📖 In-Depth Theory
Inside the Atom
Atoms are extremely small — radius approximately 1 × 10⁻¹⁰ m (0.1 nanometres).
Structure:
NUCLEUS at the centre — contains PROTONS and NEUTRONS.
ELECTRONS orbit the nucleus at different distances (energy levels/shells).
The nucleus is tiny compared to the whole atom:
Nuclear radius ≈ 1/10,000 of the atomic radius.
Most of the atom is empty space.
SIZE COMPARISON:
Atomic radius: ~1 × 10⁻¹⁰ m
Nuclear radius: ~1 × 10⁻¹⁴ m
If the nucleus were the size of a football (30 cm), the atom would be ~3 km across.
Properties of Subatomic Particles
Three subatomic particles:
PROTON:
Mass: 1 atomic mass unit (amu)
Charge: +1
Location: nucleus
NEUTRON:
Mass: 1 amu
Charge: 0 (neutral)
Location: nucleus
ELECTRON:
Mass: approximately 1/1836 amu (negligible)
Charge: −1
Location: shells (energy levels) around the nucleus
OVERALL CHARGE:
Number of protons = number of electrons in a neutral atom.
Positive protons balanced by negative electrons → atom has no overall charge.
MASS:
Almost all the mass is in the nucleus (protons + neutrons).
Electron mass is negligible.
Electron Shells and Energy Levels
Electrons occupy SHELLS (energy levels) at different distances from the nucleus.
Shells fill from the innermost outwards:
Shell 1 (closest to nucleus): maximum 2 electrons
Shell 2: maximum 8 electrons
Shell 3: maximum 8 electrons (for elements 1–20)
EXAMPLES:
Hydrogen (1 proton, 1 electron): 1 shell — 1 electron in shell 1
Carbon (6 protons, 6 electrons): 2 shells — 2 in shell 1, 4 in shell 2
Sodium (11 protons, 11 electrons): 3 shells — 2, 8, 1
ELECTRONS AND CHEMISTRY:
The arrangement of electrons determines an element's chemical properties.
Outer shell electrons are involved in chemical bonding.
Elements in the same group of the periodic table have the same number of outer shell electrons.
When an electron ABSORBS energy it moves to a higher shell (further from nucleus).
When it releases energy it moves to a lower shell — emitting radiation (light or other EM waves).
⚠️ Common Mistake
The nucleus contains protons AND neutrons — NOT electrons. Electrons orbit the nucleus in shells. The mass of an atom comes almost entirely from the nucleus (protons + neutrons) — electron mass is negligible.
📌 Key Note
Atom: nucleus (protons + neutrons) + electrons in shells. Proton: +1, mass 1. Neutron: 0, mass 1. Electron: −1, negligible mass. Protons = electrons in neutral atom. Nuclear radius ≈ 1/10,000 atomic radius — mostly empty space. Shells: 2, 8, 8...
🎯 Test Yourself
Question 1 of 2
1. Where is most of the mass of an atom concentrated?
On the outer electron shell — outermost electrons carry most of the mass
In the nucleus — protons and neutrons make up almost all the atomic mass; electrons are negligible
In the electron shells — electrons are the most numerous particles
Evenly spread throughout the atom — mass is uniformly distributed
2. A neutral atom has 8 protons. How many electrons does it have?
8 — neutral atoms always have equal numbers of protons and electrons
0 — electrons are not part of the neutral atom
4 — only half the protons are balanced by electrons
16 — electrons come in pairs so there are twice as many