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.

Example in practice

Take chlorine, in period 3 and group 17. Its position tells you a great deal: seven valence electrons (group 17), so it needs one more to complete its octet, making it a reactive non-metal that forms a −1 ion. Being high and to the right, it has high electronegativity (about 3.16 on the Pauling scale) and a small atomic radius compared with sodium at the far left of the same period.

Common mistakes

A common misconception is reading the atomic mass as a whole number of protons; the mass is a weighted average of isotopes, while the atomic number alone counts protons. Another is assuming the group number always equals valence electrons — that holds for main-group elements but not the transition metals. Confusing periods (rows) with groups (columns) is a frequent beginner slip.

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.

Why are the lanthanides and actinides shown separately below the table?

They are the f-block elements, and placing them in their proper position would stretch the table very wide and make it hard to print. To keep it compact, the two rows are pulled out and shown beneath the main body, though chemically they belong between groups 2 and 3 in periods 6 and 7.