In the last few posts, the atomic nature of the universe was discussed, Dalton’s atomic theory explained and we took a tour of the “guts” of an atom. Now let’s use all this to understand how the pieces fit together to illustrate what makes the element (and atom) hydrogen, different from carbon, oxygen and neodymium. The short story is the way the parts come together – it’s really a counting game.
A new element is discovered almost every year – but for now, a current periodic table will show 118 unique elements. (I’ve linked this image to a good table you can print out if you wish.) All periodic tables will have at least 3 pieces of information. For each element, every table should give you at least the symbol, the atomic mass and the atomic number. Some have far too much information, stuff you should be able to infer from the position on the table, they symbol, the mass, and so on, but every table – except the ones on the shoes at the top of this post – should give the three fundamental data. Of these three, the most important is the atomic number. So important is the atomic number, notice it provides the organizing principle for the arrangement of the atoms.
Simply put, the atomic number defines how many protons are in each atom. It is the number of protons – and not a thing more – that defines an element as hydrogen and not carbon. Hydrogen in any form will have 1 proton in each atom. Carbon will have 6 protons and oxygen 8 protons. This is what makes the element that particular element. If the element is in its neutral form, since protons have a positive charge, it must also contain the same number of electrons, which carry a single negative charge for each electron. It’s that simple and don’t make it more complicated.
But elements can be found in their ionic form – an element can have either a net plus charge or a net negative charge in some circumstances. Atoms and molecules that are positively charged are called cations. Atoms and molecules that have a net negative charge are called anions. (When I started, I remembered the difference by remembering that anions are negative by saying to myself, “annnn-negative.” YMMV.) For example, hydrogen might be found as H+; this is properly called the hydronium ion. The plus charge makes it a cation, and because it is not neutral, it does not contain the same number of electrons as protons. We still know it is hydrogen because it has 1 proton – the number of protons in an element never is changed, only the number of electrons. But if the hydrogen cation has a net charge of plus one, the atomic number is still 1, which means ALWAYS that it is still hydrogen because it has only one proton, but in this case, it has zero electrons. When neutral, hydrogen has one proton and one electron, but when somehow that electron gets stripped away, it no longer has a balanced charge and we know it as the hydrogen cation, or H+.
And so on. Find lithium on the periodic table – look for symbol Li. It has atomic number 3, which means it has 3 protons. No more, no less, it has three protons because it is those three protons that make lithium unique. If it is found in its neutral form, it will have 3 electrons also. But lithium is often found as a cation with a charge of plus 1, as Li+. For it to be lithium, it must have 3 protons, but to have a net charge of plus one, it must have one more proton than electron, so Li must have only 2 electrons. It’s possible to strip all the electrons from lithium and make a Li3+ ion – possible but uncommon. This general behavior is common for all of the metals in the periodic table and the metals are those elements found on the left side of the table. Those found on the right side of the table are called “nonmetals” (creative, eh?) and are usually found in either their neutral form, where the atomic number matches the number of electrons, or in a negative form, called anions.
For example, consider fluorine, with its symbol of F. We first look at the atomic number to find the number of protons and we see it has 9 protons. A neutral fluorine atom must also have 9 electrons to balance charge. However fluorine is a nonmetal, and that means it can also commonly be found as an anion, or a negatively charged ion. In the case of fluorine, it commonly is found as F–, and since it will always have 9 protons, the F– must have 1 more electron than proton, so it has 10 electrons. Sulfur has the symbol S, and has atomic number of 16 – it will always have 16 protons. Sulfur exists as a neutral element but it is also commonly found as S2-, it can carry a charge of minus 2. In that case, the proton count cannot change or the identity of the element changes, so it must have a total of 2 more electrons than protons, or 18 electrons.
So that covers counting the protons and the electrons. We will next consider the neutrons in an atom and the resulting isotope. Neutron count has no real effect on most of the common chemistry we see in day-to-day life but they are important in setting the mass of an element and it is the number of neutrons alone that dictate which isotope of an element we have in hand. In fact, it takes a tremendous effort to isolate individual isotopes, so if you have an element in hand, you most likely have a mixture of isotopes. More on that in the next post. It will be a short one.
To summarize: the identity of an element is defined by the number of protons and only the number of protons. So, the symbol of an element defines the number of protons, but that can be hard to remember, so every periodic table will list both the symbol of the element and the number of protons, given by the atomic number. Where that atomic number is located on each periodic table differs, so find hydrogen (H) and find the number 1. That locates the position of the atomic number for all elements. If the element is in a neutral form, the atomic number also defines the number of electrons. If it is in a charged form it can be either a cation or an anion, and the value of the net charge compared to the atomic number will tell you how many electrons are in that particular atom. Next time, isotopes and mass of an element.