Video Transcript
The following figure shows five silicon atoms. Only the electrons in the outer shells of the atoms are shown. Which of the following best describes why the figure does not correctly show the structure of a silicon lattice? (A) A total of four electrons have disappeared from the outer shells of these atoms. (B) A total of five electrons have disappeared from the outer shells of these atoms. (C) A lattice must contain an even number of atoms. (D) The outer shells of the atoms overlap.
Looking at our arrangement of silicon atoms, we see that indeed the outer shells do overlap. However, this is a necessary condition of the atoms forming a lattice. Because these outer shells do overlap, bonds between the silicon atoms can form where electron sharing is then enabled. Due to its electronic structure, a single atom of silicon has four electrons in its valence, or outermost shell. When silicon atoms form a lattice with overlapping outer shells, covalent bonds form between adjacent atoms, allowing electron sharing.
The overall effect of this is that for any silicon atoms in the center of the bulk material — our example, here is our central silicon atom — thanks to electron sharing, those central or bulk atoms have eight electrons in their valence shell. That’s the way it should be. But here, we see that our central silicon atom has one, two, three, four valence electrons. There are four missing, we could say, from this shell.
If we look at the other silicon atoms in our arrangement, we can see the reason for the shortage of electrons. The silicon atom at the bottom of our arrangement only has one, two, three valence electrons shown here. The same thing is true for the silicon atom to the left of our central atom; it has one, two, three valence electrons. And as we look at the silicon atom to the right and above our central atom, we see the same holds true for these.
Each one of our four outer silicon atoms is missing a valence electron. These electrons are therefore unavailable to be shared with the central silicon atom. That central atom contributes its one, two, three, four valence electrons but has none added to it through sharing with the other atoms. Answer option (A) best describes this scenario. A total of four electrons have disappeared from the outer shells of these atoms. This is the best reason why the given arrangement does not represent the structure of a silicon lattice.
