I don't understand relative isotopic mass

Can you explain please? and why does it not have units?

It's the mass of the atom compared to 1/12 of the mass of Carbon-12. Carbon 12 has a relative mass of exactly 12 because its mass is twelve twelfths of the mass of Carbon 12 and everything else is taken relative to that. It's a scale where 1 = 1/12 the mass of Carbon 12. It doesn't have units because it's the ratio between two masses so the units cancel out (kg/kg = unitless). Any ratio between things of equal unit will be unitless.

Okay so bassically you're just dividing the mass of atoms by 1 (cuz 1/12 x 12). And the mass of atoms are written as atomic mass, u? And they cancel out to produce a the RIM?

Does this apply to only isotopes? and why don't we just count the mass of p+ and neutrons and electrons instead of comparing the mass of atoms to 1/12 of c-12?

I'm not entirely sure how much detail you need to know. The reason why we don't just add the masses of protons, neutrons and electrons is that this would always give extremely small numbers so we use a relative system to make numbers more convenient for us.

At a basic level, as you've probably realised, the relative isotopic mass is pretty much the number of neutrons plus the number of electrons (since they weigh almost the same and there are 6 neutrons and 6 protons in Carbon-12 so on the relative scale 1 is very similar to the mass of a proton and neutron). However, the relative isotopic mass actually isn't the same as the number of protons and neutrons because as you may learn in the future, the mass of an atom is actually less than the sum of the masses of the protons, neutrons and electrons (as strange as that may sound). But it's still quite similar.

Relative Isotopic Mass is used to give the mass of a specific isotope. For elements you generally use the Relative Atomic Mass which is the weighted mean of the different isotopes of that element taking into account their relative abundances. So for instance if you've got a sample of chlorine from the natural environment then you'd probably want to use the relative atomic mass since that takes into account the fact that there are different isotopes in the sample but if we were talking about nuclear physics where we're discussing reactions that affect specific isotopes then we'd use the isotopic mass.

Oh okay so its bassically a way to find the weight (mass) of individual isotopes in a simpler way. So magnesium for example ways twice as heavier than c-12 therefore its relative isotopic mass is 24?

Remember that Magnesium is an element, not an isotope. An element is defined by its proton number, i.e. every atom with 12 protons is Magnesium. An isotope has a specific number of protons and neutrons so Magnesium-24 has a relative isotopic mass of 24 (technically slightly lower than 24 for the reasons I mentioned earlier which you probably don't need to worry about). Similarly Magnesium-25 has a relative isotopic mass of 25 etc.

Sorry about that. Okay so isotopes are basically atoms of an element (magnesium) but with different numbers of neutrons but same number of protons. e.g. magnesium-25. Thank you so much for the help - I have a better understanding of the topic thank you! (I just need to know the definition according to the spec but I think its better to understand it as well)