Unit 10: Radioactivity

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Atom: same number of protons and electrons → neutral.
Ion: gains or loses electrons.
- Gain electrons → negative ion.
- Lose electrons → positive ion.

The nucleus does not change in ion formation.

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Atoms consist of:
- Nucleus → very small, dense centre containing:
  - Protons (positive charge)
  - Neutrons (no charge)
- Electrons → orbit the nucleus in shells.
Most of the atom is empty space.
Size:
- Atom radius ≈ 1 × 10⁻¹⁰ m.
- Nucleus radius ≈ 1 × 10⁻¹⁵ m (100,000 times smaller)
Atomic Number (Z)

= number of protons

→ Determines the identity of the element.

→ In a neutral atom, it also equals the number of electrons.
Mass Number (A)

= number of protons + neutrons

→ Tells you the mass of the atom.

Example:

${}^{14}_{6}{C}$ = 6 protons, 8 neutrons

Particle	Relative Charge	Relative Mass	Location	Notes
Proton	+1	1	Nucleus	Determines atomic number
Neutron	0	1	Nucleus	Adds mass; stabilises nucleus
Electron	–1	1/2000	Shells	Responsible for chemical reactions

Isotopes:

Isotopes are atoms of the same element with:
- same number of protons
- different number of neutrons

Examples:

Properties:

Some isotopes are unstable, which makes them radioactive.

Aim: To investigate the structure of the atom by firing alpha particles at extremely thin gold foil.

	Findings	Conclusion
A	Most of the $\alpha$ particles passed through the foil	Atoms are mostly empty space
B	A few $\alpha$ particles were deflected from their path but continued through the foil	The nucleus of the atom has a strong positive charge
C	A small number of $\alpha$ particles rebounded	The atoms contains a small, heavy nucleus

This disproved the earlier plum pudding model and led to the nuclear model of the atom.