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ELEMENTS

Periodic table · 118 elements · 7 periods · 18 groups

Click an element to open its card: data, chemical compounds and a fun fact.

How to read the periodic table

The elements are arranged by increasing atomic number Z — the number of protons in the nucleus, equal to the number of electrons in a neutral atom.

Periods and groups

  • The period (row) tells you how many electron shells an atom has: sodium, in period 3, has electrons in three shells, K, L and M.
  • A group (column) brings together elements with a similar outer shell — which is why they react in similar ways. The alkali metals in group 1 each have one valence electron, the halogens in group 17 have seven, and the noble gases have a closed shell.
number of valence electrons = group no. (groups 1–2)  ·  group no. − 10 (groups 13–18)e.g. oxygen, in group 16, has 16 − 10 = 6 valence electrons; exception: helium has 2

Why is the table shaped like this?

Electrons fill the subshells s (2 electrons), p (6), d (10) and f (14) in turn. Hence the blocks 2, 6, 10 and 14 columns wide. The full table would have 32 columns, so the 14 lanthanides and 14 actinides (the f-block) are pulled out underneath.

Trends — check them on the property maps

  • Electronegativity increases across a period from left to right and decreases down a group. The record holder is fluorine (3.98); the lowest measured value belongs to cesium (0.79).
  • Atomic radius decreases across a period (the growing nuclear charge pulls the electrons in more strongly) and increases down a group (more shells are added).

Formulas

M = Σ ni · Ai    M(H2O) = 2 · 1.008 + 15.999 = 18.015 g/molmolar mass: the sum of the atomic masses of all the atoms in the formula
Σ ni · si = 0    H2SO4: 2 · (+1) + x + 4 · (−2) = 0 ⇒ x = +6in a neutral compound the oxidation states s add up to zero
ΔE = |EA − EB|    NaCl: 3.16 − 0.93 = 2.23by convention: ΔE < 0.4 — nonpolar covalent bond, 0.4–1.7 — polar covalent, > 1.7 — mostly ionic

Simplifications and sources: values rounded according to the CRC Handbook and IUPAC; gas densities are given at 0 °C, transition temperatures at a pressure of 1 atm. Boiling points of refractory metals differ between sources by as much as several tens of degrees. Atomic radius means the covalent radius (Cordero et al., 2008). Electronegativities of some lanthanides and heavy elements are not well established, so they are omitted. The molecule drawings are schematic: bond angles and lengths are approximate. The year of discovery is the year the element was recognized (for some of them the metal was isolated later).

Did you know…?

  • Mendeleev left gaps in his 1869 table and predicted the properties of the missing elements. Gallium (1875), scandium (1879) and germanium (1886) were discovered later — just as he had predicted.
  • About 99% of the mass of the human body is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus.
  • Two elements honor Poland: polonium (from the Latin Polonia) and copernicium (after Nicolaus Copernicus), while curium honors Marie Skłodowska-Curie and her husband.

Challenges

  • Find the element with the highest electronegativityTurn on the “Electronegativity” map and open the record holder's card.to do
  • Build table saltOpen the cards of both elements that make up NaCl one after the other — e.g. sodium, then go from the compound card to chlorine.to do
  • Go back to 1869Turn on the “Year of discovery” map and set the slider to the year Mendeleev published his table. How many elements were known then?to do
  • Find a metal that melts in your handOpen the card of a metal that melts below body temperature (37 °C) — the melting point map will help.to do
  • Discover 5 organic compoundsOpen the cards of five different organic compounds in the “Compounds” tab. Progress: 0/5to do