How the Table Is Organised
The elements are laid out in order of atomic number — the number of protons in the nucleus — and wrapped into rows so that elements with similar chemistry fall into the same column. Everything else follows from that one arrangement.
- Group (column, 1 to 18) — elements in a group share the number of electrons in their outer shell, which is what governs how they react. Group 1 metals all react violently with water; group 18 gases react with almost nothing.
- Period (row, 1 to 7) — the outermost occupied shell. Period 3 elements are filling the third shell.
- Block — which type of orbital is being filled: s, p, d or f. The block explains the shape of the table, including why the lanthanides and actinides are usually printed underneath it.
Reading an Electron Configuration
A configuration such as [Ar] 3d6 4s2 for iron means: the electron arrangement of argon, plus six electrons in the 3d subshell and two in 4s. The bracketed noble gas is shorthand for the filled inner shells.
Subshells fill in order of increasing energy rather than increasing shell number, which is why 4s fills before 3d. About twenty elements break the simple filling rule — chromium, copper, silver, gold, palladium and most of the actinides among them — because a half-filled or completely filled d or f subshell is more stable than the pattern predicts. Those exceptions are built into the results above rather than calculated from the rule.
Atomic Mass, Mass Number and Isotopes
| Term | Meaning |
|---|---|
| Atomic number (Z) | Protons in the nucleus. Defines the element. |
| Mass number (A) | Protons plus neutrons in one particular isotope. Always a whole number. |
| Standard atomic weight | The average over the isotopes as they occur naturally, weighted by abundance. Rarely a whole number. |
Chlorine's atomic weight of 35.45 is the clearest example: no chlorine atom weighs 35.45 u. The figure is the average of chlorine-35 (about 76% of natural chlorine) and chlorine-37 (about 24%).
For elements with no stable isotope — technetium, promethium and everything past bismuth — the value quoted is the mass number of the longest-lived isotope, shown as a whole number.
Element Categories
| Category | Where | Character |
|---|---|---|
| Alkali metals | Group 1 | Soft, one outer electron, react vigorously with water |
| Alkaline earth metals | Group 2 | Harder, two outer electrons, less reactive |
| Transition metals | Groups 3–12 | Filling d orbitals; multiple oxidation states, coloured compounds |
| Post-transition metals | Right of the d-block | Softer, lower melting points than transition metals |
| Metalloids | The diagonal staircase | Intermediate properties; the basis of semiconductors |
| Reactive nonmetals | Upper right | The elements of organic chemistry and life |
| Halogens | Group 17 | Seven outer electrons; the most reactive nonmetals |
| Noble gases | Group 18 | Full outer shell; almost entirely unreactive |
| Lanthanides | 57–71 | Filling 4f; the rare earths, chemically very similar to each other |
| Actinides | 89–103 | Filling 5f; all radioactive |
Trends Across the Table
- Atomic radius decreases left to right across a period (more protons pulling the same shell inward) and increases down a group (a new shell each row).
- Ionisation energy does the opposite: it rises across a period and falls down a group.
- Electronegativity rises toward the top right. Fluorine is the most electronegative element; caesium and francium the least.
- Metallic character increases down and to the left. The dividing staircase between metals and nonmetals runs from boron down to astatine.
Frequently Asked Questions
How many elements are there?
118 are named and confirmed, from hydrogen to oganesson. Ninety-four occur naturally on Earth, at least in traces; the rest exist only as products of nuclear reactions, some for a few milliseconds.
Why is hydrogen placed above the alkali metals?
Because it has one electron in an s orbital. Chemically it behaves nothing like sodium, which is why many printed tables set it apart or show it twice.
What is the difference between group and period?
Group is the column and shares chemical behaviour; period is the row and shares the outermost electron shell.
How many neutrons does an element have?
It varies by isotope. Subtracting the atomic number from the rounded atomic weight gives a reasonable figure for the commonest isotope, which is what is shown above — but it is an estimate, not a fixed property of the element.