Re: How many particles make up an atom?

Hi Mahvish,

Well the question is a bit like 'how long is a piece of string?' When people talk of the atom as being 'made' of something they talk as though the atom was a little solar system, with the nucleus in the middle and the electrons going round in little ellipses, like planets. We know the atomic world does not behave like that. The rules that govern the universe at that scale are called quantum mechanics, and the rules are very different to the ones obeyed by big lumps of matter like ourselves, or planets. The vacuum, by which I mean space without anything in it, is not empty at all. According to these subatomic rules, particles can blink in and out of existance all they want, but only for short periods of time. Thus, in effect, we have spent years and years trying to understand how the subatomic world works, only to find out that we don't even really understand empty space! This may seem like a big backward step, but we have learned much from our study of the workings of atoms (don't forget that when your grandmother was born the very existance of atoms was not accepted by all scientists!). For instance, we know that the nucleus is made up of protons and neutrons, which in turn are made up of things called 'quarks', and that the electrons don't seem (yet) to be made of anything 'smaller'. When I say smaller, and 'made of', I don't want you to get the idea that the neutron is like a little egg, which you can break open to see three little specks called quarks, or that a neutron looks just like three quarks stuck together like beads. As I said, quantum mechanics is weird. All these 'particles' really behave nothing like particles at all!!! They sometimes look like waves and sometimes like particles, but it depends on HOW YOU LOOK. Read more about this in a book like Schrodinger's Kittens by John Gribbin - it's a bit off the track of your question...

So, what's in an atom. Well, simply stated, it's just the quarks that make up the protons and neutrons (two types, called "up" and "down") and the electrons around the nucleus. But what holds them together? Why don't they just float away from each other? Gravity is far too weak to hold them together - it 's only effective when huge amounts of material are present, like the planets. The forces which hold the atom together are called the 'nuclear force' and the 'electromagnetic force'. The nuclear force sticks the quarks together inside the protons and neutrons, but the quarks inside the OTHER protons or neutrons can also feel a kind of 'left over' force coming out of all the others, so this 'leftover' nuclear force holds the nucleus itself together. The electromagnetic force holds the electrons in the atom, otherwise they'd just float away leaving a bare little nucleus on its own. Imagine if this happened. All the nuclei would feel the attraction of all the others, and they'd all stick together, so the whole earth would collapse into a single tiny ball the size of a rice grain, kind of like all the drops in a rain cloud sticking together to give a little pool of water. So, the electromagnetic force is vital for keeping matter 'inflated' to it's normal size. It's also responsible for little things like chemistry, too ;-)

The thing is, whereas we used to thing that the electromagnetic force was just ripples in something called the 'field', we now know that waves and particles are pretty much interchangeable, and that what used to be called 'short wavelength ripples in the electromagnetic field' are now called particles - photons, to be exact. The same way of describing nature has seeped into our description of the nuclear force, too. The ripples in the nuclear force field are now called gluons, and thought of as particles in the same sense as an electron is. If 'particles' like electrons start behaving like waves, and 'waves' like light start behaving like particles, we may as well give up and call them all 'particle waves' and get on with figuring out how they work. This is what physicists have done, and that's how we discovered things like the gluons - by accepting that nature behaves kind of weird, but just seeing what our theories tell us, rather than going "That can't be true. Things don't happen like that!". On our scale, maybe they don't, but inside the world of the atom, it's a bubbling, fuzzy, strange sea of electrons, gluons, quarks and photons.

Hope that helped!
Best Wishes,
Max