In silicon crystals and in most other semiconductors, the atoms are kept in their positions by covalent bonds that have certain directions in space, e.g. towards the 4 vertices of a regular tetrahedron in the case of a silicon crystal.
Because of that, the atoms are not packed together like some spheres, i.e. like the atoms in metallic aluminum or in table salt, where you may speak about the diameter of the atomic spheres. They are distributed on a lattice that has empty spaces between atoms and their bonds (i.e. the places where electrons belonging to the atoms are located with high probability).
The distance between the silicon atoms in a silicon crystal varies depending on the direction, so there is no single value that could be considered the diameter of a silicon atom.
The periodic cell of a silicon crystal has the same structure as that of cubic diamond and it has the form of a cube with 8 atoms inside it (an atom in a cube corner counts as 1/8 inside, an atom on a face counts as 1/2 inside).
While this visualization uses balls and sticks, to show the positions of the centers of the atoms, that has nothing to do with the form of the real atoms.
At most you could consider that a silicon atom has the form of the corresponding Voronoi polyhedron, in which case you would have to give several numbers, to describe its size, and not a single "across" value:
The number that I have provided, i.e. 50 Si atoms per cubic nanometer of Si crystal, can be computed by dividing 8 atoms to the volume of the cubic cell of the Si lattice. Given a volume of Si crystal, you can compute the number of Si atoms.
Like I have said, I do not know what means that 0.2 nm value, as the distance between 2 neighbor Si atoms can be larger than 0.5 nm, depending on the direction. In any case you cannot use it to compute anything about the number of atoms in a silicon device.
EDIT: I believe that you might have got your 0.2 nm from truncating the distance between 2 silicon atomic planes in the so-called "111" direction (the direction of the cube diagonal), which is the minimum distance between atomic planes in silicon.
That distance is 0.543 nm * sqrt(3) / 4 = 0.235 nm.
Because this distance is correct only for the "111" crystalographic direction, it cannot be used to compute the number of atoms in some piece of silicon, and it certainly cannot be called as the diameter of a silicon atom ("across an atom" without specifying the direction).
Because of that, the atoms are not packed together like some spheres, i.e. like the atoms in metallic aluminum or in table salt, where you may speak about the diameter of the atomic spheres. They are distributed on a lattice that has empty spaces between atoms and their bonds (i.e. the places where electrons belonging to the atoms are located with high probability).
The distance between the silicon atoms in a silicon crystal varies depending on the direction, so there is no single value that could be considered the diameter of a silicon atom.
The periodic cell of a silicon crystal has the same structure as that of cubic diamond and it has the form of a cube with 8 atoms inside it (an atom in a cube corner counts as 1/8 inside, an atom on a face counts as 1/2 inside).
A visualization from Wikipedia:
https://en.wikipedia.org/wiki/Diamond_cubic#/media/File:Diam...
While this visualization uses balls and sticks, to show the positions of the centers of the atoms, that has nothing to do with the form of the real atoms.
At most you could consider that a silicon atom has the form of the corresponding Voronoi polyhedron, in which case you would have to give several numbers, to describe its size, and not a single "across" value:
https://en.wikipedia.org/wiki/Triakis_truncated_tetrahedron
The number that I have provided, i.e. 50 Si atoms per cubic nanometer of Si crystal, can be computed by dividing 8 atoms to the volume of the cubic cell of the Si lattice. Given a volume of Si crystal, you can compute the number of Si atoms.
Like I have said, I do not know what means that 0.2 nm value, as the distance between 2 neighbor Si atoms can be larger than 0.5 nm, depending on the direction. In any case you cannot use it to compute anything about the number of atoms in a silicon device.
EDIT: I believe that you might have got your 0.2 nm from truncating the distance between 2 silicon atomic planes in the so-called "111" direction (the direction of the cube diagonal), which is the minimum distance between atomic planes in silicon.
That distance is 0.543 nm * sqrt(3) / 4 = 0.235 nm.
Because this distance is correct only for the "111" crystalographic direction, it cannot be used to compute the number of atoms in some piece of silicon, and it certainly cannot be called as the diameter of a silicon atom ("across an atom" without specifying the direction).