What does an atom really look like? A ball? Or like this, with electrons in circles around a nucleus? Maybe like this? With a nucleus in the middle and the electrons swarming around it like flies?
Yes, this picture is close to reality. The electrons can be both close to the nucleus - and very far from it. They fly around with tremendous speed - almost the speed of light. But if you tried to capture an electron - you'd discover something peculiar. It's impossible to know exactly where the electron is.
And the electrons are so small it's impossible to see them. Not even with the most powerful microscope. However, you can estimate in which area the electrons usually are. Then we find they are actually grouped at different distances from the nucleus. So we often draw the atom like this.
As a nucleus - with electrons in different shells. Almost like planets around a star. This is not an accurate picture of reality. It's a model - that makes it easier to understand how the atom works. This particular atom is Argon, a chemical element with eighteen electrons.
Argon has two electrons in the inner shell. More will not fit there. Then the atom has eight electrons in the next shell. That's also as many as can fit there. And then there are another eight electrons - in the outermost shell.
The electrons in the outermost shell are called valence electrons. They determine which chemical compounds the atom is able to form. Argon is an inert gas: It does not want to unite with other elements. Simply because the argon atom has eight electrons in its outer electron shell. Therefore, it's content...
and stable. The larger the atom - the more electron shells it can have. Very big atoms have up to seven shells. To make space for all the electrons, the atom packs more than eight into some of the shells. But never more than eight electrons in the outer shell.
The smallest atoms - hydrogen and helium - have just one electron shell. The thing is, all the atoms - but the smallest - want eight electrons in their outermost shell. This is called the Octet Rule - atoms strive to have exactly eight valence electrons. Octet! Isn't that a group of eight musicians?
Yes, and the word comes from the Latin word octo, meaning - eight.