ChemistryDifficulty 1-3

Elements

A chemical element is a substance made of atoms that all have the same number of protons — the atomic number. Carbon is every atom with 6 protons, oxygen every atom with 8. Atoms of one element can differ in their number of neutrons (isotopes), which is why an element's listed atomic mass is a weighted average rather than a whole number. 118 elements are known; 94 occur naturally on Earth.

Standard atomic weights are measured, not fixed

The IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), publishing critical evaluations since 1902 (most recently Standard Atomic Weights 2024), evaluates atomic weights for normal materials. Where isotope proportions vary measurably in nature, it reports an interval: carbon [12.0096, 12.0116] and chlorine [35.446, 35.457]. For everyday calculations it also publishes abridged values, such as 12.011 for carbon and 35.45 for chlorine.

Reconstructing chlorine's atomic weight

\bar{m}_{\mathrm{Cl}} \approx 0.758(34.96885) + 0.242(36.96590) \approx 35.45

ScienceVerse calculation with amount fractions chosen inside CIAAW's reported ranges.

Shifting the chlorine-37 fraction across its natural range moves the average only in the second decimal place — which is exactly why the interval is narrow but real.

Common misconception: An interval standard atomic weight is not an expression of measurement sloppiness; it records genuine variation in the isotopic composition of natural materials.

Full explanation — the complete reference version every reading depth is based on

What makes something an element

An element is defined by one number: the atomic number Z, the count of protons in each of its atoms. Change the number of protons and you have a different element. Historically, Antoine-Laurent Lavoisier defined an element as a substance that cannot be broken down further by chemical analysis — and showed that water is not an element, but a compound of hydrogen and oxygen.

Symbols

Every element has a one- or two-letter symbol in which only the first letter is a capital: Co is cobalt, while CO is the compound carbon monoxide. Some symbols come from Latin names, such as Na for sodium (natrium).

Isotopes and atomic mass

Atoms of the same element can have different numbers of neutrons. These versions are isotopes: carbon-12, carbon-13 and carbon-14 all have 6 protons but 6, 7 and 8 neutrons. In normal terrestrial carbon, carbon-12 is between 98.84% and 99.04% of the atoms. Frederick Soddy's investigations into the nature of isotopes were recognised by the 1921 Nobel Prize in Chemistry.

\bar{m} = \sum_i x_i \, m_i

Average atomic mass: the sum over isotopes of (fractional abundance × isotopic mass).

Worked example (ScienceVerse calculation): chlorine-35 has a mass of 34.968 8527 Da and chlorine-37 36.965 9026 Da. Taking amount fractions of 0.758 and 0.242 — inside the ranges CIAAW reports — gives 0.758 × 34.9689 + 0.242 × 36.9659 ≈ 35.45, matching chlorine's abridged standard atomic weight of 35.45.

Common misconception: No single chlorine atom has a mass of 35.45. Each atom is either chlorine-35 or chlorine-37; 35.45 is the average over a natural mixture of the two.

How many elements are there?

118 elements have been identified. The first 94 occur naturally on Earth; the remaining 24 have only been made artificially. The periodic table arranges all of them by atomic number.

Info: Elements are the starting point for the rest of chemistry: their atoms bond into molecules and compounds, and chemical reactions rearrange those atoms without ever turning one element into another.

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Sources and methodology

Each element is defined by its atomic number, the number of protons in its atoms: every carbon atom, for example, has 6 protons. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
Atoms of the same element that have different numbers of neutrons are called isotopes; carbon-12, carbon-13 and carbon-14 have 6, 7 and 8 neutrons respectively. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
In normal terrestrial materials, carbon-12 makes up an amount fraction of between 0.9884 and 0.9904 of carbon atoms and carbon-13 between 0.0096 and 0.0116. (awaiting scientific review)
- Carbon: isotopic composition and standard atomic weight — Primary dataset
The atomic mass listed for an element is a weighted average of the masses of its naturally occurring isotopes, which is why it is usually not a whole number. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
CIAAW gives the standard atomic weight of chlorine as the interval [35.446, 35.457], because the proportions of chlorine-35 and chlorine-37 vary measurably between natural and laboratory materials. (awaiting scientific review)
- Chlorine: isotopic composition and standard atomic weight — Primary dataset
118 chemical elements have been identified, of which the first 94 occur naturally on Earth and the remaining 24 are synthetic. (awaiting scientific review)
- The Periodic Table of Chemical Elements: a window on the universe — Government or standards body
Antoine-Laurent Lavoisier defined an element as a substance that cannot be broken down further by chemical analysis, and showed that water is composed of hydrogen and oxygen. (awaiting scientific review)
- Antoine-Laurent Lavoisier (Scientific Biographies) — Science media
Each element has a one- or two-letter chemical symbol with only the first letter capitalised, and some symbols come from Latin names, such as Na for sodium (natrium). (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
The IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), which has published critical evaluations of atomic weights since 1902, recommends standard atomic weights that apply to all normal materials; its most recent definitive table is Standard Atomic Weights 2024. (awaiting scientific review)
- Standard Atomic Weights 2024 — Government or standards body
Frederick Soddy received the 1921 Nobel Prize in Chemistry in part for his investigations into the origin and nature of isotopes. (awaiting scientific review)
- The Nobel Prize in Chemistry 1921: Frederick Soddy — Facts — Other (unclassified)
Carbon-12 has served since 1960 as the scale-determining reference for the unified atomic mass unit and is the basis of all atomic weights; its atomic mass is exactly 12 Da. (awaiting scientific review)
- Carbon: isotopic composition and standard atomic weight — Primary dataset
In CIAAW's Standard Atomic Weights 2024, carbon's standard atomic weight is the interval [12.0096, 12.0116] and boron's is [10.806, 10.821]. (awaiting scientific review)
- Standard Atomic Weights 2024 — Government or standards body
For elements that lack isotopes with a characteristic isotopic abundance in natural terrestrial samples, the IUPAC periodic table lists the mass number of the nuclide with the longest confirmed half-life in square brackets. (awaiting scientific review)
- IUPAC Periodic Table of Elements (release dated 4 May 2022) — Government or standards body

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

Scientific review: this version has not yet been signed off by a scientific reviewer.
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