The density of different atoms can vary more than the density of the earth’s crust and nucleus.

The density of different atoms can vary more than the density of the earth's crust and nucleus.

In 1913, Niels Bohr developed his famous atomic model. In this model the electrons rotate in circular orbits around the nucleus, occupying the lowest possible energy orbit, or the closest possible orbit to the nucleus.

However, the popular idea of ​​an atom is, to some extent, a schematic distortion, a model of understanding for laymen … an approach that little or nothing resembles reality. Atoms are not exactly how they are drawn in textbooks. In fact, the atoms are far from all the same: they are very different from each other .

Densities and shapes

As Santiago Álvarez explains in the book Of women, men and molecules, the density of atoms can vary by two orders of magnitude from one element to another, more than the density of the Earth varies from the crust to the nucleus:

If we calculate the electron cloud and nucleus densities separately, in the case of helium we find values ​​of 2×3 10 17 and 3.52 x 10 -5 kg / m 3 , respectively. That is, the density of the nucleus is about 10 21 times greater than that of its electron environment. We do not have references in the everyday world to gauge the meaning of these densities. Even the air in the stratosphere does not have a density as small as that of the electron cloud. We should go up to the thermosphere, about 100 km above the Earth’s surface, to find a comparable density. In the case of the nucleus, only a neutron star has such a high density, second only to that of black holes.


Nor are atoms always spherical . And even less is an atom within a molecule spherical. Even the atoms of the same element can differ in a series of attributes:

  • Number of neutrons (in different isotopes).
  • Number of electrons (in multiple oxidation states).
  • Degree of pairing of electrons (in various spin states).
  • Molecular environment variable in coordination number and stereochemistry that also affects shape and size.

Thus, we could affirm that the invariant property between the atoms of the same element is its atomic number (number of protons in its nucleus), the property that defines a chemical element, despite the fact that chemical reactivity and its ability to bond formation reside in valence electrons.

Daltons Symbols Various atoms and molecules as shown in John Dalton’s A New System of Chemical Philosophy (1808).

In this way, the writing of Margaret Lucas Cavendish , Duchess of Newcastle, in her book Poems and Fancies (1653) no longer produces so much blush:

  • Pointed atoms make the Fire subtle, swift and dry,
  • The Long as arrows in the air fly,
  • The Round ones turn into water, humid,
  • The Squares on Earth, with an immovable Form;
  • Square atoms make up hard Minerals,
  • The soft vegetables appropriate round atoms.

However, we need models to understand reality, because reality is not fully comprehensible :