Chemistry

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Basic Chemistry 2

by Oren Lahav

Basic Chemistry, Part 2

So, you've read lesson 1 about atoms, elements, compounds, substances, and other chemical stuff. Now we can continue!

We've discussed the protons and neutrons in an atom. But what about the electrons? Well, normally, an atom has a number of electrons that equals the number of protons and also the atomic number. So hydrogen would have 1 electron, carbon has 6, oxygen 8, etc.

There are various theories as to how electrons are situated around the atom. I'll use a slightly misleading concept that is not correct but is good enough for basic understanding. You can view the electrons as occupying certain energy levels around the atom:

Each level can fit a certain number of electrons- the first fits 2, the second fits 8, the thirds also fits 8, etc.

The periodic table, which we already looked at, is arranged in terms of these. The row represents number of orbits or shells- row one, which has hydrogen and helium (H and He) are the 2 elements that have only 1 shell. Row 2, starting at Lithium (Li) and ending at neon (Ne) has 2 shells, etc. The columns correspond to the number of electrons in the outermost shell- column 1 elements beneath hydrogen have 1 electron, column 2 elements beneath Beryllium have 2 electrons in their outermost shell, etc. Elements of column 8 (18, the middle columns 3-12 inclusive are ignored in this arrangement and elements of this area are called transition metals) have full shells.

Now, atoms don't always have to have a number of electrons equal to the number of protons. If they do, we call them neutral. However, atoms usually want to be stable and have a full shell. So looking at Oxygen, we notice that the last (Second) orbit only has 6 electrons, not 8. Oxygen would grab 2 extra electrons to have a full shell, but then it won't be neutral, it will become an ion.

There are 2 types of ions. Elements that have 3 electrons or less generally tend to give their electrons away, because it's easier to lose 1 electron than to gain 7. These atoms become cations, ions with a positive charge (because they have less negative electrons). The elements that form cations are known as Metals and can be found on the left side of the periodic table.

Elements that have 5 or more electrons generally gain a few more electrons to fill their last orbital. They form negatively charged anions. These elements are found on the right side of the periodic table, and are called Nonmetals. Note that a few molecules can also be ionically charged

Now you ask yourself, where do anions take their electrons? And where do the electrons from the cations go? The answer is simple- the cations give their electrons to the anion. For example, take sodium, Na. It has one electron in his last orbit, which it would like to get rid of and have a full orbit. At the same time chlorine, Cl, has 7 electrons and need just one more to have a full shell. So, Na would give Cl his electron, and both would be happy! This results in a bond known as an ionic bond, resulted by the attraction between the positive Na cation and the negative Cl cation. Ionic bonds, which form between a metal/hydrogen and a non-metal, result in an ionic compound, such as NaCl- better known as table salt!

So back to the periodic table. The left side has metals, which donate electrons. Elements in group 1, called alkali metals, only have 1 electron which is very easy to give away (easier than 2 or 3), so they are highly reactive. Similarly elements in group 17, called halogens, from the right side that gain electrons only need one electron, so they are also very reactive. Elements in the middle are not as reactive. Elements in group 18, the Noble gases, have a full shell already- they don't need to gain or lose any electrons, they're already good. These elements, including helium, neon, argon, etc., are nonreactive and usually do not form compounds.

Now, ionic compounds don't have to include just elements from groups 1 and 7. For example, say aluminum wants to lose 3 electrons and oxygen wants to gain 2. We can make them react- 2 aluminum atoms would combine with 3 oxygen ones to form a substance known as aluminum oxide, Al_2O_3. When writing formulas for ionic compounds, the metal always comes first, and the numbers at the bottom represent the ratio of atoms required per molecule (reduced to the simplest ratio). When naming this compounds we call them metal nonmetal-ide, like sodium chloride or potassium bromide.

There are, however, types of compounds that are not ionic. The most fun one is called a Molecular compound, and is composed out of 2 or more different nonmetals. The way this works is simple- say I have a hydrogen atom and a fluorine atom and I want to combine them. Hydrogen and fluorine are both nonmetals, so they would both want to gain an electron and fill their shell. But instead of hydrogen giving away his 1 electron and being left with nothing, which would be weird, they do something else- hydrogen gives his electron and fluorine gives one of his, and they share the 2.

Sharing 1 (or more) pairs of electrons forms a covalent bond between nonmetals. In reality, this bond is much more complex and involves hybridization of orbitals, but let's not go into that. Covalent bonds are weaker than ionic bonds, and result in molecular compounds like HF, CH_4, and the popular H_2O. These compounds have specific molecular shapes, and properties that are very different from ionic compounds:

Ionic Molecular
Ionic bonds- strong Covalent bonds- weak
Solids Solids, liquids, or gases
Hard (relatively) Soft
High boiling and melting points Low boiling and melting points
Soluble in water Insoluble in water
Conduct electricity in liquid or dissolved state Don't conduct electricity

So, these are your 2 general types of compounds. There are much more specific types too, like polar covalent. But we're not going to get into those, they're pretty boring if you as me.

Next time we may talk a bit about chemical reactions and cool stuff!

That's enough for now! See you next time

Some Photo Credits:by denn, by alicearisty, by Paul goyette, by etherealillusion.

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oLahav
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About: I don't know how to describe myself... besides, I'm way too biased in this particular topic. What's the point?

Last Updated At Dec 07, 2012
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