Question Video: Reading Number of Electrons, Protons, and Neutrons from Atomic Number and Mass Number

Video Transcript

An aluminum atom has an atomic number of 13 and a mass number of 27. How many protons are there in an atom of aluminum?

Aluminum is an element. An aluminum atom consists of a positively charged nucleus surrounded by a negatively charged electron cloud. Inside the nucleus are positively charged protons and neutral neutrons. Our first question is, how many protons are there in an atom of aluminum? As I’ve just said, those protons are to be found in the nucleus. The question has given us two properties of an aluminum atom that might help: the atomic number and the mass number.

The atomic number of an atom is the number of protons in the nucleus of that atom. For this aluminum atom and indeed all aluminum atoms, the atomic number is equal to 13. Therefore, the number of protons in an atom of aluminum is 13. I’ll just store away that value because it will come in handy later.

How many neutrons are there in an atom of aluminum?

As with protons, neutrons can be found in the nucleus of the atom of aluminum. To answer this part of the question, we’re going to need the mass number. The mass number is the number of protons and neutrons in the nucleus of the atom. For this particular atom of aluminum, the mass number is 27. Therefore, in this particular atom of aluminum, the number of protons and neutrons is 27. So we can calculate the number of neutrons by taking the number of protons away from the number of protons and neutrons. Therefore, the number of neutrons is equal to 27 protons and neutrons minus 13 protons, which we can simplify to 27 minus 13 which gives us 14 neutrons. This is the same number we would get if we took away the atomic number from the mass number. So this particular atom of aluminum has 14 neutrons.

Now, onto the last part of this question.

How many electrons are there in an atom of aluminum?

The electrons of an atom of aluminum can be found in the space around its nucleus. In order to answer this question, we need three bits of information. The first piece of information is that atoms in this context are neutral overall. Individual protons are said to have a charge of positive one and electrons are said to have a charge of negative one. Neutrons have no charge whatsoever. So we don’t need to include them in this part of the question. The overall charge for an atom or ion is always the total charge of the protons plus the total charge of the electrons.

In this case, we have 13 protons. So the charge of the protons is equal to 13 multiplied by the charge per proton positive one. The total charge of our electrons is unknown because we don’t know the number of electrons. But if we did, the total charge of electrons would be equal to the number of electrons multiplied by the charge per electron, negative one. In this case, we know that the overall charge is equal to zero. With a little bit of jiggling about, we can see that the number of protons is equal to the number of electrons from neutral particle. Therefore, for an atom of aluminum, there are 13 electrons inside it.

So we’ve demonstrated that for an aluminum atom which has the atomic number of 13 and has a mass number of 27, there are 13 protons, 14 neutrons, and 13 electrons.