Recap: Shells, Outermost Electrons and Charges
Every bond in this chapter is made by the outermost electrons (ബാഹ്യതമ ഇലക്ട്രോണുകൾ) of atoms. They are given away, taken or shared. So you must be able to write the electron configuration (ഇലക്ട്രോൺ വിന്യാസം) of any element from Z = 1 to 20 in a few seconds and read off its outermost electrons. You also need to work out the charge of a particle when it loses or gains electrons, and to know that opposite charges attract. This page revises all of that and gives you a first look at ions (അയോൺ).
- Elements, symbols, metals and non-metals (page 1).
- Nothing else: Chapter 1 (Structure of the Atom) and Chapter 2 (Periodic Table) are revised here from the start.
1. Inside an atom
| Particle | Where it is | Charge | Mass (approx.) |
|---|---|---|---|
| Proton പ്രോട്ടോൺ (p) | in the nucleus | +1 | 1 u |
| Neutron ന്യൂട്രോൺ (n) | in the nucleus | 0 | 1 u |
| Electron ഇലക്ട്രോൺ (e⁻) | in shells around the nucleus | −1 | about 1/1837 u |
- Atomic number (അറ്റോമിക നമ്പർ) Z = number of protons. It decides the element: every atom with 11 protons is sodium.
- A neutral atom has as many electrons as protons, so the charges cancel. Sodium: 11 protons (+11) and 11 electrons (−11) → charge 0.
- In the textbook's orbit drawings the nucleus is labelled like 11p 12n: 11 protons and 12 neutrons.
2. Shells and electron configuration
Electrons move around the nucleus in shells K, L, M, N (n = 1, 2, 3, 4). For the first 20 elements three rules give every configuration:
- Fill the shells from the inside out: K, then L, then M, then N.
- A shell holds at most 2n² electrons: K 2, L 8, M 18, N 32.
- The outermost shell (ബാഹ്യതമ ഷെൽ) holds at most 8 (K, when it is the only shell, holds 2). That is why potassium is 2, 8, 8, 1 and not 2, 8, 9.
The outermost shell is drawn in dark green. The neutron number shown is that of the most common atom of the element.
3. Outermost electrons and the groups
The last number of a configuration is the number of outermost electrons. Elements in the same group of the periodic table have the same number of outermost electrons, so they behave alike. These groups appear again and again in this chapter:
| Group | Outermost electrons | Examples (Z ≤ 20) | Kind |
|---|---|---|---|
| 1 | 1 | (H 1), Li 2,1 · Na 2,8,1 · K 2,8,8,1 | metals (hydrogen is a non-metal) |
| 2 | 2 | Be 2,2 · Mg 2,8,2 · Ca 2,8,8,2 | metals |
| 13 | 3 | B 2,3 · Al 2,8,3 | metalloid, metal |
| 14 | 4 | C 2,4 · Si 2,8,4 | non-metal, metalloid |
| 15 | 5 | N 2,5 · P 2,8,5 | non-metals |
| 16 | 6 | O 2,6 · S 2,8,6 | non-metals |
| 17 | 7 | F 2,7 · Cl 2,8,7 (halogens) | non-metals |
| 18 | 8 (He 2) | He 2 · Ne 2,8 · Ar 2,8,8 (noble gases) | non-metals |
Metals (groups 1, 2, 13) have few outermost electrons: 1, 2 or 3. Non-metals of groups 15–17 have many: 5, 6 or 7. The noble gases have a full outermost shell. This difference decides who gives, who takes and who shares electrons in a bond.
4. Charge = protons − electrons: ions
Protons sit deep in the nucleus and never change in a chemical reaction. Only outermost electrons can leave or arrive. When they do, the numbers no longer balance and the particle carries a charge:
charge = (number of protons) − (number of electrons)
- Sodium atom: 11 p, 11 e → charge 0. If it loses 1 electron: 11 p, 10 e → charge +1, written Na⁺.
- Chlorine atom: 17 p, 17 e → charge 0. If it gains 1 electron: 17 p, 18 e → charge −1, written Cl⁻.
A charged particle formed in this way is an ion (അയോൺ). A positive ion is a cation (കാറ്റയോൺ); a negative ion is an anion (ആനയോൺ). The charge is written at the top right: Mg²⁺ means a charge of +2 (2 electrons lost), O²⁻ a charge of −2 (2 electrons gained). The textbook draws an ion inside square brackets with its charge outside at the top right.
Try sodium (remove 1), magnesium (remove 2), chlorine (add 1) and oxygen (add 2). Each time the configuration becomes that of a noble gas. That is no accident – page 3 explains it. The lab lets you remove or add up to 3 electrons.
Your turn. Fill in the charge and the symbol of each particle. (Write + or − with the number, e.g. +2, and Mg2+ for Mg²⁺.)
5. What does “stable” mean?
An atom whose outermost shell is full – 8 electrons, or 2 when K is the only shell – is very stable (സ്ഥിരത): it has no tendency to lose, gain or share electrons. The noble gases helium (2), neon (2, 8) and argon (2, 8, 8) are like this. All other atoms try to reach such an arrangement by combining with other atoms – that is the story of page 3.
6. Opposite charges attract
Charges push and pull on each other without touching. This is called electrostatic force. Opposite charges attract; like charges repel. The force is stronger when the charges are bigger and when they are closer. You have seen it: a plastic comb rubbed in dry hair picks up bits of paper. In the chapter this force holds a Na⁺ ion and a Cl⁻ ion together in common salt.
Arrow length shows the size of the force, which grows as (charge × charge) ÷ distance². Halve the distance and the force becomes four times bigger. Distances here are in pixels, just for comparing.
- Z = protons = electrons in a neutral atom. Shells fill K → L → M → N; the outermost shell holds at most 8.
- The last number of the configuration = outermost electrons. Groups 1, 2, 13 have 1, 2, 3; groups 15, 16, 17 have 5, 6, 7; group 18 has 8 (He 2).
- Charge = protons − electrons. Losing electrons → positive ion, a cation (കാറ്റയോൺ); gaining → negative ion, an anion (ആനയോൺ). Protons never change.
- A full outermost shell (8, or 2 for He) is stable (സ്ഥിരത). Opposite charges attract; like charges repel.