Atoms differ from each other mainly by how many protons they have, which is called their atomic number. Different proton counts create different elements (hydrogen, carbon, oxygen, etc.), and variations in neutrons and electrons fineā€‘tune their behavior.

Core difference: proton number

  • Each element is defined by a specific number of protons in its nucleus, called the atomic number.
  • If you change the proton number, you no longer have the same element: for example, 1 proton = hydrogen, 6 protons = carbon, 8 protons = oxygen.
  • This proton number controls the pattern of electrons around the nucleus, which in turn sets the elementā€™s typical chemical reactions.

So, at the most basic level, what makes one atom different from another atom is the number of protons in its nucleus.

Neutrons: same element, different version

Even when proton number is the same, atoms can still differ by how many neutrons they have.

  • Atoms of the same element with different neutron counts are called isotopes (like hydrogen-1, hydrogen-2, hydrogen-3).
  • Isotopes share almost the same chemistry (same protons and electrons) but can differ in mass and stability; some isotopes are radioactive.

So neutrons give you ā€œflavorsā€ of the same element, without changing which element it is.

Electrons: chemistry and charge

Electrons orbit the nucleus and largely determine how an atom bonds and reacts.

  • In a neutral atom, the number of electrons equals the number of protons.
  • If an atom gains or loses electrons, it becomes an ion, which changes its charge and how strongly it attracts or repels other atoms, but not what element it is.

Different electron arrangements (especially in the outermost shell) are why some atoms are very reactive (like sodium) while others are very unreactive (like neon).

Zooming out: why matter feels so different

When people ask ā€œwhat makes one atom different from another?ā€ they often also wonder how different materials arise if everything is just atoms.

  • Different elements (different proton numbers) combine in different ratios and structures to form molecules and solids with very different propertiesā€”soft flesh, hard wood, shiny metal, etc.
  • Like letters in an alphabet, a small set of atom types can be arranged and combined in countless ways to build the huge variety of substances in the universe.

TL;DR: One atom differs from another mainly by its number of protons (its atomic number), which decides what element it is; differences in neutrons create isotopes, and differences in electrons change charge and chemical behavior.

Information gathered from public forums or data available on the internet and portrayed here.