Periodic Table of Elements
Click any element to view properties. Filter by category or search.
Understanding the Periodic Table
The periodic table is the most important reference tool in chemistry. First published by Dmitri Mendeleev in 1869, it organizes all 118 known elements by atomic number and groups them by shared chemical properties. The table's structure reflects the quantum mechanical behavior of electrons, making it both a practical reference and a map of atomic structure.
How to Read the Periodic Table
Each element cell displays three key pieces of information: the atomic number (number of protons), the chemical symbol (one or two letters), and the atomic weight (weighted average of all naturally occurring isotopes, in atomic mass units).
Elements are arranged in 7 periods (horizontal rows) and 18 groups (vertical columns). Elements in the same group have the same number of valence electrons and exhibit similar chemical behavior. For example, the Group 18 noble gases (He, Ne, Ar, Kr, Xe, Rn) are all colorless, odorless gases that rarely form chemical bonds.
Periodic Trends
The table reveals systematic patterns in element properties:
- Atomic radius increases going down a group (more electron shells) and decreases going across a period (stronger nuclear charge pulls electrons closer).
- Ionization energy — the energy required to remove an electron — generally increases across a period and decreases down a group.
- Electronegativity measures how strongly an atom attracts bonding electrons. Fluorine (3.98) is the most electronegative element. Electronegativity increases across a period and decreases down a group.
- Metallic character increases toward the bottom-left of the table. Francium is the most metallic element, while fluorine is the least.
Element Categories
Elements are classified into categories based on their physical and chemical properties. Metals (about 75% of elements) are generally shiny, conductive solids that lose electrons to form positive ions. Nonmetals tend to gain electrons and include essential elements like carbon, nitrogen, and oxygen. Metalloids (silicon, germanium, arsenic, and others) have intermediate properties and are widely used as semiconductors.
The transition metals (Groups 3-12) are notable for their variable oxidation states, colored compounds, and catalytic properties. The lanthanides and actinides, shown separately below the main table, include the rare earth elements used in electronics and the radioactive elements used in nuclear applications.
Using This Tool
Click any element to view its full property card, including electron configuration, density, melting and boiling points, oxidation states, and discovery information. Use the search bar to find elements by name, symbol, or atomic number. Click a category in the legend to highlight all elements of that type. The molar mass calculator uses the same atomic weight data to compute molecular weights for any chemical formula.
Frequently Asked Questions
How is the periodic table organized?
The periodic table arranges all known chemical elements by increasing atomic number (the number of protons in the nucleus). Elements are placed into rows called periods and columns called groups.
- Periods (rows) — Each period represents a new electron shell being filled. Period 1 has 2 elements, periods 2 and 3 have 8, and periods 4 and 5 have 18 each.
- Groups (columns) — Elements in the same group share the same number of valence electrons, giving them similar chemical properties. For example, Group 1 elements (alkali metals) are all highly reactive metals.
- Blocks — The table is divided into s-block, p-block, d-block, and f-block regions based on which orbital is being filled.
This arrangement reveals periodic trends in properties such as atomic radius, ionization energy, electronegativity, and electron affinity.
What are the main element categories?
Elements are classified into several categories based on their physical and chemical properties:
- Alkali metals (Group 1) — Soft, highly reactive metals that form +1 ions. Examples: lithium, sodium, potassium.
- Alkaline earth metals (Group 2) — Reactive metals that form +2 ions. Examples: magnesium, calcium, barium.
- Transition metals (Groups 3-12) — Hard metals with variable oxidation states. Examples: iron, copper, gold.
- Metalloids — Elements with properties between metals and nonmetals. Examples: silicon, germanium, arsenic.
- Nonmetals — Poor conductors that tend to gain electrons. Examples: carbon, nitrogen, oxygen.
- Halogens (Group 17) — Highly reactive nonmetals that form -1 ions. Examples: fluorine, chlorine, bromine.
- Noble gases (Group 18) — Extremely unreactive gases with full valence shells. Examples: helium, neon, argon.
What is electronegativity and how does it change across the periodic table?
Electronegativity measures an atom's ability to attract shared electrons in a chemical bond. The Pauling scale is the most commonly used scale, where fluorine has the highest value at 3.98.
Electronegativity follows two clear trends:
- Increases across a period (left to right) — As nuclear charge increases and atomic radius decreases, atoms attract bonding electrons more strongly.
- Decreases down a group (top to bottom) — Additional electron shells increase the distance between the nucleus and bonding electrons, weakening the attraction.
Electronegativity differences between atoms determine bond character: small differences produce nonpolar covalent bonds, moderate differences produce polar covalent bonds, and large differences (typically greater than 1.7) produce ionic bonds.
What is electron configuration and why does it matter?
Electron configuration describes how electrons are distributed among the orbitals of an atom. It follows three key principles:
- Aufbau principle — Electrons fill orbitals starting from the lowest energy level (1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on).
- Pauli exclusion principle — Each orbital holds a maximum of two electrons with opposite spins.
- Hund's rule — Within a subshell, electrons occupy empty orbitals first before pairing up.
Electron configuration determines chemical behavior. Elements with similar valence electron configurations behave similarly, which is why elements in the same group share chemical properties. For example, all noble gases have completely filled valence shells, making them chemically inert.
Who created the periodic table?
Dmitri Mendeleev published the first widely recognized periodic table in 1869, arranging 63 known elements by atomic weight and grouping them by similar properties. His key insight was leaving gaps for elements not yet discovered — he predicted the properties of gallium, scandium, and germanium years before their discovery.
Henry Moseley refined the table in 1913 by organizing elements by atomic number rather than atomic weight, resolving inconsistencies in Mendeleev's arrangement. The modern periodic table contains 118 confirmed elements, with oganesson (element 118) being the most recently named in 2016.