Grade 10 general science Matter and Chemical Reactions – The Periodic Table Notes
Matter & Chemical Reactions — Subtopic: The Periodic Table
Specific learning outcomes (SLOs)
- a) Examine electron arrangement of an atom of an element.
- b) Classify elements into groups and periods of the periodic table.
- c) Explain the stability and electron affinity of atoms of elements.
- d) Learn formation for the first 20 elements of the periodic table.
- e) Formulate chemical formulae of common compounds.
- f) Write balanced equations of chemical reactions.
- g) Appreciate the importance of atomic structure in the development of the periodic table.
1. Quick recap: Atom structure and electron arrangement
The atom has a nucleus (protons and neutrons) and electrons arranged in shells (energy levels). The shells are commonly named K (1st), L (2nd), M (3rd), N (4th)… The maximum electrons per shell (simple model): K = 2, L = 8, M = 18 (but for first 20 elements M holds up to 8 before 3d fills).
Electron arrangement examples (shells):
- H (1): 1 → K: 1
- He (2): 2 → K: 2 (stable)
- Li (3): 2,1 → K:2, L:1
- O (8): 2,6 → K:2, L:6
- Na (11): 2,8,1 → K:2, L:8, M:1
2. The Periodic Table — groups, periods and types
Elements are arranged by increasing atomic number. Rows = periods (energy levels). Columns = groups (similar chemical properties because of the same number of valence electrons).
Important groups (examples):
- Group 1: Alkali metals (Li, Na) — 1 valence electron
- Group 2: Alkaline earth metals (Be, Mg) — 2 valence electrons
- Group 17: Halogens (F, Cl) — 7 valence electrons, very reactive
- Group 18: Noble gases (He, Ne, Ar) — full valence shell, very stable
Period rules:
- Period number = number of electron shells for main-group elements.
- Across a period: atomic size decreases, nuclear charge increases, ionization energy generally increases.
3. First 20 elements — table (atomic number, symbol, electron shells, configuration & group/period)
| Z | Element | Shells | Electron config | Group / Period / Type |
|---|---|---|---|---|
| 1 | H (Hydrogen) | 1 | 1s1 | Period 1 / nonmetal |
| 2 | He (Helium) | 2 | 1s2 | Group 18 / Period 1 / noble gas |
| 3 | Li (Lithium) | 2,1 | 1s2 2s1 | Group 1 / Period 2 / metal |
| 4 | Be (Beryllium) | 2,2 | 1s2 2s2 | Group 2 / Period 2 / metal |
| 5 | B (Boron) | 2,3 | 1s2 2s2 2p1 | Period 2 / metalloid |
| 6 | C (Carbon) | 2,4 | 1s2 2s2 2p2 | Period 2 / nonmetal |
| 7 | N (Nitrogen) | 2,5 | 1s2 2s2 2p3 | Period 2 / nonmetal |
| 8 | O (Oxygen) | 2,6 | 1s2 2s2 2p4 | Period 2 / nonmetal |
| 9 | F (Fluorine) | 2,7 | 1s2 2s2 2p5 | Group 17 / Period 2 / halogen |
| 10 | Ne (Neon) | 2,8 | 1s2 2s2 2p6 | Group 18 / Period 2 / noble gas |
| 11 | Na (Sodium) | 2,8,1 | 1s2 2s2 2p6 3s1 | Group 1 / Period 3 / metal |
| 12 | Mg (Magnesium) | 2,8,2 | ...3s2 | Group 2 / Period 3 / metal |
| 13 | Al (Aluminium) | 2,8,3 | ...3s2 3p1 | Period 3 / metal |
| 14 | Si (Silicon) | 2,8,4 | ...3s2 3p2 | Period 3 / metalloid |
| 15 | P (Phosphorus) | 2,8,5 | ...3s2 3p3 | Period 3 / nonmetal |
| 16 | S (Sulfur) | 2,8,6 | ...3s2 3p4 | Period 3 / nonmetal |
| 17 | Cl (Chlorine) | 2,8,7 | ...3s2 3p5 | Group 17 / Period 3 / halogen |
| 18 | Ar (Argon) | 2,8,8 | ...3s2 3p6 | Group 18 / Period 3 / noble gas |
| 19 | K (Potassium) | 2,8,8,1 | ...4s1 | Group 1 / Period 4 / metal |
| 20 | Ca (Calcium) | 2,8,8,2 | ...4s2 | Group 2 / Period 4 / metal |
4. Stability, valence electrons and electron affinity
Atoms are most stable when they have a full valence shell (the octet rule: eight electrons in outer shell for many main-group elements). Noble gases already have full shells and are inert.
- Electron affinity: tendency of an atom to gain an electron. Halogens (e.g., Cl) have high electron affinity — they easily gain one electron to form Cl−.
- Metals (left side) tend to lose electrons to become positive ions (cations). Nonmetals (right side) tend to gain electrons to become negative ions (anions).
- Atoms become more stable when forming ionic or covalent bonds to achieve noble-gas electron arrangements.
Example: Sodium (Na: 2,8,1) loses 1 electron → Na+ (2,8) (stable). Chlorine (Cl: 2,8,7) gains 1 electron → Cl− (2,8,8) (stable). Na+ and Cl− form ionic NaCl.
5. How to write formulae of common compounds (simple rules)
- For ionic compounds: combine ions so total positive charge = total negative charge. Write cation first, anion second. Simplify to smallest whole-number ratio.
- For covalent compounds (nonmetals): use prefixes or write element symbols and indicate bonding pairs (learn Lewis dot later).
Examples:
- Sodium chloride: Na+ and Cl− → NaCl
- Magnesium oxide: Mg2+ and O2− → MgO
- Calcium chloride: Ca2+ and Cl− → CaCl2 (Ca2+ + 2Cl− → CaCl2)
- Water: H (nonmetal) covalently bonds with O → H2O
- Carbon dioxide: C shares with O → CO2
6. Writing balanced chemical equations (simple examples)
Balance mass: number of atoms of each element must be same on both sides.
Examples:
- Formation of water (combustion-like): 2H2 + O2 → 2H2O
- Formation of sodium chloride: 2Na + Cl2 → 2NaCl
- Combustion of methane: CH4 + 2O2 → CO2 + 2H2O
- Magnesium + oxygen → magnesium oxide: 2Mg + O2 → 2MgO
7. Bonding visuals — simple dot-cross idea
Use dots (electrons of one atom) and crosses (electrons of the other) to show sharing/transfer.
Example — NaCl (ionic transfer):
Na• + ••Cl••••••• → Na+ [••Cl•••••••]−
Example — H2 (covalent sharing):
H• + •H → H: H (shared pair)
8. Fit to Kenyan curriculum & relevance for age 15
This content aligns with Kenyan secondary school science: identifying element properties, writing formulae and equations, and linking atomic structure to chemical behavior. Practical context: salts used in cooking (NaCl), building materials (Ca compounds), fertilizers (N, P, K), and indicators of water quality (Cl).
9. Suggested learning experiences (classroom & practical)
- Hands-on models: Use beads/ball-and-stick or clay to build Bohr-like models of elements (show shells and valence electrons for first 20 elements).
- Group activity: Give each group a set of element cards (Z, symbol, shells). Ask them to place elements into groups and periods and explain why.
- Flame tests (demonstration, teacher-led with safety): Flame colours for Na (yellow), K (lilac), Ca (brick red). Discuss link to electron excitation.
- Ionic vs covalent sorting game: Provide compound names and have learners classify and write formulae (NaCl, H2O, CO2, MgO, NH3).
- Balancing equations workshop: Provide unbalanced equations, learners balance and explain steps. Include real examples (combustion, formation of salts).
- Research task: How atomic structure led to the periodic table (Mendeleev’s ideas vs modern arrangement). Short presentation per group.
- Homework: Write electron arrangements and predict formulae for simple combinations (e.g., Al and O → ?).
Safety note: All practicals must follow school safety rules. Flame tests and chemical handling must be teacher-supervised with goggles and minimal reagents.
10. Classroom assessment ideas
- Short test: write electron arrangement for given elements (Z = 7, 11, 16), name group and period.
- Practical: construct a model of sodium chloride and explain electron transfer and stability.
- Worksheet: write formulae for magnesium oxide, calcium chloride, aluminum oxide (show ionic charges), and balance 4 equations.
- Project: Poster on the first 20 elements showing uses in Kenya (e.g., N and P in fertilisers, Ca in cement).
Quick reference — common formulae & balanced reactions
- Common formulae: NaCl, MgO, CaCl2, H2O, CO2, NH3, CH4
- Balanced reaction examples (for practice):
- 2H2 + O2 → 2H2O
- CH4 + 2O2 → CO2 + 2H2O
- 2Na + Cl2 → 2NaCl
- 2Mg + O2 → 2MgO