# Video: Understanding the Atomic Number and the Mass Number

An uncharged atom has an atomic number of 6 and a mass number of 11. How many protons does that atom have? How many neutrons does that atom have? How many electrons does that atom have?

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### Video Transcript

An uncharged atom has an atomic number of six and a mass number of 11. How many protons does that atom have? How many neutrons does that atom have? And how many electrons does that atom have?

So to make sure we don’t miss anything out, let’s underline all the important bits of the question. So what we’ve got is an uncharged atom. This part is really easy to miss. We also know that it’s got an atomic number of six and a mass number of 11. But, what do we mean by atomic number and mass number?

Atomic number is defined as the number of protons in a nucleus. And mass number is defined as the number of nucleons, that’s protons and neutrons, in a nucleus. By the way, protons and neutrons are called nucleons because you find them in the nucleus of an atom. So based on that, we’ve got enough information to answer our question.

First of all, how many protons does that atom have? Well, we know that it’s got an atomic number of six. And the atomic number is just the number of protons. So these two numbers are identical. It has six protons.

Secondly, how many neutrons does that atom have? Well, we’ve got the mass number and we’ve got the atomic number. Now the mass number is the total number of nucleons. That’s protons and neutrons put together. And that total number is 11. But we know that out of those 11, six of them are protons. So the remaining number are neutrons. So in order to find that out, what we need to do, is to do 11 minus six. And that ends up being five. So we’ve got five neutrons. And that does make sense. If we’ve got six protons and five neutrons, then together we’ve got 11 nucleons. That’s protons and neutrons together. And hence, the mass number is 11.

Finally, we need to work out how many electrons that atom has. Now in order to answer this, we need to look at the very first piece of information that we’ve been given in this question. The fact that the atom is uncharged. If the atom is uncharged, then the total overall charge of the atom is zero. That’s what uncharged means. So in order to do that, all of the positive charges in the atom have to be cancelled by all of the negative charges. Or, in other words, the number of negative charges in the atom has to be exactly the same as the number of positive charges in the atom. That way, each of those negative charges can cancel out each one of those positive charges. So we know that we’ve got six protons in the atom. And those protons are positively charged. Therefore, we’ve got six positive charges in the atom. We also know that electrons are negatively charged. So it’s the electrons that’ll be cancelling out all other protons. Hence, the number of electrons has to be the same as the number of protons. So we’ve got six electrons in the atom. And that’s the final answer to our question.