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Introduction & Structure
Introduction & Structure
The periodic table is one of the most powerful tools in all of science. It organises every known element — from the lightest gas to the heaviest synthetic atom — into a single, elegant framework that reveals patterns in behaviour, bonding, and reactivity. Understanding how to read the periodic table is the first step toward mastering chemistry.
A Brief History
- 1789 — Antoine Lavoisier published the first list of 33 chemical elements, grouped into gases, metals, non-metals, and earths
- 1829 — Johann Wolfgang Döbereiner noticed "triads" of elements with similar properties (e.g. Li, Na, K)
- 1862 — Alexandre-Émile Béguyer de Chancourtois arranged elements by atomic weight on a spiral, the first geometric representation
- 1864 — John Newlands proposed the "Law of Octaves", noting repeating properties every eight elements
- 1869 — Dmitri Mendeleev published his periodic table, arranged by atomic weight, famously leaving gaps for undiscovered elements
- 1871 — Mendeleev predicted the properties of gallium (eka-aluminium), scandium (eka-boron), and germanium (eka-silicon) — all later confirmed
- 1913 — Henry Moseley determined that atomic number, not atomic weight, is the true organising principle
- 2016 — Elements 113 (Nh), 115 (Mc), 117 (Ts), and 118 (Og) were officially named, completing period 7
How the Table is Organised
Groups (Columns)
The periodic table has 18 groups (vertical columns). Elements in the same group share similar chemical properties because they have the same number of electrons in their outermost shell (valence electrons).
| Group | Name | Valence Electrons | Example Elements |
|---|---|---|---|
| 1 | Alkali metals | 1 | Li, Na, K |
| 2 | Alkaline earth metals | 2 | Be, Mg, Ca |
| 3–12 | Transition metals | Variable | Fe, Cu, Zn |
| 13 | Boron group | 3 | B, Al, Ga |
| 14 | Carbon group | 4 | C, Si, Ge |
| 15 | Pnictogens | 5 | N, P, As |
| 16 | Chalcogens | 6 | O, S, Se |
| 17 | Halogens | 7 | F, Cl, Br |
| 18 | Noble gases | 8 (2 for He) | He, Ne, Ar |
Periods (Rows)
The table has 7 periods (horizontal rows). Each period corresponds to the highest energy level (principal quantum number) occupied by electrons in the ground-state atoms of those elements.
- Period 1 — 2 elements (H, He)
- Period 2 — 8 elements (Li to Ne)
- Period 3 — 8 elements (Na to Ar)
- Period 4 — 18 elements (K to Kr)
- Period 5 — 18 elements (Rb to Xe)
- Period 6 — 32 elements (Cs to Rn), including the lanthanides
- Period 7 — 32 elements (Fr to Og), including the actinides
How to Read an Element's Entry
Each element box on the periodic table typically shows:
- Atomic number — the number of protons in the nucleus (top of the box)
- Chemical symbol — one or two letters, e.g. Fe for iron, Au for gold
- Element name — the full name
- Relative atomic mass — the weighted average mass of all naturally occurring isotopes
For example, carbon:
| 6 |
|---|
| C |
| Carbon |
| 12.011 |
The atomic number uniquely identifies each element. No two elements share the same atomic number.
Metals, Non-Metals, and Metalloids
The periodic table can be broadly divided into three categories:
Metals
- Found on the left-hand side and in the centre of the table
- Good conductors of heat and electricity
- Malleable (can be hammered into shape) and ductile (can be drawn into wire)
- Tend to lose electrons to form positive ions (cations)
- Include alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides
Non-Metals
- Found on the upper right-hand side of the table
- Poor conductors of heat and electricity (insulators)
- Brittle as solids, many are gases at room temperature
- Tend to gain electrons to form negative ions (anions)
- Include hydrogen, carbon, nitrogen, oxygen, the halogens, and the noble gases
Metalloids (Semi-Metals)
- Found along the staircase line separating metals from non-metals
- Have properties intermediate between metals and non-metals
- Many are semiconductors, making them essential in electronics
- Include boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), and polonium (Po)
Why the Periodic Table Matters
The arrangement of the periodic table allows chemists to:
- Predict the properties of elements they have not yet studied in detail
- Understand why certain elements react vigorously while others are inert
- Classify elements into families with shared characteristics
- Design new materials, medicines, and technologies by understanding elemental behaviour
The periodic table is not merely a chart on a classroom wall — it is a map of the building blocks of the entire universe.