Mendeleev's gaps
By the mid-1800s about 60 elements were known, with no good way to organize them. In 1869 Russian chemist Dmitri Mendeleev noticed that when elements were ordered by atomic weight, similar properties recurred periodically, and he built a table. His genius lay in leaving gaps where nothing fit and predicting the properties of missing elements. The one he called eka-silicon was discovered in 1886 as germanium, with an atomic weight and density almost exactly as forecast. Germany's Lothar Meyer drew up a similar table around the same time, but Mendeleev's bold predictions earned him the title father of the periodic table, and element 101, mendelevium, bears his name.
How to read the table
In 1913 Britain's Henry Moseley used X-ray spectra to show that the true order of elements is not atomic weight but the number of protons in the nucleus, the atomic number. Rows (periods) reflect the number of electron shells, and columns (groups) reflect outer-electron arrangement, so elements in a group behave alike. Group 1 alkali metals such as sodium and potassium react violently with water, group 17 halogens such as fluorine and chlorine are reactive nonmetals, and group 18 noble gases such as helium and neon barely react at all. Metals fill the left and middle, nonmetals the upper right, with metalloids like silicon along the border; the two rows set apart below are the lanthanides and actinides.
118 elements and their origins
In 2016 IUPAC confirmed the names nihonium (113), moscovium (115), tennessine (117) and oganesson (118), completing period 7. Most elements heavier than uranium (92) are made in accelerators, and many have a half-life under a second. Natural elements formed in stages across cosmic history: hydrogen and helium just after the Big Bang, elements up to iron through nuclear fusion in stars, and heavy ones such as gold and platinum in supernovae and neutron star collisions. The carbon and oxygen in our bodies were forged in stars long ago.
FAQ
- Why is the periodic table shaped that way?
- It follows the order in which electron orbitals (s, p, d, f) fill. The s-block has 2 columns, the p-block 6 and the d-block 10; the 14-column f-block is placed below so the table does not become too wide.
- Could element 119 be made?
- Teams in Japan, the US and Russia are attempting to synthesize elements 119 and 120. Success would open period 8, but their extremely short lifetimes make confirmation very difficult.
Sources reviewed: 2026-10-26
References
- Mendeleev, D. (1869). On the relationship of the properties of the elements to their atomic weights. Zhurnal Russkogo Khimicheskogo Obshchestva, 1, 60–77.
- Moseley, H. G. J. (1913). The high-frequency spectra of the elements. Philosophical Magazine, 26(156), 1024–1034.
- IUPAC (2016). IUPAC is naming the four new elements nihonium, moscovium, tennessine, and oganesson. iupac.org.