- The **atomic nucleus** is the tiny, dense core of every [[Atom]] — about 1/100,000th of the atom's diameter, yet holding more than 99.9% of its [[mass]]. It is built from [[Proton]]s and [[Neutron]]s, together called *nucleons*, held against the protons' electrical repulsion by the strong nuclear force.+ The **atomic nucleus** is the tiny, dense core of every [[Atom]] — thousands of times smaller across than the atom around it, yet holding more than 99.9% of its [[mass]]. It is built from [[Proton]]s and [[Neutron]]s, together called *nucleons*, held against the protons' electrical repulsion by the strong nuclear force.- | share of the atom's volume | about a quadrillionth |+ | size next to the atom | for [[hydrogen]], the atom's size scale is ~63 000× the proton's charge radius |+ | share of the atom's volume | roughly one part in 10¹⁴ |+ The size row is CODATA 2022 arithmetic: a Bohr radius of 5.291 772 × 10⁻¹¹ m against a proton charge radius of 8.4075 × 10⁻¹⁶ m. Cube that ratio and you get the volume row. An atom is overwhelmingly empty.+- Ernest Rutherford found the nucleus in 1911. Geiger and Marsden had fired alpha particles at gold foil; a few bounced nearly straight back, which a diffuse atom could never do. Rutherford's conclusion: the atom's positive charge and mass sit concentrated in a core "exceedingly small compared with the diameter of the atom".+ Ernest Rutherford announced the nucleus in 1911. Geiger and Marsden had fired alpha particles at gold foil; a few bounced nearly straight back, which a diffuse atom could never do.++ What Rutherford actually wrote, in the *Philosophical Magazine* that May, is more careful than the textbook version:++ > Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents.++ Read the words again. He calls it a **central charge** — a phrase he uses about thirty times in that paper. The word *nucleus* does not appear in it once. He was claiming something narrower than "atoms have a core": he was arguing that the deflections come from the charge acting as a single object rather than from its pieces. The name, and the picture everyone now carries, came afterwards.
History of Atomic nucleus
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- The **atomic nucleus** is the tiny, dense core of every [[Atom]] — about 1/100,000th of the atom's diameter, yet holding more than 99.9% of its mass. It is built from [[Proton]]s and [[Neutron]]s, together called *nucleons*, held against the protons' electrical repulsion by the strong nuclear force.+ The **atomic nucleus** is the tiny, dense core of every [[Atom]] — about 1/100,000th of the atom's diameter, yet holding more than 99.9% of its [[mass]]. It is built from [[Proton]]s and [[Neutron]]s, together called *nucleons*, held against the protons' electrical repulsion by the strong nuclear force.- | held together by | the strong force, at nucleon range |+ | held together by | the [[strong force]], at nucleon range |
+ The **atomic nucleus** is the tiny, dense core of every [[Atom]] — about 1/100,000th of the atom's diameter, yet holding more than 99.9% of its mass. It is built from [[Proton]]s and [[Neutron]]s, together called *nucleons*, held against the protons' electrical repulsion by the strong nuclear force.+ ## The shape of the idea++ - The number of protons (Z) decides **which element** the atom is.+ - The number of neutrons (N) decides **which [[Isotope]]** of that element it is.+ - The mass number A = Z + N is the count of nucleons.++ ## Properties++ | property | value |+ |---|---|+ | typical radius | 1–10 femtometres (10⁻¹⁵ m) |+ | share of the atom's mass | > 99.9% |+ | share of the atom's volume | about a quadrillionth |+ | held together by | the strong force, at nucleon range |++ ## Binding energy++ ++ The [[Nuclear binding energy]] per nucleon peaks near iron. That single curve explains both nuclear power sources: **fusing** nuclei lighter than iron releases energy (stars), and **splitting** nuclei heavier than iron releases energy (reactors).++ ## Discovery++ Ernest Rutherford found the nucleus in 1911. Geiger and Marsden had fired alpha particles at gold foil; a few bounced nearly straight back, which a diffuse atom could never do. Rutherford's conclusion: the atom's positive charge and mass sit concentrated in a core "exceedingly small compared with the diameter of the atom".