the atomic nucleus
The atomic nucleus is the tiny, dense knot of matter at the very centre of every atom, made of protons and neutrons packed tightly together, with the electrons swarming far outside it. Everyday image: if an atom were the size of a sports stadium, the nucleus would be a marble at the centre spot, yet that marble holds almost all of the atom's mass. The question it answers: what is inside an atom, and where is its weight and its positive charge concentrated?
Precisely, the nucleus contains protons (positively charged) and neutrons (uncharged), collectively called nucleons, bound together by the strong nuclear force, which overpowers the electrical repulsion between the like-charged protons at very short range. The number of protons, called the atomic number Z, decides which element the atom is; the total number of nucleons is the mass number A = Z + N, where N is the number of neutrons. A typical nucleus is only about 10^-15 metres across, roughly one hundred-thousandth the size of the whole atom, yet it carries over 99.9 percent of the atom's mass.
Why it matters: the nucleus was discovered in 1911 when Rutherford fired alpha particles at gold foil and found a few bouncing straight back, revealing a tiny hard core. Everything about radioactivity, nuclear energy, and the origin of the chemical elements in stars comes down to what nuclei do. Honest note: the nucleus is not a static clump; its protons and neutrons are in constant motion, and whether a given nucleus is stable or radioactive depends delicately on the balance of protons and neutrons it holds.
A helium nucleus is two protons plus two neutrons bound together; it is so tightly bound and stable that it is emitted whole in alpha decay. Its four nucleons carry essentially all of a helium atom's mass, while the two electrons contribute almost nothing.
Almost all the atom's mass sits in a nucleus a hundred-thousandth its size.
The atomic number Z (the proton count) alone decides the element; change Z and you change what element it is, which is exactly what radioactive decay does.