- An **atom** is the smallest piece of a chemical element that still behaves like that element. Every atom has two parts: a tiny, heavy [[Atomic nucleus]] built from [[Proton]]s and [[Neutron]]s, wrapped in a cloud of [[Electron]]s. A neutral atom carries exactly as many electrons as protons, so its total [[Electric charge]] is zero.+ An **atom** is the smallest piece of a [[chemical element]] that still behaves like that element. Every atom has two parts: a tiny, heavy [[Atomic nucleus]] built from [[Proton]]s and [[Neutron]]s, wrapped in a cloud of [[Electron]]s. A neutral atom carries exactly as many electrons as protons, so its total [[Electric charge]] is zero.- Almost all of an atom's mass is in the nucleus. Almost all of its volume is empty space where the electrons live.+ Almost all of an atom's [[mass]] is in the nucleus. Almost all of its volume is empty space where the electrons live.- | Bohr radius a₀ (hydrogen's size scale) | 5.291 772 105 44 × 10⁻¹¹ m | CODATA 2022 |+ | Bohr radius a₀ ([[hydrogen]]'s size scale) | 5.291 772 105 44 × 10⁻¹¹ m | CODATA 2022 |
History of Atom
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- - Blow a hydrogen atom up until its proton is a 1 cm marble. The electron cloud around it is then roughly 1.3 km across. That is a marble in the middle of a small town.+ - Blow a hydrogen atom up until its proton is a 1 cm marble. The electron cloud around it is then about 630 m across — roughly six city blocks. That is one marble in the middle of a neighbourhood, and nothing else.
+ An **atom** is the smallest piece of a chemical element that still behaves like that element. Every atom has two parts: a tiny, heavy [[Atomic nucleus]] built from [[Proton]]s and [[Neutron]]s, wrapped in a cloud of [[Electron]]s. A neutral atom carries exactly as many electrons as protons, so its total [[Electric charge]] is zero.+ Almost all of an atom's mass is in the nucleus. Almost all of its volume is empty space where the electrons live.++ ++ ## What it is made of++ | part | how many in a neutral atom | charge | where it sits |+ |---|---|---|---|+ | [[Proton]] | Z — the atomic number | +1 e each | nucleus |+ | [[Neutron]] | A − Z, where A is the mass number | 0 | nucleus |+ | [[Electron]] | Z | −1 e each | cloud around the nucleus |++ Three counts, three different meanings:++ - **Change Z** and you have a different element. Z *is* the element.+ - **Change the neutron count** and you have a different [[Isotope]] of the same element. Same chemistry, different mass and different nuclear stability.+ - **Change the electron count** and you have an ion — an atom with a net charge.++ ## Scale: mostly nothing++ | quantity | value | source |+ |---|---|---|+ | Bohr radius a₀ (hydrogen's size scale) | 5.291 772 105 44 × 10⁻¹¹ m | CODATA 2022 |+ | proton rms charge radius | 8.407 5 × 10⁻¹⁶ m | CODATA 2022 |+ | ratio of the two | ≈ 62 900 | the two rows above |+ | proton-to-electron mass ratio | 1836.152 673 426 | CODATA 2022 |+ | atomic mass constant *u* | 1.660 539 068 92 × 10⁻²⁷ kg | CODATA 2022 |++ Two ways to feel those numbers:++ - Blow a hydrogen atom up until its proton is a 1 cm marble. The electron cloud around it is then roughly 1.3 km across. That is a marble in the middle of a small town.+ - In that same hydrogen atom, the electron carries about 0.054 % of the mass. The proton carries the other 99.95 %.++ ## How many kinds there are++ NIST's table of atomic weights covers **118 elements**, from hydrogen (Z = 1) to oganesson (Z = 118). It lists **354 nuclides** in all and gives a measured natural abundance for **288** of them — those are the nuclides you can find on Earth without building them first. The remaining ones only exist in reactors, accelerators, or stars.++ ## How we worked this out++ 1. **1897** — J. J. Thomson finds the [[Electron]], the first proof that atoms have parts.+ 2. **1909–1911** — Geiger and Marsden fire alpha particles at gold foil. Most pass straight through; a few bounce almost straight back. Rutherford announces the nucleus in 1911: the positive charge and nearly all the mass are packed into a speck at the centre.+ 3. **1913** — Bohr explains why atoms emit sharp spectral lines, by allowing electrons only certain orbits.+ 4. **1932** — Chadwick finds the [[Neutron]], which explains why isotopes of one element differ in mass.+ 5. **1920s onward** — quantum mechanics replaces orbits with orbitals: clouds of probability, not paths.++ ++ ## Where to read next++ - [[Atomic nucleus]] — the speck at the centre, and the force that holds it together+ - [[Proton]] and [[Neutron]] — the two nucleons, and what they are made of+ - [[Isotope]] — same element, different neutron count+ - [[Quark]] — one level further down