What Is the Periodic Table?
The periodic table is an organised chart of all 118 known chemical elements, arranged in order of increasing atomic number (number of protons). It was first designed by the Russian scientist Dmitri Mendeleev in 1869. He noticed that elements with similar properties appeared at regular ("periodic") intervals when arranged by mass.
The periodic table is one of the most important tools in chemistry because it allows scientists to predict the properties of elements and understand how they react.
Hover over an element to see its full name and atomic mass. Values in parentheses are for the most stable isotope.
Rows and Columns
The table is arranged into periods (rows) and groups (columns):
Periods (horizontal rows):
- Elements in the same period have the same number of electron shells.
- Period 1 has 2 elements; Period 2 and 3 have 8 elements each.
- As you move across a period, the atomic number increases by 1 each time.
Groups (vertical columns, numbered 1β18):
- Elements in the same group have the same number of electrons in their outer shell.
- This gives them very similar chemical properties.
- Group 1 elements (alkali metals) all react vigorously with water. Group 17 (halogens) all form salts. Group 18 (noble gases) are all unreactive.
Mendeleev was so confident in his table that he deliberately left gaps for elements that had not yet been discovered. He even predicted the properties of these missing elements! When they were later found β for example, gallium (1875) and germanium (1886) β their properties matched Mendeleev's predictions very closely.
Metals, Non-Metals, and Metalloids
The periodic table is divided into broad regions:
- Metals (left and centre): shiny, conduct electricity and heat, malleable (can be hammered into shapes), mostly solid at room temperature. Examples: iron, copper, gold, sodium.
- Non-metals (right side): dull (or gases), poor conductors, brittle if solid. Examples: carbon, oxygen, sulfur, chlorine.
- Metalloids (along the dividing line): have properties of both metals and non-metals. Examples: silicon, germanium.
Key Groups to Know
| Group | Name | Properties | Examples |
|---|---|---|---|
| 1 | Alkali metals | Soft metals, very reactive with water | Lithium, sodium, potassium |
| 2 | Alkaline earth metals | Reactive metals | Calcium, magnesium |
| 17 | Halogens | Reactive non-metals, form salts | Fluorine, chlorine, bromine |
| 18 | Noble gases | Unreactive gases | Helium, neon, argon |
You read the periodic table like a book β left to right, top to bottom. Each element's box shows its atomic number (top), symbol (middle), and name (sometimes printed below the symbol).
Real world: Group 1 alkali metals in your kitchen
Sodium (Na) and potassium (K) are both in Group 1 of the periodic table. They behave similarly because they both have one outer electron. Table salt is sodium chloride (NaCl), and "salt substitute" products use potassium chloride (KCl) β same position in the periodic table, very similar chemical behaviour, which is why one can substitute for the other in food.
Real world: Halogens in disinfection and cooking
Chlorine, bromine, and iodine are all in Group 17 (halogens). Chlorine is added to swimming pool water to kill bacteria. Iodine is used in antiseptic wound treatments. Both work by oxidising microorganism cell membranes β they behave similarly because they are in the same group and have similar outer-shell electron arrangements.
Real world: Periodic trends in fireworks
The bright colours in fireworks are produced by specific metal ions: strontium (Sr, Group 2) gives red, barium (Ba, Group 2) gives green, copper (Cu, transition metal) gives blue. Chemists use the periodic table to predict which elements will produce which colours based on their electron configurations and how they emit light.
Where do you see this in real life? The periodic table is behind every chemical product label, alloy specification, and medical compound. Understanding groups and periods lets you predict that lithium and sodium will both react violently with water, or that neon and argon will both be completely safe in light fittings.
Practice Problems
An element is in Period 3 and Group 1. How many electron shells does it have, and how many electrons are in its outer shell?
Show answer
Why do elements in the same group of the periodic table have similar chemical properties?