How many elements should an accurate periodic table include?

An accurate table includes all 118 confirmed elements, from hydrogen (H, atomic number 1) to oganesson (Og, atomic number 118). Elements 1 through 94 occur naturally on Earth, some only as trace amounts, while elements 95 through 118 have only been produced synthetically in laboratories.

Where do the atomic weights on this table come from?

The atomic weights are the IUPAC 2021 abridged values from the Commission on Isotopic Abundances and Atomic Weights (CIAAW). For elements with a stable, natural isotopic composition — like carbon (12.011) or iron (55.845) — the table shows the conventional standard atomic weight. For radioactive elements with no stable isotope, it shows a bracketed mass number, such as Tc [97] or Og [294].

What does a value in square brackets mean?

A bracketed value is the mass number of the element's most stable (longest-lived) isotope, not a measured standard atomic weight. Radioactive elements such as technetium, promethium, and every element from polonium (84) onward have no standard atomic weight because they lack a stable isotope with a fixed natural abundance, so IUPAC lists the mass number of the most stable isotope instead.

What do the colours on the table mean?

Each colour marks one of ten element categories: alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloids, reactive nonmetals, halogens, noble gases, lanthanides, and actinides. Grouping by colour makes periodic trends easy to see, and you can click any category in the legend to highlight only those elements.

Why are the lanthanides and actinides shown as separate rows?

The lanthanides (elements 57 to 71) and actinides (elements 89 to 103) are f-block elements that chemically belong in group 3 of periods 6 and 7. Placing them inline would make the table 32 columns wide, so by long-standing convention they are pulled out into two rows beneath the main table, with placeholders marking where they fit.

An Accurate Periodic Table: Where the Numbers Come From

An accurate periodic table is one whose standard atomic weights come from a single, internationally recognised authority and whose layout matches the convention chemists use. The Interactive Periodic Table uses the IUPAC 2021 abridged standard atomic weights published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW), and lays out all 118 elements in the standard 18-column, 7-period grid with the lanthanides and actinides drawn as two separate rows beneath the main body — the same arrangement found in every chemistry textbook and journal reference. Every cell shows the atomic number, element symbol, name, and standard atomic weight, with each element colour-coded by its chemical category so the shape of the table can be read at a glance. Clicking an element opens a detail panel with the full record — atomic number, symbol, standard atomic weight, category, period, group, and electron block — all delivered directly in the browser, with no install and no sign-in. Because the weights come from a named IUPAC publication date, you always know exactly which revision of the data you are looking at rather than an unsourced rounded guess.

periodic table accurate — An Accurate Periodic Table: Where the Numbers Come From

What "Accurate" Means for Periodic Table Data

An accurate periodic table is defined by two things: a data source with international authority, and a clear convention for marking values that are not directly measured standard weights. Many classroom posters and printable PDFs round atomic weights to whole numbers — carbon (12.011) becomes 12, iron (55.845) becomes 56, copper becomes 64 — and that rounding is fine for memorisation but it discards the precision used in real calculations. A standard atomic weight is the weighted average of an element's stable isotopic masses, accounting for the natural abundance of each isotope on Earth. For elements with a stable natural composition, the figure shown — like 12.011 for carbon or 196.97 for gold — is called the conventional standard atomic weight. For elements that have no stable isotope, no such weighted average exists, and a proper reference shows the mass number of the longest-lived isotope in square brackets instead, matching IUPAC's own convention on its official table.

Where the Standard Atomic Weights Come From

The atomic weights on the table are the IUPAC 2021 abridged values published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW), the body of the International Union of Pure and Applied Chemistry that formally evaluates isotopic data and recommends atomic-weight values for publication. CIAAW reviews new measurements periodically and updates the table of standard atomic weights when better abundance data becomes available, which is why values shift slightly between revisions. The 2021 numbers, for instance, differ from older 2013 values by small amounts for a handful of elements. Because CIAAW is the single named source, the values on the table match what appears in textbooks, peer-reviewed journals, and chemical safety data sheets. That traceability is what separates a reference table from a poster that has been copy-pasted through several internet revisions without anyone tracing the numbers back to their origin.

How to Pull an Accurate Value From the Table

To get an accurate atomic weight or symbol for any element, work through these steps in the Interactive Periodic Table:

Open the table in your browser. The data is delivered directly with the page — nothing to install and no sign-in required.
Scan the 118-element grid (hydrogen at the top left, oganesson at the bottom right), or type a name, symbol, or atomic number in the search box. Matching cells light up while the rest of the table dims, which makes the lookup visually obvious.
Read the values on the highlighted cell: atomic number, symbol, element name, and standard atomic weight are all displayed at a glance.
Click or tap the element to open the detail panel for the full record: atomic number, symbol, standard atomic weight, category, period, group, and electron block (s, p, d, or f).
When you need a group of elements rather than one, click the matching label in the colour legend instead — every element of that family stays highlighted while the rest fades, making periodic trends such as the s-block on the left or the f-block beneath the main table easy to see.

To see why the unrounded weight matters, take iron's 55.845 figure. For a 100.000 g sample of pure iron, the number of moles is mass divided by atomic weight. Substituting the IUPAC value gives 100.000 g ÷ 55.845 g/mol = 1.7907 mol (to five significant figures). Using the rounded 56 instead would give 1.7857 mol — a 0.28% bias that compounds quickly when scaled across larger sample masses or carried through a stoichiometry chain.

What the Bracketed Mass Numbers Tell You

A trustworthy periodic table is honest about what it does not know. For elements that have no stable isotope, there is no natural composition to average over, so no standard atomic weight can be defined. Examples include technetium (atomic number 43), promethium (61), and every element from polonium (84) onward through oganesson (118), including all the laboratory-made superheavies. The bracketed figure is the mass number of the longest-lived known isotope of that element — for technetium the cell reads Tc [97], and for oganesson the cell reads Og [294]. The brackets tell you immediately that the figure is a reference mass number for one isotope rather than a measured average of Earth's natural abundance, which is exactly the convention IUPAC uses on its own official table.

The distinction between measured and reference values is summarised below:

Element group	Value shown	Example	Meaning
Stable, multi-isotope	Conventional standard atomic weight	Carbon (12.011), Iron (55.845)	Weighted average of stable isotopic masses using Earth's natural abundances
Stable, monoisotopic	Conventional standard atomic weight	Gold (196.97)	Single stable isotope whose atomic mass equals the standard weight
Primordial radioactive, characterised abundance	Conventional standard atomic weight	Thorium (232.04), Uranium (238.03)	IUPAC-published value based on known terrestrial composition
No stable isotope (synthetic or short-lived)	Mass number in square brackets	Technetium [97], Oganesson [294]	Mass number of the longest-lived known isotope, not a measured weight

Accuracy Limits at the Edges of the Table

Beyond atomic weights, the position of an element on the table itself carries information that holds across every revision: the column (group) hints at how many valence electrons the atom uses in bonding, the row (period) shows which electron shells are being filled, and the colour band places the element in its chemical category. That structural accuracy depends on electron configuration rather than measured weights, so it stays stable even when a new IUPAC revision shifts a number by a few parts per ten thousand.

The hard limit on accuracy sits with the superheavy synthetic elements at the far end of the table. Some have only ever been produced a few atoms at a time, and their chemistry is mostly predicted from the table position rather than measured in a flask. The bracketed mass number is the best figure available — it represents the longest-lived isotope known at the time of the IUPAC publication. When synthesised atoms live longer or new isotopes are produced, those bracketed numbers will be revised in future CIAAW updates, and the table will reflect the next revision when it is released. The brackets say clearly: this is not a measured average, it is the best available reference for a single isotope.

A related limit affects a small number of light elements whose isotopic composition varies enough in nature that IUPAC does not give them a single conventional weight; the table still shows the values IUPAC does publish, so the source chain stays consistent from hydrogen through oganesson.