Interactive Periodic Table
Click any of the 118 elements to see its atomic number, standard atomic weight, and category. A 3D diagram compares a small, simple atom to a large, many-electron atom, and charts show two of the periodic table's most important trends: atomic radius and electronegativity.
Reviewed by the ToolNestr Editorial Team — July 2026
Click any element below to see its details.
Small atom vs. large atom
1. Small atom (e.g., helium)
Few electron shells — a compact atom with electrons held tightly by the nucleus.
2. Large atom (e.g., caesium)
Many electron shells — a much larger atom with its outermost electron loosely held.
Periodic trend graphs
How it works
The core idea in one line: an element's column tells you how it behaves chemically, and its row tells you how big and reactive it is compared to its neighbors — together, position on the table alone predicts most of an element's chemistry.
Atomic radius: decreases → across a period, increases ↓ down a group
more protons pull valence electrons in tighter across a period; extra electron shells push the radius out down a group
Electronegativity: increases → across a period, decreases ↓ down a group
atoms hold electrons more tightly as nuclear charge increases across a period, but less tightly as electrons sit farther from the nucleus down a group
The periodic table isn't just a list — it's arranged so that elements sharing a column (group) have the same number of valence electrons, and therefore very similar chemical behavior. Moving left to right across a row (period), protons increase steadily, pulling the outer electrons in tighter — atoms shrink and hold their electrons more strongly. Moving top to bottom down a group, each step adds a whole new electron shell, so atoms get larger and their outermost electrons, now farther from the nucleus, become easier to remove.
Worked example 1 — reading chlorine's tile
Given: Chlorine sits in period 3, group 17 (the halogens), with atomic number 17.
Chlorine's non-round atomic weight is one of the clearest classroom examples of why atomic weight is an abundance-weighted average, not a single isotope's mass.
Worked example 2 — comparing sodium and potassium (same group)
Given: Sodium (Z=11) and potassium (Z=19) are both in group 1, the alkali metals.
This is the core logic of the periodic table: elements in the same column react similarly, and reactivity trends shift predictably as you move down that column.
The ten element categories on this table
Every element belongs to one family based on shared chemical behavior.
| Category | Example elements |
|---|---|
| Alkali metals ★ | Li, Na, K, Rb, Cs, Fr |
| Alkaline earth metals | Be, Mg, Ca, Sr, Ba, Ra |
| Transition metals | Fe, Cu, Ag, Au, Zn |
| Post-transition metals | Al, Ga, Sn, Pb, Bi |
| Metalloids | B, Si, Ge, As, Sb, Te |
| Nonmetals | H, C, N, O, P, S, Se |
| Halogens | F, Cl, Br, I, At |
| Noble gases | He, Ne, Ar, Kr, Xe, Rn |
| Lanthanides | La through Lu |
| Actinides | Ac through Lr |
★ Reference row. Category assignment is the single fastest way to predict how an unfamiliar element will behave chemically.
Where the periodic table actually matters
🔋 Battery and materials chemistry
Lithium's position as the lightest alkali metal with the smallest atomic radius in its group makes it uniquely suited to high energy-density batteries — a direct consequence of trends read straight off the periodic table.
💊 Pharmaceutical and biochemistry research
Understanding which elements behave similarly (same group) helps chemists predict how substituting one atom for another in a drug molecule might change its properties, well before running an experiment.
⚛️ Nuclear medicine and energy
Radioactive elements like technetium-99m (used in medical imaging) and uranium (used in nuclear power) are identified and handled based on their periodic table position and known isotope behavior.
🏗️ Materials and alloy design
Engineers select transition metals for alloys based on periodic trends in hardness, conductivity, and reactivity — for example, chromium and nickel are added to steel because their periodic properties resist corrosion.
Common misconceptions
"Atomic weight and atomic number are the same thing."
Atomic number is a whole number counting protons (defines the element); atomic weight is the abundance-weighted average mass of all naturally occurring isotopes, and is almost never a whole number.
"Elements in the same period (row) are chemically similar."
It's elements in the same group (column) that share similar chemistry, because they have the same number of valence electrons. Elements in the same period instead show a trend — properties change steadily from one side of the table to the other.
"The periodic table is a fixed, unchanging list."
New elements are still being confirmed and named — element 118 (oganesson) wasn't officially named until 2016. Ongoing nuclear physics research continues to search for even heavier, so-far-unconfirmed elements.
"Metalloids are just weak metals."
Metalloids (like silicon and germanium) have properties genuinely intermediate between metals and nonmetals — most famously, they act as semiconductors, conducting electricity under some conditions but not others, which is precisely why they underpin all modern computer chips.
Formula sources & further reading
The formulas here are standard, traceable to:
- • IUPAC — Standard Atomic Weights (periodic official reference). iupac.org
- • OpenStax, Chemistry 2e — Chapter 6, "Electronic Structure and Periodic Properties" (free, peer-reviewed).
- • CRC Handbook of Chemistry and Physics — standard reference atomic weights and radii.
Atomic weights follow IUPAC standard values (rounded); bracketed values are the mass number of the most stable known isotope for elements with no stable isotopes. Results are rounded for display.
How to use this calculator
Click any tile
The details panel shows that element's symbol, name, atomic number, and standard atomic weight.
Check the category legend
Colors group elements by chemical family — a fast way to predict likely behavior.
Follow a row or column
Move across a period to see gradual trend shifts, or down a group to compare chemically similar elements.
Related tools
Frequently asked questions
How is the periodic table organized?
Elements are arranged by increasing atomic number (proton count), in rows called periods and columns called groups. Elements in the same group share similar chemical behavior because they have the same number of valence electrons.
Why are the lanthanides and actinides shown as a separate block?
They are shown below the main table purely to keep it a manageable width — they actually belong in period 6 (lanthanides, starting after lanthanum) and period 7 (actinides, starting after actinium), pulled out below so the table isn't excessively wide.
What is atomic weight, and why isn't it a whole number?
Atomic weight is the weighted average mass of all naturally occurring isotopes of an element, so it reflects both each isotope's mass and its natural abundance — which is why it's essentially never a clean whole number, chlorine's 35.45 being a classic example.
What do the bracketed atomic weights mean, like [98] for technetium?
Brackets indicate the element has no stable isotopes — the number shown is the mass number of its longest-lived or most studied radioactive isotope instead of a natural-abundance-weighted average.
How many elements are there, and are more being discovered?
118 elements are currently confirmed and named, filling the table through period 7. Elements above 118 have not yet been synthesized, though searches for period 8 elements are an active area of nuclear physics research.