Video Transcript
The diagram shows a lattice of silicon atoms that an atom of phosphorus is added to. An atom of phosphorus has five valence electrons. An atom of silicon has four valence electrons. How many covalent bonds does the phosphorus atom form with the silicon atoms adjacent to it? How many free electrons are added to the lattice when the phosphorus atom is added? What is the net relative atomic charge of the phosphorus after it is added to the lattice?
Let’s start with this question about the number of covalent bonds that phosphorus forms. Recall that in a silicon lattice, like we see here, each silicon atom typically forms one covalent bond with each of its four neighboring atoms. For instance, this silicon atom here has one, two, three, four covalent bonds with the four atoms adjacent to it. Each silicon atom has its own four valence electrons. And so, also having four covalent bonds allows each silicon atom to have a full eight electrons in its outermost electron shell. This sharing of outermost electrons also happens when phosphorus is added to a silicon lattice.
From the diagram, we can see that phosphorus has a full eight electrons in its outermost shell. And like a central silicon atom, there are four pairs of electrons being shared with the four adjacent silicon atoms. Each shared pair of electrons constitutes a covalent bond. And therefore, phosphorus forms four covalent bonds with the silicon atoms adjacent to it. Now let’s think about the number of free electrons that are added to the lattice in this scenario.
We know that the phosphorus atom came with five valence or outermost electrons. We also know that phosphorus forms four covalent bonds with the neighboring silicon atoms, and this contributes four extra electrons to the outermost shell of phosphorus. Recall that this outermost shell can only fit eight electrons in it. So, only four out of the five valence electrons that phosphorus came with can fit in the shell once the covalent bonds are formed. This means that one electron that phosphorus came with has no room in the shell, so it gets donated to the lattice as a free electron. Therefore, one free electron is added to the lattice when the atom of phosphorus is added.
Finally, what’s the net relative charge of the phosphorus atom now? Well, we know that the phosphorus was added to the lattice with five valence electrons, meaning it was neutral or had no net charge. But once added to the silicon lattice, we know that one of its electrons became freed, meaning it’s no longer bound to the phosphorus atom. Since phosphorus lost one electron which has a relative charge of negative one, the phosphorus is now a positively charged ion with a net relative charge of positive one.
