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So, how does all this explain why a sodium atom reacts differently from a lithium atom?
Before this, you would have said “they are different materials so of course they will react differently” and you would be right. Now that you know that the amount of protons determines the element, something doesn’t add up anymore.
Look at the scale of an atom: the distance between the electron and the nucleus of a hydrogen atom is 5,29177.10-11 m (according to Niels Bohr). The radius of its nucleus (a single proton) is still a bit unknown but is believed to be around 0,8768.10-15 m. Both are extremely small numbers of course but this does mean that the atom is about 60 000 times as large as the nucleus.
To put this into perspective; When the nucleus would be as large as a marble (2 cm wide), the electron would be 600 metres away! In a hydrogen molecule (H2) there would be 1,2 km between the two nuclei!
So, how would one of the nuclei determine whether it wants to react with something that is so far away? How would it know whether the other atom is hydrogen or helium?
The answer is that the nucleus doesn’t determine this at all. Whether two atoms bond together is determined by the electrons.
Molecules are bonded together with electrons.
You’ve seen the picture to the right (Wikipedia) but that picture is not accurate at all. Not only is the scale completely wrong, the electrons don’t just fly around the nucleus, they are organised in “shells”.
Look at this picture of the sodium atom. A sodium atom contains eleven electrons. These electrons occupy three shells. The innermost shell can only hold two electrons, the second and third shell can both hold eight electrons. As you can see, the first and second shell are filled (ten electrons) and the last electron is put in the third shell.
For reasons we won’t get into, an atom really, really wants to have his outermost shell filled. This means that sodium won’t mind if he loses that one electron in the outer shell, then the second shell is the outermost shell and that one is filled. This explains why a sodium atom can become a positive sodium ion, he’d love to lose one negative particle.
Another way of filling the outermost shell is to find another atom and share a couple of electrons. Let’s take the hydrogen atom, it only has one electron so his outermost shell isn’t filled (it can hold two). When he meets another hydrogen atom, they can get really close so that the electrons are shared.
This way, both atoms can claim that their outermost shell is filled. Of course, they have to stick together now, otherwise one or both of the atoms wouldn’t have a filled shell anymore. This bond is called a covalent bond. A hydrogen atom is said to have one valance electron (or a valance of one), one electron to share means that he can only bond with one other atom at a time.
Of course, it doesn’t have to be the same element that an atom bonds with, the amount of electrons that an atom lacks determines how ready it is to bond with other atoms. This explains why oxygen (needs only 2 more) can form only two bonds while carbon (needs 4 for more) can form four bonds.
But wait, doesn’t this mean that a Na+ ion is going to react the same as a Ne atom? Both have 10 electrons and the amount of electrons per shell remains the same.
First of all; The Na+ ion has a charge, meaning it will repel other positively charged ions while a Ne atom doesn’t do that.
Also; Protons are positively charged and the electrons are negatively charged so when there is one more positive charge in the nucleus, the electrons in orbit are going to notice that. The shells will still contain the same amount of electrons but the “height” of the orbits will change. The bigger shells contain subshells and when the amount of protons changes, the relation between those subshells is changed. All in all, one proton more or less will wreak havoc on the structure of the electrons and in turn, drastically change the way the atom reacts to other atoms.
To summarize: The amount of protons determines how the atom is going to react chemically by influencing the electrons in orbit. In the upper classes, we’ll also learn that the electrons in their shell determine how the atom reacts to light.
Further in this booklet, you’ll learn that the amount of protons determines the amount of neutrons in the nucleus, determining the mass of the atom.
It might sound a bit philosophical but there is no deeper determinant to what something is, then how it reacts to its surroundings. In this way the amount of protons determines which element the atom is.
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