Coaching Centre
Elements and the periodic table
What an element is, chemical symbols and where they come from, how Mendeleev organised the elements, groups and periods, metals, non-metals and metalloids on the periodic table, and how new elements are discovered and named.
In 1871, Dmitri Mendeleev left gaps in his periodic table and announced that elements nobody had ever seen would one day fill them. He even predicted what they would be like. Four years later gallium was discovered, and it matched his predictions almost exactly.
What is an element?
An element is a pure substance made of only one kind of atom: every atom in it has the same number of protons. Elements can't be broken down into simpler substances by chemical reactions, because they are already the simplest substances there are. Cut a lump of gold in half again and again, down to a single atom, and that atom is still gold.
Scientists know of 118 elements. About 90 are found in nature; the rest have only been made in laboratories and nuclear reactors. The heaviest are so unstable that they break apart within a fraction of a second of being made.
Chemical symbols
Each element has a chemical symbol of one or two letters. The first letter is always a capital and the second is always lower case, so Co is cobalt but CO is carbon monoxide, a compound of carbon and oxygen.
Most symbols come from the English name: H for hydrogen, C for carbon, Ca for calcium. But elements known since ancient times often use their Latin names:
| Element | Symbol | From |
|---|---|---|
| Gold | Au | aurum |
| Silver | Ag | argentum |
| Iron | Fe | ferrum |
| Copper | Cu | cuprum (from Cyprus) |
| Lead | Pb | plumbum (hence "plumber") |
| Sodium | Na | natrium |
| Potassium | K | kalium |
| Mercury | Hg | hydrargyrum, "liquid silver" |
Why bother with symbols? Because they are the same in every language. Helium is Helio in Spanish, Hel in Polish and written in completely different scripts in Chinese, Arabic and Korean, but it is He to every scientist on Earth. Symbols also make formulas quick to write: H2O is much faster than "two hydrogen atoms joined to one oxygen atom".
The periodic table
By the 1860s about 60 elements were known, and chemists noticed that some behaved alike. In 1869 the Russian chemist Dmitri Mendeleev arranged them in a table so that elements with similar properties lined up. His masterstroke was leaving gaps for undiscovered elements. When gallium (1875), scandium (1879) and germanium (1886) were found with the properties he had predicted, the periodic table was accepted around the world.
The modern periodic table lists the elements in order of atomic number (number of protons).
- The vertical columns are called groups. Elements in the same group have similar properties. Group 1 metals (lithium, sodium, potassium) are soft and react violently with water; Group 18, the noble gases (helium, neon, argon), hardly react at all.
- The horizontal rows are called periods.
- Metals are on the left and in the middle, non-metals are on the right, and a thin staircase of metalloids (such as silicon) runs between them.
- Colour by metals and non-metals. Roughly what fraction of the elements are metals?
- Colour by state. Find the only two elements that are liquid at room temperature. Where are most of the gases?
- Choose "When discovered" and drag the year back to 1700. How many elements were known? What about in 1869, when Mendeleev made his table?
- Click Ga, Ge, Po, Es and Og. What were they named after?
New elements
Elements beyond uranium (atomic number 92) are made by smashing atoms together in particle accelerators. Only a few atoms are made at a time, and they last for less than a second. The last four to be confirmed were named in 2016: nihonium (Nh, 113, after Japan), moscovium (Mc, 115, after Moscow), tennessine (Ts, 117, after Tennessee) and oganesson (Og, 118, after the scientist Yuri Oganessian). Older textbooks and posters show temporary names like "ununtrium" instead. Scientists are now trying to make element 119.
Test yourself
The chemical symbol for iron is
From the Latin ferrum.
The vertical columns of the periodic table are called
Elements in a group have similar properties.
The modern periodic table arranges elements in order of
That is, the number of protons.
Why did scientists accept Mendeleev's table?
Successful predictions are strong evidence for a scientific model.
Which element is a metalloid?
Silicon has some properties of metals and some of non-metals.
Which symbol is written correctly?
First letter capital, second letter lower case.
Use the periodic table explorer to sort these elements.
Gold has the symbol . The periodic table was first created by . The rows of the table are called . Metals are on the side of the table. There are known elements.
Explain why scientists use chemical symbols instead of the full names of elements. Give two reasons and an example.
Chemical symbols are the same in every country and language, so scientists everywhere can understand each other's work. For example, helium is "Helio" in Spanish and "Hel" in Polish, but every scientist writes it as He. Symbols are also much shorter than full names, which makes it possible to write chemical formulas quickly: H₂O shows at a glance that water is two hydrogen atoms joined to one oxygen atom.