properties of elements are the periodic functions of their atomic weight."

When elements are arranged in increasing order of atomic weight, properties recur periodically. Vertical columns = Groups · Horizontal rows = Periods.

⚠️ Problem with Mendeleev's table: Carbon has isotopes C-12, C-13, C-14 — separate spaces would be needed for each! Scientists later proved atomic weight is NOT the fundamental property of elements.

363–364

Henry Moseley & Modern Periodic Table

English scientist Henry Moseley (1913 AD) discovered that element properties depend on atomic number, not atomic weight.

⚛️ Modern Periodic Law
"The physical and chemical properties of elements are the periodic function of their atomic number."

Moseley's table = Modern Periodic Table (Long Form). Elements arranged in increasing atomic number. Same properties → same group (vertical); increasing atomic number → same period (horizontal).

~ ~ ~

Modern Periodic Table — Color-coded by Element Type

Alkali Metals

Alkaline Earth

Transition Metals

Other Metals

Metalloids

Non-metals

Halogens

Noble Gases

365–366

Characteristics of Modern Periodic Table

Period	No. of Elements	Nature
First	2	Very Short
Second	8	Short
Third	8	Short
Fourth	18	Long
Fifth	18	Long
Sixth	32	Very Long
Seventh	32	Very Long

• 1Elements arranged in increasing order of atomic number.
• 2There are 7 periods and 18 groups.
• 3Divided into 18 groups per the IUPAC system.
• 4Metals → left; Non-metals → right; Metalloids → middle.
• 5Alkali metals (IA), Alkaline earth metals (IIA), Transition metals (IIIB–IIB), Halogens (VIIA), Noble gases (Group 0/18).
• 6Lanthanides: La(57)–Lu(71). Actinides: Ac(89)–Lr(103). Both placed separately below main block.
• 7Transition metals: Groups IB–VIIB + VIIIB (10 columns), between metals and nonmetals.
• 8Elements classified as s, p, d, or f blocks based on sub-shell electronic configuration.

Metals, Nonmetals & Metalloids

⚙️ Metals

• Left side of table
• Groups IA–IIIA (except B)
• Good conductors of heat & electricity
• Alkali metals (IA): Li, Na, K — ns¹
• Alkaline earth (IIA): Mg, Ca — ns²
• Transition: Fe, Co, Ni, Au, Ag, Hg, Zn

🌿 Non-metals

• Right side of table
• Groups VA, VIA, VIIA, 0(18)
• Halogens (VIIA): F, Cl, Br, I
• Noble gases (0): He, Ne, Ar — inert
• Most reactive nonmetal: Fluorine

💎 Metalloids

• Middle of table
• Properties of both metals & non-metals
• Poor/semiconductors of electricity
• Examples: Si, Ge, Bi

367–373

Electronic Configuration — Sub-shells

Each orbit (shell) is divided into sub-shells. Sub-shells have orbitals where electrons live.

Sub-shells per Shell K → 1s  |  L → 2s, 2p  |  M → 3s, 3p, 3d  |  N → 4s, 4p, 4d, 4f

Sub-shell	Max Electrons
s	2
p	6
d	10
f	14

📐 Aufbau's Principle — Filling Order 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p …

Electronic Configuration of First 20 Elements

#	Element	Sym	K	L	M	N	Sub-shell Config
1	Hydrogen	H	1				1s¹
2	Helium	He	2				1s²
3	Lithium	Li	2	1			1s², 2s¹
4	Beryllium	Be	2	2			1s², 2s²
5	Boron	B	2	3			1s², 2s²2p¹
6	Carbon	C	2	4			1s², 2s²2p²
7	Nitrogen	N	2	5			1s², 2s²2p³
8	Oxygen	O	2	6			1s², 2s²2p⁴
9	Fluorine	F	2	7			1s², 2s²2p⁵
10	Neon	Ne	2	8			1s², 2s²2p⁶
11	Sodium	Na	2	8	1		1s², 2s²2p⁶, 3s¹
12	Magnesium	Mg	2	8	2		1s², 2s²2p⁶, 3s²
13	Aluminium	Al	2	8	3		1s², 2s²2p⁶, 3s²3p¹
14	Silicon	Si	2	8	4		1s², 2s²2p⁶, 3s²3p²
15	Phosphorus	P	2	8	5		1s², 2s²2p⁶, 3s²3p³
16	Sulphur	S	2	8	6		1s², 2s²2p⁶, 3s²3p⁴
17	Chlorine	Cl	2	8	7		1s², 2s²2p⁶, 3s²3p⁵
18	Argon	Ar	2	8	8		1s², 2s²2p⁶, 3s²3p⁶
19	Potassium	K	2	8	8	1	1s², 2s²2p⁶, 3s²3p⁶, 4s¹
20	Calcium	Ca	2	8	8	2	1s², 2s²2p⁶, 3s²3p⁶, 4s²

368–370

Periodic Trends

Lithium (Li)
K=2, L=1

Sodium (Na)
K=2, L=8, M=1

Chlorine (Cl)
K=2, L=8, M=7

A. Valency

In a period (left → right), valencies: 1, 2, 3, 4, 3, 2, 1, 0 (IA → VIIA → 0). All elements in a group share the same valency.

B. Atomic Size

Along a Period →

⬅ Decreases

More protons pull electrons in; shell count stays the same.

Down a Group ↓

⬇ Increases

Extra shell added with each new period down the group.

C. Electropositivity & Electronegativity

Electropositivity (metallic)

Decreases → period
Increases ↓ group

Tendency to lose valence electrons → form cations (+)

Electronegativity (non-metallic)

Increases → period
Decreases ↓ group

Tendency to gain electrons → form anions (−)

D. Chemical Reactivity

• Metals: decreases → period  |  increases ↓ group
• Non-metals: increases → period  |  decreases ↓ group
• Rightmost element in period = inert gas
• Most reactive metal: Cesium (Cs)  |  Most reactive nonmetal: Fluorine (F)

Third period: Na = most reactive metal · Cl = most reactive nonmetal · Ar = inert gas 🧪

374–376

✏️ Exercise — Questions & Answers

Section 1: Choose the Correct Option

a In which group do elements with electronic configuration 1s², 2s²2p³ lie?

(i) IIIA(ii) IIIB(iii) VA(iv) VB

✅ Answer: (iii) VA
Configuration 1s², 2s²2p³ has 5 valence electrons (2+3). The last electron enters the p sub-shell → p-block with 5 valence electrons → Group VA.

b Elements between groups IIA and IIIA are called?

(i) Alkali metals(ii) Transition metals(iii) Alkaline earth metals(iv) Rare Earth metals

✅ Answer: (ii) Transition metals
Groups IIIB to IIB (10 columns) lie between IIA and IIIA. These are d-block transition metals.

c Which nonmetal is the most reactive?

(i) Fluorine(ii) Chlorine(iii) Bromine(iv) Iodine

✅ Answer: (i) Fluorine
Fluorine has the smallest atomic size in group VIIA and highest electronegativity — it gains electrons most readily, making it the most reactive nonmetal.

d Which is the most reactive metal?

(i) Lithium(ii) Sodium(iii) Potassium(iv) Cesium

✅ Answer: (iv) Cesium
Cesium is the largest alkali metal listed. Going down group IA, atomic size increases → electrons lost more easily → Cesium is most reactive.

e Which group do inert gases belong to?

(i) 0 / 18(ii) IA(iii) VIIB(iv) VIIA

✅ Answer: (i) Group 0 (18)
Noble gases have completely filled valence shells (octet) — they are chemically inert. Placed in Group 0 or 18.

f Correct order of increasing reactivity of metals?

(i) Be < Mg < Ca(ii) Na < Li < K(iii) Mg < Al < Si(iv) C < O < N

✅ Answer: (i) Be < Mg < Ca
Going down group IIA, atomic size increases → electrons lost more easily → metal reactivity increases: Be < Mg < Ca.

g Among A, B, C, D shown in the periodic table, which can form acidic oxide?

✅ Answer: (iii) C
Acidic oxides are formed by nonmetals. Element C is positioned on the right side of the table (nonmetal region). Nonmetal oxides are acidic (e.g., CO₂, SO₂, P₂O₅).

378–379

Section 2: Give Reason

a Classification of elements is necessary.

With 118 elements of varying properties, classification groups similar elements together. This makes studying easier, helps predict properties of undiscovered elements, and organizes chemical knowledge systematically.

b Atomic size increases from top to bottom in a group.

Going down a group, each element gains a new shell. More shells → outermost electrons are farther from the nucleus → atomic size increases. (e.g., Li has K, L shells; Na has K, L, M shells → Na > Li)

c Atomic size decreases from left to right in a period.

In a period, the number of shells stays constant but nuclear charge (protons) increases. More protons attract electrons more strongly, contracting the atom. So atomic size decreases left → right.

d Hydrogen is a non-metal but kept with metals in the modern periodic table.

Hydrogen has 1 valence electron (1s¹) like alkali metals and can lose it to form H⁺. Although hydrogen is a nonmetal, its electronic similarity to Group IA elements places it there in the modern periodic table.

e Potassium is more reactive than sodium.

Both K and Na are in Group IA. K is below Na, so K has a larger atomic size (extra M and N shells). The valence electron of K is farther from the nucleus and is lost more easily → K is more electropositive and reactive.

f Fluorine is more reactive than chlorine.

Both are in Group VIIA. F is above Cl → smaller atomic size → higher electronegativity. F gains electrons more readily than Cl, making F the more reactive halogen.

g Metallic character decreases and non-metallic character increases across a period.

Across a period, nuclear charge increases while shell count remains the same. Electron-losing ability (metallic character) decreases because electrons are held tighter. Electron-gaining ability (non-metallic character) increases accordingly.

h Inert gases are kept in group 0 of the modern periodic table.

Noble gases (He, Ne, Ar, Kr, Xe, Rn) have completely filled valence shells (octet / duplet for He). This makes them highly stable — they do not participate in chemical reactions. Their valency = 0 → Group 0 (18).

i Atomic size decreases from Li to Ne (second period).

Li to Ne are all in Period 2 — all have 2 shells (K and L). Moving from Li(Z=3) to Ne(Z=10), the nuclear charge increases while shells stay the same. Increasing nuclear pull contracts the atoms → size decreases steadily.

379

Section 3: Write the Differences

a Group vs Period

Group	Period
Vertical column	Horizontal row
Same number of valence electrons	Same number of electron shells
18 groups in modern PT	7 periods in modern PT
Atomic size increases downward	Atomic size decreases left → right
Same valency throughout	Valency changes across the period

b Chlorine vs Sodium

Chlorine (Cl)	Sodium (Na)
Non-metal	Metal
Group VIIA, Period 3, Z=17	Group IA, Period 3, Z=11
7 valence electrons	1 valence electron
Gains 1e⁻ → Cl⁻ (electronegative)	Loses 1e⁻ → Na⁺ (electropositive)
Most reactive nonmetal in period 3	Most reactive metal in period 3
1s², 2s²2p⁶, 3s²3p⁵	1s², 2s²2p⁶, 3s¹

c Elements of Group IA vs Group VIIA

Group IA (Alkali Metals)	Group VIIA (Halogens)
Metals	Non-metals
1 valence electron (ns¹)	7 valence electrons (ns²np⁵)
Lose electrons — electropositive	Gain electrons — electronegative
Form cations (+)	Form anions (−)
Reactivity increases down the group	Reactivity decreases down the group
Li, Na, K, Rb, Cs	F, Cl, Br, I

d Electronegativity vs Electropositivity

Electronegativity	Electropositivity
Tendency to gain electrons	Tendency to lose electrons
Forms anions (−)	Forms cations (+)
Property of non-metals	Property of metals
Increases → period	Decreases → period
Decreases ↓ group	Increases ↓ group
Fluorine = highest	Cesium = highest

380

Section 4: Answer the Following

a State the modern periodic law.

Modern Periodic Law (Henry Moseley, 1913):
"The physical and chemical properties of elements are the periodic functions of their atomic number." When elements are arranged in increasing atomic number, their properties repeat periodically.

b How many groups and periods in the modern periodic table?

The modern periodic table has 18 groups (vertical columns) and 7 periods (horizontal rows).

c Write the position of sodium. Why is it an alkali metal?

Position: Group IA (1), Period 3, Atomic number 11.
Electronic config: 1s², 2s²2p⁶, 3s¹ (1 valence electron, ns¹)
Alkali metal because: It dissolves in water to form a strong alkali (NaOH). It readily loses 1 electron to form Na⁺, a property shared by all Group IA elements — called alkali metals.

d Write positions of alkali metals, alkaline earth metals, inert gases, transition metals, lanthanides and actinides.

Element Type	Position	Examples
Alkali Metals	Group IA (1)	Li, Na, K, Rb, Cs
Alkaline Earth Metals	Group IIA (2)	Be, Mg, Ca, Sr, Ba
Transition Metals	Groups IIIB–IIB (3–12)	Fe, Co, Ni, Cu, Zn
Noble/Inert Gases	Group 0 / 18	He, Ne, Ar, Kr, Xe, Rn
Lanthanides	Below Period 6 (Z = 57–71)	Ce, Nd, Sm … Lu
Actinides	Below Period 7 (Z = 89–103)	Th, U, Np … Lr

e Write the electronic configuration of sulphur and its position.

Sulphur (S), Z = 16
Shell-based: K=2, L=8, M=6
Sub-shell: 1s², 2s²2p⁶, 3s²3p⁴
Position: Period 3 (3 electron shells), Group VIA (6 valence electrons). Non-metal with valency 2.

f On which basis are group and period separated in the modern periodic table?

Group: Determined by the number of valence electrons. Elements with the same valence electron count are in the same group.
Period: Determined by the number of electron shells. Elements with the same number of shells are in the same period.
The overall arrangement is based on increasing atomic number.

g In which group do F, Cl, Br lie? Which is most reactive?

F, Cl, Br lie in Group VIIA (17) — the Halogens. All have 7 valence electrons and need 1 more to complete their octet.
Fluorine (F) is most reactive — smallest atomic size, highest electronegativity, gains electrons most readily.
Reactivity order: F > Cl > Br

h If you could improve the modern periodic table, what changes would you make?

1. Hydrogen placement: Give H its own group or place it at Group IVA, since it can both gain and lose 1 electron — unlike true alkali metals.
2. Integrate Lanthanides & Actinides into the main body (Periods 6 & 7) to show their actual position clearly.
3. Color coding: Add systematic colors for s, p, d, f blocks and element states (solid/liquid/gas) at room temperature.
4. More data: Include electronegativity values, common oxidation states, and everyday uses directly on each element's cell.

380

⭐ Quick Revision Summary

📌 Key Scientists Mendeleev → arranged by atomic weight → Mendeleev's Periodic Law
Henry Moseley (1913) → arranged by atomic number → Modern Periodic Law

📌 Structure of Modern Periodic Table 118 elements · 7 Periods · 18 Groups
Metals (left) · Metalloids (middle) · Non-metals (right)
Lanthanides (57–71) & Actinides (89–103) placed separately below

📌 Sub-shell Filling Order (Aufbau's Principle) 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p

📌 Periodic Trends Atomic Size: → decreases  |  ↓ increases
Electropositivity: → decreases  |  ↓ increases
Electronegativity: → increases  |  ↓ decreases
Metal reactivity: → decreases  |  ↓ increases
Non-metal reactivity: → increases  |  ↓ decreases

🎯 Most reactive metal = Cs (Cesium)  |  Most reactive nonmetal = F (Fluorine)
Noble gases = Group 0 · Valency = 0 · Don't react!
Halogens = Group VIIA · 7 valence e⁻ · Most reactive nonmetals

Summary