Do atoms of the same element differ in size?

Question

I know that atoms can differ in size based on how many protons and electrons it has. But assuming that the atoms had the same amount of mass, could there be any measurable difference in size between them. I know that change in size at a quantum level is both very relative and very negligible, but I want to know anyway.

Answer 1

Atoms of the same element have the same size unless they are ions of that element. If the element forms a positively charged ion it will be smaller than the neutral atom. If the element forms a negatively charged ion, it will be larger than the neutral atom.



====== Follow up ======



"Cogito" says "yes", that atoms of the same element have different sizes and then goes on to refute that notion. Then don't say "yes." Perhaps he is confusing mass and atomic radius. Even if you consider the "size" (nuclear radius) of the nucleus, it is but a tiny, tiny fraction of the size (atomic radius) of the atom. Therefore, the various isotopes of an element will be the same size, but not the same mass. The atomic radius of a free atom is indeed a nebulous concept. It's like measuring the size of a cloud in the sky. It doesn't have a clear-cut boundary, it just tapers off to the point where there isn't any more cloud. Atoms are similar in concept.



On the other hand, the notion of atomic radius of atoms in a homonuclear diatomic molecule is more well-defined. It is possible to measure the distance between atom centers. The atomic radius is therefore, half the distance between the nucleii. Atomic radii can also be measured in more complex compounds.



But as for your original question, the "size" or the average atomic radii of the atoms of the same element in the same environment does not vary.

Answer 2

Yes. Even if you ignored the electron shells and only looked at the nucleus, the answer is still yes. A single element can come in a variety of isotopes that will change the mass / size of the nucleus.



Personally I think defining the size of an element by the distance of the furthest electron shell is as absurd as defining the size of a star by the distance of its furthest planet. Its nonsense. The element doesnt change with more or fewer electrons, regardless of their energy states. The element itself is the same, ergo it cannot and ought not be defined by its electrons. Its the protons in the nucleus that define the element. The size of an element is no more defined by its valence shell as the size of our sun is defined by the distance of the planet Pluto.



You need to be more rigorous with your meaning of "size". Volume is of little importance as almost all of the volume contained within the outer valence shell is empty space. Thats true of any element and any isotope and any ionization. If you mean "at what distance can electrostatic forces interact" then, well, the answer is infinite. Electrostatic forces have the same reach as gravity. So again we lack rigor with the definition of size.



Perhaps I am just too layman to understand the notion but I dont see how such an arbitrary concept as size has any value in chemistry. "The reach of electrostatic forces" I get. Probability and statistics and confidence comes into play. There are no clear cut and dry borders. Ive always thought that measurements of elemental size (not the nucleus) was a childs game.

Answer 3

Atoms of the same element, with the same number of electrons, can have different amounts of energy. This can be from vibration or rotation. This would result in _very_ small differences in sizes.