Question Video: Bonding of the Outermost Electrons in Pure Silicon Physics • 9th Grade

An atom of Si is part of an object made of Si atoms, as shown in the diagram. Only the electrons in the outermost shells of the atom are individually represented. How many of the electrons in the outermost shell of an atom in the object form covalent bonds with adjacent atoms?

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Video Transcript

An atom of silicon is part of an object made of silicon atoms, as shown in the diagram. Only the electrons in the outermost shells of the atom are individually represented. How many of the electrons in the outermost shell of an atom in the object form covalent bonds with adjacent atoms?

Okay, so we’re told that in this diagram, all of the atoms represented are silicon atoms. And we can think of these red circles as the nuclei of these silicon atoms. And then the smaller blue dots around them represent valence electrons. That is, they’re electrons in the outermost shells of the respective atoms. Our question asks, how many of these electrons in the outermost shell, that is, these valence electrons, of an atom that is in the object form covalent bonds with adjacent atoms? To answer this question, there’s something important we’ll need to know about silicon.

When we have a neutral silicon atom, say that this is the nucleus of that atom, then that silicon atom possesses four electrons in its outermost shell. And we might represent that like this. This is the natural state of a neutral silicon atom when it’s all by itself. We see in our sketch though that we have many silicon atoms represented here. They’re all arranged in this orderly formation called a lattice. We can see that lattice in the even rows and columns of all these silicon atoms.

When they’re arranged this way, adjacent silicon atoms, that is, atoms that are right next to one another in the lattice, are actually capable of sharing electrons between them. So, for example, if we had a second silicon atom right here that also had its own four valence electrons, then the two electrons in this outermost energy level here and here, right between the adjacent atoms, could be shared by a bond called a covalent bond. Through this sharing, we could say that both of these silicon atoms possess both of these electrons. And this would mean that for each of these atoms individually, whereas it used to have four electrons in its outermost shell, now it has five.

If we take this idea of electron sharing and apply it to the silicon atoms in our object, specifically for the interior silicon atoms, that is, the ones in the object, like, for example, this one right here, we can see that even more electron sharing goes on. That’s because this particular silicon atom is adjacent to this atom and this atom and this one and this one. It shares a side, we could say, with four other identical silicon atoms. And therefore, rather than one instance of electron sharing, like we saw here with our two adjacent atoms, there are now four opportunities for that to happen for our interior silicon atom. And we see the result of this for this highlighted interior atom right here.

Notice that it has one, two, three, four, five, six, seven, eight electrons in its outermost shell. That has happened, we can say, because the four original electrons in this individual silicon atom’s outermost shell now have four other electrons added to their total, one each, thanks to sharing between this silicon atom and the four adjacent to it. That is, this atom here shares one electron with this silicon atom, as does this atom here, as does this one, and this one. It’s through the sharing process that an interior silicon atom, such as this one, is able to populate a full or a complete outermost shell with eight electrons.

So then, we’ve taken the four original valence electrons, and through the formation of covalent bonds, we’ve added four more to them. This is true for all of the atoms inside this object, that is, in its interior.

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