Isotopa

Meet Isotopa, the same element as always, just carrying a few extra neutrons.

An element is set by its number of protons, the atomic number Z. Carbon always has 6. But the number of neutrons N can vary, and atoms of one element with different neutron numbers are called isotopes. The mass number A counts both, so A = Z + N. Chemistry depends almost entirely on the electrons, which match the protons, so isotopes of an element behave nearly the same chemically. Frederick Soddy introduced the word isotope in 1913.

Carbon-12 has A = 12, so N = 12 − 6 = 6. Carbon-14 has A = 14, so N = 14 − 6 = 8: two extra neutrons. Carbon-14 is radioactive, with a half-life of about 5,730 years, while carbon-12 is stable.

Isotopes are everywhere in science: radiocarbon dating, medical tracers, uranium-235 for reactors, and heavy water containing deuterium, the hydrogen isotope with one neutron.

A common mix-up: the mass number is a whole-number count, not the atom's exact mass. Atomic masses differ slightly because of nuclear binding energy, which is why tables list values like 12.000 and 14.003.