Periodic Table

What is the periodic table?

The periodic table organizes all 118 known chemical elements by increasing atomic number (number of protons), arranged in rows (periods) and columns (groups) that reveal patterns in chemical properties. Elements in the same group have similar chemical behavior because they share the same number of valence electrons.

Dmitri Mendeleev published the first widely recognized periodic table in 1869, predicting the existence and properties of several undiscovered elements based on gaps in his arrangement. The modern table has been refined with quantum mechanical understanding but retains Mendeleev's fundamental insight that element properties recur periodically.

Table structure

• Periods (rows): Elements in the same period have the same number of electron shells. Period 1 has 2 elements, periods 2-3 have 8, period 4-5 have 18, and periods 6-7 have 32.
• Groups (columns): Elements in the same group have the same number of valence electrons and similar chemistry. Group 1 (alkali metals) are highly reactive; Group 18 (noble gases) are nearly inert.
• Blocks: The table is divided into s-block (groups 1-2), p-block (groups 13-18), d-block (transition metals), and f-block (lanthanides and actinides) based on which orbital the last electron fills.

How to use this tool

Click on any element to see detailed information: atomic number, atomic mass, electron configuration, electronegativity, ionization energy, common oxidation states, melting and boiling points, density, and discovery year. Use the color-coding to visualize element categories, blocks, or property trends.

Periodic trends

Several properties change predictably across the table. Atomic radius increases down a group and decreases across a period. Electronegativity (tendency to attract electrons) increases across a period and up a group, with fluorine being the most electronegative element. Ionization energy (energy to remove an electron) follows a similar pattern, increasing across and up.

Frequently asked questions

How many elements are there?

As of 2024, 118 elements have been confirmed and named. Elements 1-94 occur naturally (though some, like technetium and promethium, are extremely rare). Elements 95-118 are synthetic, created in particle accelerators. Elements beyond 118 are theoretically possible but have not yet been synthesized.

Why are lanthanides and actinides shown separately?

The lanthanides (elements 57-71) and actinides (89-103) are placed below the main table purely for layout convenience. If included in their proper positions, the table would be 32 columns wide, which is impractical for printing and display. They belong in period 6 and period 7 respectively, between groups 3 and 4.