Recap: Ions, Valency & Electronegativity സംയോജകത
Redox reactions are all about electrons moving from one atom to another. In Chapter 3 you saw sodium give an electron to chlorine and magnesium give two electrons to oxygen. This page brings back exactly the ideas that Chapter 4 builds on: what an ion is and how its charge is found, how electrons are transferred in NaCl and MgO, valency, why metals lose and non-metals gain electrons, how electronegativity decides where a shared pair goes, and how to write formulae from the charges of ions.
- Electron configurations up to calcium (2, 8, 8, 2) and the octet idea from Chapter 3.
- Counting atoms in formulae (page 1).
1. Ions and their charges
An atom has equal numbers of protons (+) and electrons (−), so it is neutral. If it loses electrons it has more protons than electrons and becomes a positive ion (a cation, കാറ്റയോൺ). If it gains electrons it becomes a negative ion (an anion, ആനയോൺ). Only electrons move – the protons in the nucleus never change.
2. Electron transfer in NaCl and MgO
Sodium (2, 8, 1) has one electron more than an octet; chlorine (2, 8, 7) is one short. Magnesium (2, 8, 2) has two extra; oxygen (2, 6) is two short. So electrons move from the metal to the non-metal, and the ions formed attract each other: an ionic bond (അയോണിക ബന്ധനം).
Look at the two half-equations again: Na → Na⁺ + e⁻ (an electron is lost) and Cl + e⁻ → Cl⁻ (an electron is gained). In Chapter 4 these get names: losing electrons is oxidation (ഓക്സീകരണം) and gaining electrons is reduction (നിരോക്സീകരണം).
3. Which atoms lose and which gain?
Atoms with 1, 2 or 3 outermost electrons – the metals (ലോഹങ്ങൾ) – find it easier to lose them. Atoms with 5, 6 or 7 – the non-metals (അലോഹങ്ങൾ) – find it easier to gain the few they are short of. Carbon and silicon (4) usually share. Noble gases already have an octet.
Hydrogen is special: it usually shares its one electron (H₂, HCl, H₂O); in acids it is shown as H⁺. Beryllium often shares too (higher classes).
4. Valency
The valency (സംയോജകത) of an element is the number of electrons one of its atoms gives, takes or shares when it combines. Fill in the table – it is the same idea as Table 4.6 on page 7.
Sodium and chlorine both have valency 1, but sodium loses an electron and chlorine gains one. The valency alone cannot tell you which. That is why Chapter 4 introduces the oxidation number (page 7), which carries a sign: Na +1, Cl −1.
5. Electronegativity – who gets the shared pair?
When two non-metals share a pair of electrons (a covalent bond, സഹസംയോജകബന്ധനം), the atoms may not share equally. Electronegativity (ഇലക്ട്രോനെഗറ്റിവിറ്റി) is the ability of an atom in a bond to attract the shared pair towards itself (Pauling's scale: F 3.98 is the highest). The more electronegative atom gets a slight negative charge (δ−) and the other a slight positive charge (δ+). If the difference is 1.7 or more, the pair is usually pulled across completely and the bond is ionic (this is a rough guide: HF, with a difference of 1.78, is still a polar covalent molecule).
To find oxidation numbers in covalent compounds (page 7) the book pretends every bond is ionic: the shared pair is given completely to the more electronegative atom. In HF, fluorine gets the pair → F −1, H +1.
6. Writing formulae from the charges of ions
A compound is electrically neutral, so the total positive charge must equal the total negative charge. A quick way is the criss-cross method: the number of the charge of each ion becomes the subscript of the other; then cancel any common factor. Groups of atoms that act as one ion (radicals such as OH⁻ and SO₄²⁻) go in brackets when there is more than one.
- Charge = protons − electrons. Losing electrons → positive ion (cation, കാറ്റയോൺ); gaining → negative ion (anion, ആനയോൺ).
- NaCl: Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻. MgO: Mg → Mg²⁺ + 2e⁻ and O + 2e⁻ → O²⁻.
- Metals lose electrons (1, 2 or 3); non-metals gain them (to reach 8).
- Valency (സംയോജകത) is a number of electrons with no sign – it does not say lose or gain.
- In a shared bond the more electronegative atom pulls the pair towards itself (δ−).
- Total charge in a compound = 0: Mg²⁺ + 2Cl⁻ → MgCl₂; 2Al³⁺ + 3O²⁻ → Al₂O₃.