Two atoms have six protons each, but different neutron counts. What are they?

Different isotopes of carbon. Exactly. The shared proton count fixes carbon; differing neutron counts distinguish isotopes.

What does the 14 in carbon-14 count?

Protons plus neutrons. Exactly. Mass number counts the nucleons, so carbon-14 has six protons and eight neutrons.

What problem did Soddy's concept address?

Different atomic weights could belong to chemically equivalent forms of one element. Exactly. The classification recognized a distinction that later nuclear theory explained more fully.

From the archive · September 29, 2026

The same element with a different nucleus

What can change inside an atom without changing which element it is?

01 · Word

Isotope

Pronounced EYE-suh-tohp

noun

An atomic form of an element with the same proton count but a different neutron count from another form

Examples

Carbon-14 is an isotope of carbon, as are carbon-12 and carbon-13.
Changing an atom's isotope changes its neutron count; changing its element changes its proton count.

Origin

The name refers to occupying the same place in the periodic table. Frederick Soddy introduced the isotope concept in 1913 to account for forms with different atomic weights but essentially the same chemical behavior. The later neutron-based explanation made the distinction inside the nucleus clear.

02 · Idea

One identity can contain several versions

An element's identity is set by its number of protons. Add or remove a neutron and that identity remains, although the atom's mass changes. The result can also change nuclear stability. Chemical similarity therefore does not mean two atoms must behave identically in every respect.

Carbon makes the distinction easy to count. Carbon-12 contains six protons and six neutrons; carbon-14 has six protons and eight neutrons. Both remain carbon. The number after the name counts protons plus neutrons, rather than giving the number of protons alone.

Same element does not mean same nucleus.

Separate the property that defines a category from the properties that vary within it.

Limits

Isotopes are not perfectly identical chemically in every setting: mass differences can affect reaction rates and physical behavior. Nor is every isotope radioactive. The category describes nuclear composition, not a universal instability or hazard.

03 · Moment

Different weights claim the same place

Glasgow, 1913; Frederick Soddy formulates the isotope concept

Working on radioactivity, Soddy confronted forms that resisted separation by their chemical behavior while differing in atomic weight and radioactive properties. Treating every distinct radioactive form as a wholly different chemical element created a classification problem. The isotope concept allowed more than one atomic form to occupy the same elemental position.

Scientist: Frederick Soddy
Year: 1913
Puzzle: Chemical similarity, different weights

The caveat

Soddy's 1913 formulation should not be narrated as though he already possessed the complete modern neutron model. The historical insight and its later physical explanation belong to different stages of the story.

This reorganized the question. Chemists did not need a new place in the periodic table for every different atomic weight. Later understanding of nuclear structure explained the distinction through neutron number. The useful category came before the familiar classroom picture of protons and neutrons made it easy to draw.

The connection

Proton number fixes elemental identity, while differences in neutron number create isotopes with different masses and nuclear properties.