A map of the elements
The periodic table is a chart of all the known chemical elements, arranged so that elements with similar properties line up together. It is one of the most useful reference tools in science because the position of an element tells you a lot about how it is likely to behave, even before you learn anything else about it.
There are 118 confirmed elements. About 90 occur naturally on Earth in measurable amounts, and the heaviest ones have been made in laboratories and exist for only fractions of a second.
How it came about
By the middle of the nineteenth century, chemists knew several dozen elements and had noticed that certain ones resembled each other. In 1869, the Russian chemist Dmitri Mendeleev arranged the known elements by atomic weight and grouped them by similar chemical behavior. His key move was to leave gaps where no known element fit and to predict that undiscovered elements would eventually fill them. Later discoveries matched several of his predictions, which helped establish the table.
A further correction came in the early twentieth century. Experiments, notably by Henry Moseley, showed that the right ordering is by atomic number, the number of protons in an atom’s nucleus, rather than by weight. That is how the modern table is organized.
Reading one box
Each element has a box, and most tables show at least these four things:
- Atomic number, the number of protons. It defines the element: every atom with 6 protons is carbon.
- Symbol, one or two letters, such as C for carbon or Na for sodium (from its Latin name, natrium).
- Name, such as carbon or sodium.
- Atomic mass, the average mass of the element’s atoms, in atomic mass units. It is an average because most elements exist as a mix of atomic isotopes with different numbers of neutrons.
For example, oxygen has atomic number 8, symbol O, and an atomic mass of roughly 16. Iron is 26, Fe. Gold is 79, Au. Some tables add extras, such as electron arrangements or color-coding by type.
Periods: the rows
The horizontal rows are called periods, numbered 1 through 7. Moving left to right along a period, the atomic number increases by one each step.
A period number tells you how many electron shells an atom of that element uses to hold its electrons, in simplified terms. Hydrogen and helium are in period 1, and sodium through argon are in period 3. Rows get longer further down because later shells can hold more electrons.
Groups: the columns
The vertical columns are groups, numbered 1 through 18 in the standard modern system. Elements in the same group tend to behave similarly because they have the same number of electrons in their outermost shell, and those outer electrons mostly decide chemical behavior.
Some well-known groups:
| Group | Name | General traits |
|---|---|---|
| 1 | Alkali metals (excluding hydrogen) | Soft, very reactive metals; react vigorously with water |
| 2 | Alkaline earth metals | Reactive metals, less so than group 1 |
| 17 | Halogens | Reactive nonmetals; readily form salts with metals |
| 18 | Noble gases | Colorless gases that rarely react |
Sodium (period 3, group 1) and potassium (period 4, group 1) are both soft metals that react strongly with water. Chlorine (period 3, group 17) and fluorine (period 2, group 17) are both reactive nonmetals. Hydrogen sits in group 1 for its electron count but is a gas and behaves quite differently, so it is usually treated as a special case.
Metals, nonmetals and in-between
A zigzag line toward the right side of the table separates metals on the left from nonmetals on the right. Most elements are metals. They typically conduct heat and electricity well and can be shaped without shattering. Nonmetals such as oxygen, sulfur and carbon are more varied. Elements along the dividing line, such as silicon and germanium, are called metalloids and have mixed properties, which is part of why silicon is useful in electronics.
The transition metals fill the central block (groups 3 to 12) and include iron, copper and gold. Below the main table are two separate rows, the lanthanides and actinides. They are placed there only to keep the table from becoming extremely wide.
Trends across the table
Because the layout reflects electron structure, several properties change in a fairly regular way:
- Atomic size generally gets smaller across a period (left to right) and larger down a group.
- Reactivity of metals tends to increase down a group, while reactivity of nonmetals tends to decrease down a group.
- Ease of removing an electron generally increases across a period and decreases down a group.
These are tendencies, not strict rules, and there are exceptions. They are still a good first guide when thinking about chemical reactions, such as which elements are likely to combine.
Using the table
You do not need to memorize it. A practical approach is to learn the pattern: the rows are periods, the columns are groups, and neighbors in a column are chemical relatives. From there, a table can help predict how an unfamiliar element behaves, compare materials, or work out the formula of a simple compound. For example, sodium is in group 1 and chlorine in group 17, which fits the familiar 1:1 compound sodium chloride, ordinary table salt.
The periodic table is still being extended and refined as new superheavy elements are produced, but its underlying logic has stayed the same since atomic number became its organizing principle.