How can you know how many atoms an element has?

Answer

Hi Sanne,

I think you’re confusing a few concepts, although it’s not easy to tell them apart.

Perhaps the most difficult is distinguishing between an element and an atom. An atom is in fact just the combination of a nucleus (composed of protons and neutrons) with an electron cloud around it. However, if we consider an atom with, for example, 2 protons and 2 neutrons in the nucleus and 2 electrons around it, we are talking about an element (in this case Helium). Although there are countless atoms on Earth and beyond, only over a hundred different elements have been identified on Earth to date. In other words, an element is a specific kind of atom.
Compare it to cars. An atom is like a car: a combination of 4 wheels, a steering wheel, some seats and body. There are also very many and very many different types and models. An element is then like the type of car: a Volkswagen Golf, or an Audi A4. There are already a lot less of these, and the model points to the specific properties, just as the element points to its specific chemical properties.

Then there is the difference between an atom and a molecule. A molecule is nothing more than the joining of atoms (of the same or different element(s)). For example, oxygen (O) is an atom, but oxygen gas (O₂) a molecule. When there is only 1 element in a molecule (as in oxygen gas), we call it a simple substance. When there are multiple elements in a single molecule (as in water, H₂O), we call this a compound.

Now, if I understand you correctly, your question is how many atoms of a specific element exist in a particular simple substance. In other words, why phosphorus occurs as tetraphosphorus (P₄) but fluorine as difluor (F₂)?

The answer to that is that you can’t just determine that from the periodic table of elements. So you should in fact learn it by heart.

However, when you look at composite substances, for example organic compounds, you can predict what the molecule will look like more quickly on the basis of the electron configurations.

Conclusion:
If I’ve understood your question correctly, you’ll have to memorize it…

Regards,
Benjamin