Atomic Structure
Models of the atom, electron configuration, periodic table
Models of the Atom
Our understanding of the atom evolved over time:
• Dalton — solid indivisible sphere
• Thomson — 'plum pudding' model (electrons in positive dough)
• Rutherford — nucleus with orbiting electrons
• Bohr — electrons in specific energy levels
• Modern/Quantum — electron cloud model
Subatomic Particles
An atom consists of:
• Protons (p⁺) — positive charge, in the nucleus
• Neutrons (n⁰) — no charge, in the nucleus
• Electrons (e⁻) — negative charge, orbiting in energy levels
Atomic number (Z) = number of protons
Mass number (A) = protons + neutrons
In a neutral atom: protons = electrons
Example
Electron Configuration
Sodium (Na): Z = 11
Electrons fill energy levels: 2 in level 1, 8 in level 2, 1 in level 3
Electron configuration: 2.8.1
Valence electrons (outer shell): 1 → sodium easily loses 1 electron to form Na⁺
Chlorine (Cl): Z = 17 → 2.8.7 → gains 1 electron to form Cl⁻
Note
The Periodic Table
Elements are arranged by increasing atomic number. Groups (columns) share the same number of valence electrons and similar properties. Periods (rows) show increasing energy levels. The periodic table was developed to organise elements logically.
Key Vocabulary
AtomThe smallest particle of an element that retains its properties
ProtonA positively charged particle in the nucleus
NeutronA neutral particle in the nucleus
ElectronA negatively charged particle orbiting the nucleus
Atomic numberThe number of protons in an atom's nucleus
Valence electronAn electron in the outermost energy level
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