Science & Engineering
Interactive Periodic Table
Browse all 118 elements with their atomic number, symbol, name, category, and atomic mass. Search by name, symbol, or category.
How to read the periodic table
Each element cell typically shows:
- Atomic number: the number of protons in the nucleus (defines the element)
- Symbol: 1–3 letter abbreviation (H, He, Li, Na, Fe...)
- Atomic mass: weighted average of naturally occurring isotopes
Organization
- Periods (rows): elements in the same period have the same number of electron shells.
- Groups (columns): elements in the same group have the same number of valence electrons and similar chemical properties.
- Blocks: s, p, d, f blocks correspond to which subshell the outermost electrons occupy.
Special groups
- Group 1 (Alkali metals): highly reactive; 1 valence electron
- Group 17 (Halogens): very reactive nonmetals; form salts with metals
- Group 18 (Noble gases): largely inert; complete outer electron shells
Periodic trends
Several properties change predictably across rows and columns:
| Property | Trend across a period (->) | Trend down a group (↓) |
|---|---|---|
| Atomic radius | Decreases (more protons pull electrons closer) | Increases (more electron shells) |
| Ionization energy | Increases (harder to remove electrons) | Decreases (outer electrons are farther from nucleus) |
| Electronegativity | Increases (Fluorine is highest at 3.98) | Decreases |
| Metallic character | Decreases (metals on left, nonmetals on right) | Increases (heavier elements tend to be more metallic) |
Discovery timeline highlights
- 1669: Phosphorus - first element isolated by a known individual (Hennig Brandt)
- 1869: Dmitri Mendeleev publishes the first periodic table, predicting gaps for undiscovered elements
- 1898: Pierre and Marie Curie discover Polonium and Radium
- 1940: Plutonium synthesized - first transuranic element (Glenn Seaborg's team)
- 2016: Four new elements (113–118) officially named: Nihonium, Moscovium, Tennessine, Oganesson
Reading atomic mass
Atomic mass (also called atomic weight) is a weighted average of all naturally occurring isotopes of an element. For example, chlorine has two stable isotopes: ³⁵Cl (75.77%) and ³⁷Cl (24.23%), giving an average atomic mass of 35.45 u. For radioactive elements with no stable isotopes (like Technetium or all elements above Bismuth), the mass number of the most stable or best-known isotope is listed in brackets, e.g. [98] for Technetium.