ICSE Class 10 Chemistry • Chapter 01
Modern Periodic Law (Moseley, 1913): "The physical and chemical properties of elements are the periodic functions of their atomic numbers."
Atomic Number (Z): Number of protons in the nucleus of an atom.
| Feature | Description |
|---|---|
| Periods | 7 horizontal rows (Period 1-7) |
| Groups | 18 vertical columns (Group 1-18) |
| Period Number | = Number of shells in the atom |
| Group Number | = Number of valence electrons (for main group elements) |
| Category | Groups | Characteristics |
|---|---|---|
| Alkali Metals | Group 1 (Li, Na, K, Rb, Cs, Fr) | 1 valence electron, very reactive, form +1 ions |
| Alkaline Earth Metals | Group 2 (Be, Mg, Ca, Sr, Ba, Ra) | 2 valence electrons, reactive, form +2 ions |
| Halogens | Group 17 (F, Cl, Br, I, At) | 7 valence electrons, very reactive, form -1 ions |
| Noble Gases | Group 18 (He, Ne, Ar, Kr, Xe, Rn) | 8 valence electrons (2 for He), inert/stable |
| Transition Metals | Groups 3-12 | Variable valency, colored compounds |
Definition: Half the distance between the nuclei of two adjacent atoms of the same element bonded together.
Trend Across Period (→): DECREASES
Reason: Same shell, but nuclear charge increases → electrons pulled closer to nucleus.
Trend Down Group (↓): INCREASES
Reason: New shells added → outermost electrons farther from nucleus.
Example: Arrange in increasing order of atomic size: Na, Mg, Al, Si
All are in Period 3. Atomic size decreases across a period.
Order: Si < Al < Mg < Na
Definition: Tendency of an atom to lose electrons and form positive ions (cations).
Trend Across Period (→): DECREASES
Reason: Smaller size, higher nuclear pull → harder to lose electrons.
Trend Down Group (↓): INCREASES
Reason: Larger size, weaker nuclear pull → easier to lose electrons.
Definition: Tendency of an atom to gain electrons and form negative ions (anions).
Trend Across Period (→): INCREASES
Reason: Smaller size, need fewer electrons to complete octet.
Trend Down Group (↓): DECREASES
Reason: Larger size, less attraction for incoming electrons.
Definition: Energy required to remove the outermost electron from an isolated gaseous atom.
$M(g) + \text{Energy} \rightarrow M^+(g) + e^-$
Unit: kJ/mol or eV
Trend Across Period (→): INCREASES
Reason: Smaller size → electrons held more tightly → more energy needed.
Trend Down Group (↓): DECREASES
Reason: Larger size → outermost electron loosely held.
Higher Ionisation Energy = Less Metallic Character
Noble gases have the highest ionisation energy in each period (stable octet).
Definition: Energy released when an electron is added to an isolated gaseous atom.
$X(g) + e^- \rightarrow X^-(g) + \text{Energy}$
Trend Across Period (→): INCREASES (becomes more negative)
Reason: Smaller size → greater attraction for incoming electron.
Trend Down Group (↓): DECREASES
Reason: Larger size → less attraction for additional electron.
Exception: Chlorine has higher electron affinity than Fluorine!
Reason: Fluorine is so small that the incoming electron faces strong repulsion from existing electrons in the compact 2p orbital.
Definition: The tendency of an atom to attract the shared pair of electrons towards itself in a covalent bond.
Most Electronegative Element: Fluorine (4.0 on Pauling scale)
Trend Across Period (→): INCREASES
Trend Down Group (↓): DECREASES
| Property | Across Period (→) | Down Group (↓) |
|---|---|---|
| Atomic Size | Decreases | Increases |
| Metallic Character | Decreases | Increases |
| Non-metallic Character | Increases | Decreases |
| Ionisation Energy | Increases | Decreases |
| Electron Affinity | Increases | Decreases |
| Electronegativity | Increases | Decreases |
Memory Trick: "MAID" → Metallic character and Atomic size Increase going Down!
Everything else (Non-metallic, IE, EA, EN) does the opposite.
Periodicity: The repetition of similar properties at regular intervals when elements are arranged by increasing atomic number.
Cause: Similar electronic configurations of outermost shell after regular intervals.
Example: Li (2,1), Na (2,8,1), K (2,8,8,1) all have 1 valence electron → similar properties.
| Term | Definition |
|---|---|
| Atomic Radius | Half the internuclear distance between two bonded atoms |
| Ionisation Energy | Energy to remove outermost electron from gaseous atom |
| Electron Affinity | Energy released when electron is added to gaseous atom |
| Electronegativity | Tendency to attract shared electrons in a bond |
| Valence Electrons | Electrons in the outermost shell |
BOARD State the variation in the following properties across a period and down a group: (i) Ionisation potential (ii) Electron affinity
BOARD Arrange in order of increasing atomic size: Li, Na, K, Rb. Give reason.
HOTS Why is the atomic radius of sodium greater than magnesium even though both are in the same period?