[Rutherford]

Which of the parallelograms in the figure below are unit cells?

I thought A and E, because the points that represent atoms are regularly connected with lines that represent bonds. To my surprise, D is also a unit cell. I don't understand how. What type of unit cell is it?

[Borek]

What is unit cell?

[Rutherford]

The smallest part of a crystal that has the properties of the crystal.

[Rutherford]

In D, there is only 1 atom and the lines that represent the box (cube) aren't passing through it, so how can it be regarded as a unit cell?

[Borek]

The smallest part of a crystal that has the properties of the crystal.

Not exactly. It is the smallest part that will give you a crystal when repeated.

[Rutherford]

In that sense, a single atom when repeated in all 3 directions can produce a crystal.
In that sense, why C can't be regarded as a unit cell?

[sjb]

In that sense, a single atom when repeated in all 3 directions can produce a crystal.
In that sense, why C can't be regarded as a unit cell?

The lines in C go outside the middles of the points, |o  o|, repeated will give |o  o|o  o|o  o| (moving horizontally), rather than e.g. Θ--Θ which repeated will give

Θ--Θ--Θ
|  |  |
|  |  |
Θ--Θ--Θ

(best viewed in monospace font, e.g. Courier or similar; please excuse gratuitous use of thetas etc)

[curiouscat]

In that sense, a single atom when repeated in all 3 directions can produce a crystal.
In that sense, why C can't be regarded as a unit cell?

If you repeat just an atom you get this:

oooooooooooo
oooooooooooo
oooooooooooo
oooooooooooo

What you want is this:
o    o    o    o    o

o    o    o    o    o

o    o    o    o    o

o    o    o    o    o

A crystal structure is not just its constituent atoms but also the empty space between the atoms because the empty space is what keeps the atoms in unique locations characteristic of that particular crystal structure.

Ergo an atom is not a unit cell (unless what you are trying to represent is a close contact heap).

[curiouscat]

The smallest part of a crystal that has the properties of the crystal.

Not exactly. It is the smallest part that will give you a crystal when repeated.

Isn't that a primitive unit cell?

Can't a unit cell be any part that will give you a crystal when repeated?

Not sure. But am curious to know the difference.

[Rutherford]

Then I assume that D is an atom with empty space around it (as it is represented). In C there won't be any space between atoms, in B there are no atoms, so A, D and E are correct. Thanks to everyone for helping me here.

[Borek]

Not exactly. It is the smallest part that will give you a crystal when repeated.

Isn't that a primitive unit cell?

Can a unit cell be any part that will give you a crystal when repeated?

Good questions.