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I am currently studying Introduction to Solid State Physics, 8th edition, by Kittel. Chapter 1 provides the following figure and accompanying explanations:

enter image description here

It also says the following:

An ideal crystal is constructed by the infinite repetition of identical groups of atoms. A group is called the basis. The set of mathematical points to which the basis is attached is called the lattice. The lattice in three dimensions may be defined by three translation vectors $\mathbf{a}_1$, $\mathbf{a}_2$, $\mathbf{a}_3$, such that the arrangement of atoms in the crystal looks the same when viewed from the point $\mathbf{r}$ as when viewed from every point $\mathbf{r}^\prime$ translated by an integral multiple of the $\mathbf{a}$'s: $$\mathbf{r}^\prime = \mathbf{r} + u_1 \mathbf{a}_1 + u_2 \mathbf{a}_2 + u_3 \mathbf{a}_3. \tag{1}$$ Here $u_1, u_2, u_3$ are arbitrary integers The set of points $\mathbf{r}^\prime$ defined by (1) for all $u_1, u_2, u_3$ defines the lattice. The lattice is said to be primitive if any two points from which the atomic arrangement looks the same always satisfy (1) with a suitable choice of the integers $u_1$.

Here are some lattice points I drew:

enter image description here

enter image description here

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It doesn't seem to me that any of these lattice points are primitive; that is, I don't think that any of these lattice points satisfy $\mathbf{r}^\prime = \mathbf{r} + u_1 \mathbf{a}_1 + u_2 \mathbf{a}_2 + u_3\mathbf{a}_3$. The irregular pattern present in this space lattice makes it difficult to see how this could be done.

So, in order to satisfy figure 3c, what are the lattice points of interest?

I would greatly appreciate it if people would please take the time to explain this.

EDIT

enter image description here

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  • $\begingroup$ How about a cell larger than your 1st try but smaller than the last one? $\endgroup$
    – marcin
    Jun 19 '20 at 16:10
  • $\begingroup$ @marcin Is that the cell that you were referring to? But isn't there still a gap? $\endgroup$ Jun 19 '20 at 18:55
  • $\begingroup$ A cell, above all other definitions, is the thing that repeats. Does your first tentative cell repeat to the right and to the left? $\endgroup$ Jun 19 '20 at 19:53
  • $\begingroup$ The cell after EDIT looks good to me. There won't be a gap (unless I overlooked something) $\endgroup$
    – marcin
    Jun 19 '20 at 19:58
  • $\begingroup$ @marcin shouldn’t the cells be making contact, so that where one ends another begins? $\endgroup$ Jun 19 '20 at 20:05
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I think the cell after EDIT is correct. Here I added a few neighboring cells:

image with more unit cells

Admittedly, the atom in the right bottom corner is missing. Apparently the author didn't consider it important.

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You have to take care about the definition of lattice and crystal structure. Quoting your text "An ideal crystal is constructed by the infinite repetition of identical groups of atoms. A group is called the basis. The set of mathematical points to which the basis is attached is called the lattice". So the crystal structure is constituted of a basis (an atom or a group of atoms) attached to a lattice.

By definition a lattice is an infinite array of mathematical points in space, in which each point has identical surroundings to all others. your Fig. 3a represents a lattice. Conversely your fig 3c does not represent a lattice; in fact in the figure caption it is stated that the points represent atoms. In this case case each pair of points represent your basis, not lattice points. You're asked to find the lattice points. The cell after EDIT is correct; in that case lattice points are at the same position of one of the two points forming the pair.

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