What Is an Isotope? Atoms, Mass, and Examples

Why does the periodic table list chlorine's mass as 35.5 when you can't have half a particle? The answer is isotopes — and once you understand them, a lot of small mysteries about atoms suddenly make sense.

The short answer: isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Same element, different mass.

A quick atom refresher

Every atom is built from three particles:

  • Protons — positive charge; the number of them defines which element you have (this is the atomic number).
  • Neutrons — no charge; they add mass.
  • Electrons — negative charge; they orbit the nucleus.

The key fact: protons define the element. Change the protons and you have a different element entirely.

What makes an isotope

Isotopes come from changing the neutron count while keeping the protons the same. Because the protons don't change, it's still the same element — it just weighs a little more or less.

We label isotopes by their mass number (protons + neutrons):

  • Carbon-12 has 6 protons and 6 neutrons.
  • Carbon-14 has 6 protons and 8 neutrons.

Both are carbon (6 protons), but carbon-14 is heavier because of the two extra neutrons.

Classic examples

Hydrogen has three well-known isotopes:

IsotopeProtonsNeutrons
Protium (normal hydrogen)10
Deuterium11
Tritium12

Carbon has carbon-12, carbon-13, and carbon-14 — all with 6 protons, differing only in neutrons.

Why isotopes matter

They explain those "decimal" atomic masses. The atomic mass on the periodic table is a weighted average of all an element's naturally occurring isotopes. Chlorine is about 75% chlorine-35 and 25% chlorine-37, which averages to roughly 35.5 — hence the non-whole number.

Some isotopes are radioactive. These are called radioisotopes. Carbon-14, for example, decays at a steady rate, which is the basis of carbon dating for fossils and artifacts. Other radioisotopes are used in medical imaging and cancer treatment.

Do isotopes behave differently?

Chemically, almost identically — chemistry is driven by electrons, and isotopes have the same number of those. They differ slightly in physical properties (heavier isotopes move a bit more slowly), and some are radioactive while others are stable.

Common mistakes to avoid

  • Don't confuse isotopes with ions. An isotope has a different number of neutrons. An ion has a different number of electrons (giving it a charge). Different particles, different concept.
  • Mass number isn't the same as atomic mass. Mass number is the whole-number count of protons + neutrons in one atom; atomic mass (on the table) is the averaged value across isotopes.

FAQ

Do isotopes have the same number of protons?
Yes — that's what keeps them the same element. They differ only in neutrons.

Why isn't atomic mass a whole number?
Because it's a weighted average of an element's isotopes, which have different masses (e.g., chlorine ≈ 35.5).

What is a radioisotope?
An isotope with an unstable nucleus that decays over time, releasing radiation. Carbon-14 is a famous example.

Are isotopes the same element?
Yes. Same number of protons = same element, regardless of the number of neutrons.

The takeaway

Isotopes are simply different "weights" of the same element — same protons, different neutrons. They're why periodic-table masses aren't whole numbers, and why tools like carbon dating exist.

Pair this with The Periodic Table and What Is the Mole? to see where atomic mass comes from.

Comments

Post a Comment

Popular posts from this blog

What is Pepsin?

Image
In Your Body... Pepsin is a digestive enzyme that performed the best in the acidic condition (pH 2) produced by the gastric juice. It digests and breaks down protein (e.g. meat, seeds, eggs, dairy products, etc.). Keep in mind that pepsin digests proteins (not carbohydrates, not fat) into peptides. These peptides are then either absorbed in the intestine or further break down by enzymes from pancreas. In small intestine (pH around 7), pepsin is no longer effective and no longer digests proteins. Hormones Hormones that regulate digestion and hunger such as gastrin and secretin stimulates the production of pepsinogen, the proenzyme of pepsin. In other words, when pepsinogen is activated, it becomes pepsin.
Read more

The structure of Chimeric Antigen Receptor (CAR) III - Transmembrane Domains

Image
Transmembrane Domains Among the four components of a chimeric antigen receptor (CAR), the transmembrane domain is the least studied one. How different transmembrane domains affect and contribute to each other's functions is also not well studied. That being said, we will go through the function of a transmembrane domain of a CAR, the CD3ζ transmembrane domain, and the CD8α transmembrane domain.  Introduction of the transmembrane domain The transmembrane domain is where the transmembrane proteins are located. In a CAR, the transmembrane domain has the function to anchor the CAR to the T cell membrane. How strong the CAR is anchored to the T cell membrane affects the CAR expression level and stability.  Currently, the commonly used transmembrane domain is derived from proteins such as CD3ζ, CD4, CD8α, and CD28. The CD3ζ transmembrane domain Among the transmembrane domains mentioned, the CD3ζ transmembrane may improve CAR-mediated T cell action as the CD3ζ transmembrane medi...
Read more

Current Limitations of CAR-T Cell Therapy I - Antigen Escape

Image
Antigen Escape Antigen Escape is one of the most concerning challenges during the clinical development of CAR-T cell therapy. A robust initial response is often followed by a relapse in many trials. This is often due to the downregulation or the complete loss of target antigens on the tumor cells. Introduction Antigen escape describes the development of tumor resistance to a single antigen targeting CAR construct. After the initial encounter with the CAR-T cells, the malignant cells mutate and show partial or complete loss of the target antigen expression.  In patients with ALL treated with CD19-targeted CAR-T cell therapy, a downregulation or loss of CD19 antigens was observed.  In patients with myeloma treated with BCMA-targeted CAR-T cell therapy, a downregulation or loss of BCMA expression was observed.  In patients with glioblastoma, a solid tumor treated with IL13Ra2-targeted CAR-T cell therapy, a downregulation of IL13Ra2 expression was observed.  Improve...
Read more