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Updated February 13, 2023
  Size and energy of atoms

Image: Atomic orders of magnitude.
In the world of the infinitely small, the orbitals of the electron can take different characteristic forms depending on the nature of the atom. For example the orbitals of hydrogen have a spherical shape, the orbitals of oxygen have the shape of two drops of water, the orbitals of iron have the shape of four drops of water. This shape of the atomic orbital defines the size of the atom.
The characteristic sizes of atoms or the distances between nuclei in molecules are of the order of angstroms (one tenmillionth of a millimetre) in agreement with experiment. We can say that the atoms are separated from each other by a few angstroms.
However, the electronic cloud of an atom does not have a well-defined dimension because it is a superposition of atomic orbitals of a probabilistic nature. There is therefore no very precise measurement of the size of atoms because the shape of this region of atomic space depends on the energy of the electron and its angular momentum. The characteristic energies of atoms are of the order of an electron-volt.
The sizes and energies of atoms or molecules are obviously too small to be directly observable at our scale. But their effects can be amplified and made visible thanks to the colossal number of atoms that make up the world on our scale.
1 mole of hydrogen (1H) weighs 1 g.
1 mole of iron (56Fe) weighs 56 g.
This amount of matter is made up of 6.02 x 1023 elementary entities.
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