[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to use the atomic and the mass numbers to work out the number of protons neutrons and electrons in atoms you should then be able to describe what's meant by an isotope and an ion in the last video we looked at the structure of atoms we saw that atoms contain a central nucleus and this contains protons which are positive and neutrons which are neutral they've got no charge around the nucleus we've got electrons and they're in energy levels remember that electrons are negative now every element has a symbol and these always have two numbers in this video we're going to learn how to use them I'm showing you here the symbol for the element sodium you can see that we've got a large number in this case 23 and a smaller number in this case 11 the smaller numbers called the atomic number and this tells us the number of protons so as you can see a sodium atom has 11 protons in its nucleus now it's really important to understand that all atoms of sodium have 11 protons if an atom doesn't have 11 protons then it's not sodium now as we saw before protons have a positive charge but atoms have got no overall charge and that's because the number of electrons is equal to the number of protons that means that the negative charges on the electrons can cancel out the positive charges on the protons so the atom is neutral overall going back to the symbol for sodium we can see that because sodium atoms contain 11 protons they also contain 11 electrons now the larger number is called the mass number this tells us the total number of protons and neutrons added together to calculate the number of neutrons we subtract the atomic number from the mass number so in the case of sodium the mass number is 23 subtracting 11 from 23 tells us that sodium atoms have got 12 neutrons now there is one important point about neutrons that we need to look at sometimes atoms of the same element have got different numbers of neutrons we call these Isotopes and I'm showing you two isotopes of sodium here as you can see they both have an atomic number of 11 and that's because all sodium atoms have got 11 protons however the isotope on the left has got 12 neutrons whereas the isotope on the right has got 13 neutrons a lot of Isotopes are radioactive and we're going to look at radioactivity in later videos now as we said before the number of electrons in an atom is the same as the number of protons and that means that atoms have got no overall charge however atoms can lose electrons from their outer energy level and I'm shown you that here for sodium so the sodium atom has 11 protons and 11 electrons as you can see it's got no overall charge when a sodium atom loses one electron from its outer energy level it now has 11 protons but only 10 electrons so it's got an overall charge of plus one scientists call atoms with a charge like this an ion so this is now called a sodium ion so remember that when an atom loses one electron the ion now has a one positive charge if an atom loses two electrons then the ion has a two positive charge now in chemistry we saw that atoms can also gain electrons to form negative ions but that's not required in the physics specification remember you'll find plenty of questions on Atomic and mass numbers Isotopes and ions in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to use the atomic and mass numbers to work out the number of protons neutrons and electrons in atoms you should then be able to describe what's meant by an isotope and an on [Music]