Atoms and Elements

What is an atom?  

- The smallest particle of matter that cannot be broken further.  

- Too small to be seen individually.  

- Mostly space, with a nucleus and electrons moving around it.  

Elements  

- An element contains only one kind of atom.  

- 118 elements known (90 natural, rest man‑made).  

- Each element has a symbol for ease of use.  

Structure of the Atom  

- Nucleus → cluster of protons (+) and neutrons (no charge).  

- Electrons (–) move around the nucleus.  

- These are called subatomic particles.  

- Measured in atomic mass units (amu).  

Key numbers:  

- Proton number (atomic number): number of protons.  

- Nucleon number (mass number): protons + neutrons.  

- Example: Sodium → 11 protons, 12 neutrons → nucleon number = 23.  

Isotopes & Radioactivity  

- Isotopes: atoms of the same element with the same protons but different neutrons.  

  - Example: Carbon‑12, Carbon‑13, Carbon‑14.  

- Radioisotopes: unstable isotopes that decay, releasing radiation (alpha, beta, gamma).  

  - Examples: Carbon‑14, Cobalt‑60, Potassium‑40.  

Uses:  

- Leak detection in pipes (Geiger counter).  

- Radiotherapy (Cobalt‑60 kills cancer cells).  

- Sterilisation of medical equipment.  

- Food preservation (spices, fruits).  

- Carbon dating (age of remains).  

- Nuclear power fuel.  

Harmful effects: 

- Radiation kills body cells → vomiting, hair loss, bleeding gums, cancer risk.  

Electron Arrangement  

- Electrons are arranged in shells around the nucleus.  

- Rule: (2n^2) electrons per shell.  

- Example: Sodium → (2,8,1).  

- Period number = number of shells.  

- Group number = number of outer electrons.  

- Group VIII (0 group) → stable, unreactive (noble gases).  

Metals vs Non‑Metals  

Metals:  

- Good conductors of heat/electricity.  

- High melting/boiling points (solid at room temperature, except mercury).  

- Hard, strong, malleable, ductile, shiny, sonorous.  

- Form positive ions, react with oxygen → basic oxides.  

Non‑Metals:  

- Poor conductors (except graphite).  

- Low melting/boiling points (often gases).  

- Brittle, dull, low density.  

- Form negative ions, react with oxygen → acidic oxides.  

Exceptions:  

- Mercury → liquid metal.  

- Sodium/potassium → soft metals.  

- Graphite → non‑metal but conducts electricity.  

- Diamond → hardest material, high melting point.  

- Hydrogen → non‑metal but forms a positive ion.  

Uses  

- Metals: copper (wires), aluminium (planes, rockets), iron/steel (structures, vehicles).  

- Non‑metals: oxygen & nitrogen (air), hydrogen & oxygen (water), carbon compounds (life), silicon dioxide (sand).