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Honors Chemistry

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Atoms, Ions and Isotopes

How we represent atoms

When we label an atom, the most important thing is it's atomic number, which is the number of protons. The number of protons determines the atoms identity. Carbon always has 6 protons. There is no form of carbon that has a different number of protons. If I were to add a proton to carbon, I would have 7 protons, and that would be some form of nitrogen (which has 7 protons, always. The atomic number usually is listed below the symbol for the element and to the left (the 6 is the number of protons, and, did I mention: carbon always has 6 I stressing that enough--how much you want to bet that SOMEONE will label something carbon that does not have 6 protons)? The second number, written as a left superscript, is the mass number. It is the total number of nucleons (protons and neutrons) in the nucleus. It is NOT the number of neutrons, but you can determine the number of neutrons from the number. To get the number of neutrons, you subtract the number of protons from the mass number. You see, while electrons do have a mass, it is too small to affect the mass of the atom much. So carbon has 6 protons, 6 neutrons and 6 electrons (you know that because carbon, like all atoms, is electrically neutral). If it has 6 protons and has no net charge, there must be 6 electrons. There are forms of carbon that have 7 or even 8 neutrons. They would 13C or 14C be and respectively. We would say the names "carbon 13" or "carbon 14." The most common isotope is known as "carbon 12."
Why don't we have to specify the number of protons? Because, calling it carbon already does that (you may recall that all forms of carbon have 6 protons...Okay, I'm done with that for a while). Isotopes have more or less identical chemistry. That's not surprising since they have only changed in the nucleus (which is the marble in the center of the stadium, while it is the electrons in the cheap seats that do the chemistry).
You can remove or add electrons from an atom without changing what type of element it is (but not, really, now I'll stop). When you add an electron, since the electron carries a negative charge, the result is a negative ion, known as an "anion" (Pronounced "AN-i-on"). An example would be which is "chloride" or "chlorine-minus. Also, you can remove an electron from an atom, which makes it a net positive. And example would be Na+ (sodium plus). That is a cation (pronounced CAT-i-on…you don't pronounce the "-tion" as you would at the end of "nation" or "caution."

Sample questions (answers below):
1 Suppose I start with nitrogen-15, which has 7 protons. What would the symbol for that be?
2 Suppose I add 3 electrons, what would the symbol for that?
3 Suppose I started with nitrogen 15 and took away 2 neutrons, what would the symbol for that be?
4 Finally, suppose I started with the result from question 3 and took away a proton (left the same number of neutrons and electrons), what would the symbol for that be?

Borrowed from

Here is a link to a periodic table you can download

A Model for chemistry:
First, let's remember that chemistry will happen at the level of changes in the number of or interactions among electrons. Remember too that everything happens because of energy. Everything that happens will do so because it satisfies our second law and spreads out energy (Usually through enthalpy, or "heat"). So, it stands to reason that when bonds form, electrons will end up in a lower potential energy state, releasing energy to the surroundings. note also that there are elements called "Noble gases," which do not engage in chemistry at all. They are the ones in the last column on the right. It stands to reason that they, or rather their electrons, are already in as low an energy state as they can get to. Maybe chemistry happens in ways that get other elements to an arrangement of electrons more nearly like noble gases. That will provide our first working model for how bonds will form.

What I just told you is true as far as it goes. I've not offered you any deep explanation, which I will later.
The elements in the column topped by Helium (blue, on the far right), generally don't "do" chemistry. They are already in the lowest potential energy state they can be.
Sodium has 11 protons and 11 electrons. If it loses one electron, it will 10, like Neon (which is a "noble gas"). It is then Sodium-plus (Na+). Sodium is explosive…very dangerous stuff. But, Na+ is mostly harmless…in fact, you need a fair bit in your diet, though probably not as much as you have).
Magnesium is a metal (you saw me burn some). But, you need the ion in your diet also. It's in the second column (Mg). It has 12 protons and 12 electrons normally. It could lose 2 electrons to get 10, again like Ne). It would be Mg2+, sometimes written Mg++.
On the other side of the table, the easiest way to get to a noble-gas number of electrons is to gain 1 or 2 electrons. So, Chlorine, which has 17 protons and 17 electrons, can gain one electron have 18, the same as the nearest noble gas, Argon.
You may already have realized that if you mixed Sodium with chlorine, sodium could trade one electron to Chlorine, and you would have Na+ and Cl-, which stick together because they are charged. That's known as sodium chloride. you may have heard of it…table salt.

This is how we form a class of compounds known as ionic compounds. There is the the issue of forming what we call covalent bonds, which are found in true "molecules." We will work on that soon.
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