Identity and isotopes

What can change while hydrogen stays hydrogen?

Identify isotopes by proton and neutron counts without confusing mass number with standard atomic weight.

Every hydrogen nucleus has one proton. Hydrogen-1 has no neutron, hydrogen-2 has one, and hydrogen-3 has two. These are isotopes of the same element. Changing neutron count changes the isotope; changing proton count changes the element. Only the listed examples are represented in this activity.

Explore the model (optional)

EXPLORE THE MODEL

Change neutrons. Preserve identity.

Nucleus bookkeeping diagram. The electron and nucleus are not shown to a common physical scale.

Neutrons0

ElementHydrogen · 1 proton

IsotopeProtium

Mass number1

StabilityStable isotope

Proton count fixes elemental identity. Neutron count changes the isotope; mass number is not standard atomic weight.

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HOW DO WE KNOW?

Evidence & limits

Reference science + bounded teaching model

What this interaction establishes

Identify isotopes by proton and neutron counts without confusing mass number with standard atomic weight.

Nucleus bookkeeping diagram. The electron and nucleus are not shown to a common physical scale.

Source bindings for this explanation

Start here

Sources: S03, S04.

Explore the science

The mass number is the integer total of protons and neutrons in a particular nucleus. An isotope's measured atomic mass is a different quantity, and an element's standard atomic weight describes accepted values for normal terrestrial materials. Hydrogen's standard atomic weight can be an interval because isotope proportions vary among such materials. Hydrogen-1 and hydrogen-2 are stable; hydrogen-3, called tritium, is radioactive. Radioactivity is not determined by simply making the on-screen nucleus larger.

Sources: S03, S04.

Go deeper

A periodic-table weight should not be rounded and treated as the neutron count of every atom. For example, hydrogen-2 still has one proton even though its mass number is two. This lesson constrains the neutron selector to named isotopes so that invented configurations are not silently presented as stable. Isotope abundance depends on the sample; a teaching mass spectrum with selected proportions is synthetic unless a specific measured dataset and preparation are identified.

Sources: S03, S04.

Limitations

The nucleus display is schematic and not a nuclear stability solver. Mass number, isotope mass and standard atomic weight are distinct quantities. A synthetic isotope mixture does not establish a natural sample’s composition.

Sources: S03, S04.

Keep these boundaries in mind

The nucleus display is schematic and not a nuclear stability solver.
Mass number, isotope mass and standard atomic weight are distinct quantities.
A synthetic isotope mixture does not establish a natural sample’s composition.

Inspect the references

Supplemental primary reference ↗Supplemental primary reference ↗

Scientific explanations with linked references. Model calculations retain their stated assumptions and limits.

Optional: check your understanding

MAKE THE CONNECTION YOUR OWN

Test your reasoning

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recognition

Which nucleus is hydrogen-2?

One proton and one neutronTwo protons and no neutronsOne proton and two neutrons

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mechanism

A hydrogen nucleus has one proton and two neutrons. Enter its mass number.

Your answer mass number

evidence

A table displays an interval for hydrogen’s standard atomic weight. What should you infer?

Some individual hydrogen atoms have fractional neutron countsThe atomic number is uncertainNormal terrestrial isotope proportions can vary within the stated convention

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transfer

Two carbon atoms each have six protons, but one has six neutrons and the other seven. What are they?

Different isotopes of carbonCarbon and nitrogenThe same isotope with different colors

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Water is not hydrogen gas Next: Atomic transitions and light