Grade 10 chemisty Inorganic Chemistry – Periodicity Notes

Periodicity — Inorganic Chemistry (Notes for age 15)

These notes follow the specific learning outcomes: trends in physical and chemical properties of Groups I, II, VII (halogens), VIII (noble gases) and across Period 3 (Na → Ar). Simple classroom activities and Kenyan-relevant examples are included.

Specific learning outcomes (short)

• a) Describe trends in physical properties in Groups I, II, VII, VIII and across Period 3: ionization energy, electron affinity, melting & boiling points, ductility, malleability, electrical conductivity.
• b) Investigate chemical properties of Group I & II (reaction with oxygen, chlorine, cold water, steam, dilute acids).
• c) Describe halogens (Cl, Br, I): physical & chemical properties, displacement reactions, bleaching action.
• d) Explain trends across Period 3.
• e) Outline applications of elements in Groups I, II, VII, VIII and Period 3.

Key terms (quick)

• Periodicity — repeating trends in properties across the periodic table.
• Ionization energy (IE) — energy to remove an electron from an atom (↑ across a period, ↓ down a group generally).
• Electron affinity (EA) — energy change when an atom gains an electron (more negative = greater tendency to gain e−).
• Atomic radius — size of atom (↓ across a period, ↑ down a group).
• Metallic character — how easily an atom loses electrons (↓ across a period, ↑ down a group).

Visual: Period 3 row (Na → Ar)

Na

Mg

Al

Si

P

S

Cl

Ar

General trends explained (simple)

• Across a period (left → right): atomic radius decreases, IE increases, electron affinity generally becomes more negative (non-metals want electrons), metallic character decreases.
• Down a group (top → bottom): atomic radius increases, IE decreases, elements become more metallic (easier to lose electrons), reactivity of metals increases, reactivity of halogens decreases.

Physical properties: what to expect

Focus properties: ionization energy, electron affinity, melting & boiling points, ductility, malleability, electrical conductivity.

• Metals (Groups I & II): generally good electrical conductors, malleable, ductile. As you go down a metal group, conductivity and ductility generally increase; melting points may drop (Group 1 metals have low mp that decrease down the group).
• Halogens (Group VII): exist as diatomic molecules (Cl2 gas, Br2 liquid, I2 solid), low electrical conductivity, poor ductility/malleability; reactivity decreases down the group.
• Noble gases (Group VIII): monoatomic gases, very low reactivity, very low conductivity, very low melting/boiling points (increase slightly down the group).

Trends across Period 3 (Na → Ar) — explained

• Atomic radius: decreases from Na to Ar (because rising nuclear charge pulls electrons closer).
• Ionization energy: increases across; Na has low IE, Ar highest in the period.
• Electron affinity: becomes more negative up to Cl (Cl has high tendency to gain e−); noble gas Ar has little tendency to gain electrons.
• Melting/boiling points: metals (Na, Mg, Al) have metallic bonding — melting points generally high (Al higher). Si (giant covalent) has very high melting point. P, S, Cl are molecular (low mp). Ar is a very low mp/boiling point gas.

Group I (alkali metals) — chemical behaviour and examples

Members: Li, Na, K, Rb, Cs (focus on Li, Na, K for school experiments).

• With cold water: M + H2O → MOH + H2 (vigorous; increases down group). Example: 2Na + 2H2O → 2NaOH + H2↑. Safety: only small pieces under teacher supervision; hydrogen is flammable.
• With steam: produce oxide and H2 (faster reaction than with cold water for some metals): 2Mg + O2 → 2MgO (example for group 2; group 1 often oxidise in air).
• With oxygen (air): metals form oxides (Li forms Li2O; Na forms Na2O/Na2O2; K can give K2O, KO2). Metals tarnish and burn to give oxide (bright flame for K/Na).
• With chlorine: 2Na + Cl2 → 2NaCl (salts formed; ionic solids).
• With dilute acids: metal + acid → salt + H2 (e.g., Mg + 2HCl → MgCl2 + H2). For Group 1: very fast and vigorous.

Group II (alkaline earth metals) — chemical behaviour

Members: Be, Mg, Ca, Sr, Ba (focus: Mg, Ca).

• Reactivity: less reactive than Group 1; reactivity increases down the group.
• With cold water: Mg reacts very slowly with cold water; Ca reacts with cold water producing Ca(OH)2 and H2 (Ca + 2H2O → Ca(OH)2 + H2). Mg reacts readily with steam to give MgO and H2.
• With oxygen: form oxides (2Mg + O2 → 2MgO; 2Ca + O2 → 2CaO).
• With chlorine: form ionic chlorides (MgCl2, CaCl2).
• With dilute acids: react to give salt + H2 (e.g., Ca + 2HCl → CaCl2 + H2).

Halogens (Group VII): Cl, Br, I — properties & reactions

• Physical states: Cl2 = greenish gas; Br2 = reddish-brown liquid; I2 = dark solid (sublimes to violet vapour).
• Reactivity: decrease down the group (Cl > Br > I). They gain electrons to form X− ions.
• Displacement reactions: a more reactive halogen displaces a less reactive halide from solution:
  • Cl2 + 2KBr → 2KCl + Br2 (Cl2 displaces Br−; solution turns brown/orange due to Br2).
  • Cl2 + 2KI → 2KCl + I2 (brown/black iodine formed).
• Bleaching action: chlorine reacts with water to form hydrochloric acid and hypochlorous acid:
  Cl2 + H2O ⇌ HCl + HClO (HClO = bleaching agent)
  Hypochlorite (ClO−, e.g., NaClO in household bleach) is the active bleaching/disinfecting species.

Group VIII (noble gases)

• Monoatomic gases (He, Ne, Ar...). Very low reactivity because of full outer shells. Used in lighting (neon signs, argon in bulbs), and in inert environments for welding.

Simple classroom investigations (suitable for age 15, follow safety rules)

1. Demonstrate displacement: Add small piece of chlorine solution (chlorine is dangerous—use dilute bleach solution and iodide/starch paper method) or demonstrate with safer halogen displacement using household bleach (NaClO) and potassium iodide solution—observe brown iodine and test with starch (blue-black).
2. Group 1 reaction (teacher demo only): drop a tiny piece of sodium on water under supervision (in a tray), observe fizzing and movement, then extinguish and discuss products (NaOH + H2). Students record observations instead of doing themselves.
3. Magnesium + steam demonstration (teacher demo): Mg ribbon heated and passed into steam — white oxide forms and H2 produced (test with burning splint).
4. Bleaching test: add a drop of household bleach to damp coloured paper or dyes to show bleaching (explain hypochlorite action).

Safety reminders:

• Always wear goggles and gloves. Reactions may produce heat, flammable hydrogen, or toxic gases (chlorine).
• Group 1 metals must be handled by teacher only. Do not perform open flames near hydrogen evolution.
• Dispose chemicals as instructed by teacher; never pour reactive metals into sink.

Applications (Kenyan context examples)

• Group I: Sodium (Na) in NaCl — table salt and food preservation; potassium (K) compounds (KCl, KNO3) in fertilizers (important for tea, maize, horticulture in Kenya).
• Group II: Calcium compounds (CaO, CaCO3) in cement and building (housing, roads); magnesium in alloys and agriculture (Mg in fertilisers).
• Halogens (Group VII): Chlorine used to disinfect water (important for safe drinking water in schools); bleach (NaClO) used for laundry and sanitation.
• Noble gases (Group VIII): Argon in welding and lighting (industry); neon in signage.
• Period 3 elements: Aluminium (lightweight cans, cooking foil, building), Silicon (electronics — chips and solar panels), Phosphorus (fertilisers — major agricultural use), Sulfur (sulphuric acid — industry), Chlorine (water treatment), Argon (light bulbs).

Quick summary table (trends)

Property	Across period (→)	Down group (↓)
Atomic radius	decreases	increases
Ionization energy	increases	decreases
Electron affinity	generally more negative (nonmetals)	less tendency to gain e−
Metallic character	decreases	increases

Revision questions (short)

1. Explain why ionization energy increases across Period 3.
2. Write an equation for magnesium reacting with steam and state observations.
3. Predict what happens when chlorine water is added to potassium bromide solution. Write the chemical equation.
4. List two Kenyan uses of calcium compounds.

Answers (brief)

1. Increased nuclear charge pulls electrons closer; atomic radius decreases, so more energy is needed to remove an electron.
2. Mg + H2O (steam) → MgO + H2↑. Observation: white oxide forms, H2 gas is produced (test with a lighted splint — pop sound).
3. Cl2 + 2KBr → 2KCl + Br2 — solution turns brown/orange as bromine forms (Cl displaces Br−).
4. Calcium oxide/calcium carbonate used in cement and building; calcium compounds used in making soil lime and animal feed supplements.

Use these notes to guide lessons and practicals. Teachers should lead demonstrations of the reactivity of alkali metals and halogen displacement reactions for safety. Encourage learners to connect element uses to local industries (agriculture, construction, water treatment).