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Bonding
Bonding
Covalent Bonding
Covalent Bonding
Covalent bonds exist between atoms when those atoms are sharing some of their outer (valence) electrons. These atoms share these electrons in order to reach a more stable form by filling their outer (valence) electron shell. Basically, every atom (with the exception of hydrogen and helium, which have 2) have 8 spots in their outer orbital (electron shell) that they'd like to have filled. So atoms will share, steal, or give electrons in order to fill that outer shell. Covalent bonds are represented by lines connecting the atomic symbols of the elements in a Lewis dot structure (see below).
Depending on how many pairs of electrons are shared between the atoms, there are different kinds of covalent bonds. If a single pair of electrons is shared, it is a single covalent bond. If two pairs of electrons are shared, it is a double covalent bond. And finally, if three pairs of electrons are shared, it is a triple covalent bond.
Double Covalent Bond
Ionic Bonds
Ionic Bonds
Ionic bonds occur when atoms are a little more selfish...or more selfless. Instead of sharing, atoms that form an ionic bond either steal or give away electrons from or to one another. This means that one atom lost an electron and one gained an extra electron. If an atom lost an electron (remember, electrons have negative charges), that is like losing a negative. If you subtract (or lose) a negative, you become positive. So the atom that lost an electron is now a positive particle, known as a positive ion (cation).
The atom that gained an electron took on an extra negative particle. So, if it was neutral before this bond formed, it is now negative. So it is a negative ion (anion). Remember, an ion is a charged particle.
Electronegativity and Polarity
Electronegativity and Polarity
Electronegativity
Electronegativity
So why would some of these bonds involve stealing of electrons? Why can't we all just share? Well, if you remember from basic chemistry, some atoms are more electronegative than others. This means that they are more attracted to electrons than others - in other words, they are greedier with their electrons. For this section, I will be stating that atoms 'want' electrons. This is just an easier way to imagine this phenomenon - obviously atoms don't actually want anything.
If an atom is greedy enough (compared to the other bonding atom), then the first atom may steal the electron away. This would be like the ionic bond above. Chlorine REALLY wanted that electron. Sodium didn't really care too much because it would be happy to get rid of the electron (then its outer shell would be full - exactly what it wants).
So what if atoms want the electron equally? Well, in that case, they are going to share the electron pair right in the middle. This is commonly seen with carbon and hydrogen. They both want the electron a similar amount, so the electron will stay in the middle of them most of the time. This would be considered a nonpolar covalent bond. It is covalent because the electron is shared, not given away/stolen. It is nonpolar because the electrons stay in the middle most of the time. They're both playing tug-of-war for that electron pair, but they are equally strong.
Polarity
Polarity
But what if one atom REALLY wants that electron pair, and the other is just kind of lukewarm about it? Well, let's look at an example. In the case of water, there are 2 hydrogen atoms, each bonded to an oxygen. Oxygen LOVES electrons, so it's going to be putting its all into that tug-of-war. Hydrogen doesn't put up much of a fight. As a result, the electrons are going to spend more time near the oxygen atom.
What happens if somebody really negative spends a lot of time around you? You probably get a little negative too. Not as negative as they are, but at least a little more cynical. Oxygen, in this example, does the same thing. The electrons spend more time with oxygen, so oxygen has a partial negative charge. It's not an ion, so it's not an ionic bond. It is just slightly negative. If the electrons are spending more time with oxygen, they are spending less time with hydrogen. So, the hydrogens will have a slightly positive charge. This is known as a polar covalent bond.
These partial charges in polar covalent bonds are typically represented with the symbols shown here.
Hydrogen Bonding
Hydrogen Bonding
So now that we these partial charges, it begs the question: why does that matter? Well, do you remember the old phrase "opposites attract"? That often is used to describe electricity. Positive attracts negative and vice versa. So now the hydrogen is partially positive, it will be attracted to anything nearby that is negative... or even just partially negative.
So what might be around a water molecule? Well, in a single droplet of water, there are approximately 1.67 x 10^21 molecules of water - that's a lot. Often, there are many other water molecules near one. When they are near on another, they take on a formation similar to that shown here.
The partial negative charges of an oxygen are attracted to the partial positive charges of a nearby molecule's hydrogen atoms, and vice versa. This causes them to form numerous weak bonds between the partially charged atoms. These are known as hydrogen bonds. They are represented by the dashed lines in the image.
They are really weak - nowhere near as strong as a covalent bond. But there are lots of them. I mean LOTS them. Remember how many molecules of water are in just a single drop of water? Now imagine that they're all holding hands.
Water's Properties - All Because of Hydrogen Bonding???
Water's Properties - All Because of Hydrogen Bonding???
Water is truly an amazing substance, and there are so many reasons why. But here I will highlight some of the things that make water so incredible. The good thing is that these are all just logical consequences of that polarity and hydrogen bonding you just read about.
High Specific Heat Capacity
High Specific Heat Capacity
Have you ever been to the beach? If so, I want you to think about that experience for a moment. It is the early afternoon, you just got to the beach. What do you feel as you walk across the beach? The sand is HOT... perhaps too hot. But that's okay, you can take a dip in the water. But wait... why is the water so much cooler? Well, the ocean water has a higher specific heat capacity (and it's also very massive, but let's focus on one thing at a time).
This means that it takes a LOT more energy to warm up a sample of water than something with a lower specific heat capacity. So the sand and the water have been in the sunlight for just about the same amount of time. But the water heats up much more slowly, so it is still cold from the night before. The sand is very quick to go up in temperature when provided with solar energy, though.
What about going to the beach at night? You are walking across the sand and you notice that it feels quite cool, sometimes even cold. Why is that? Well, sand is also quick to LOSE its energy because of its low heat capacity. Water, on the other hand, holds on to its heat for much longer. So, to summarize - water takes a lot longer to heat up, but it holds onto that heat for a long time. Sand is quick to heat up and quick to cool down.
But why? Well, basically all of those hydrogen bonds we mentioned before get broken all the time. Some sunlight hits the surface of the water, and that energy is devoted to breaking some of those hydrogen bonds. And then some of them reform. Basically, the energy is spent breaking these hydrogen bonds instead of raising the temperature of the water.
Cohesion
Cohesion
Water attracts water. Remember how those partially negative oxygen atoms were attracted to those partially positive hydrogen atoms? Well that means that those two molecules are harder to separate than they would be without that hydrogen bond. This basically means that water acts like glue with itself. This is known as cohesion. ('co' means together, or with - think about a coworker, someone you work with).
This is why water forms droplets instead of separating far more, and also why when you put two water droplets together, they combine into one larger droplet.
Furthermore, this cohesion actually means that the surface of water is almost like a film. If your mass was small enough or distributed over a large enough surface area, you could actually walk on water. Now, obviously that's pretty far fetched for us, but that's how numerous insects rest on top of the water's surface. One lizard can even run along the top of the water.
Adhesion
Adhesion
Water doesn't just stick to itself. It can stick to numerous surfaces quite well for the same reason as in cohesion - polarity and hydrogen bonding. If water is attracted to something other than water, it is known as adhesion ('ad' means different - think about an adversary). This is partially why you can drink water out of a straw. In that example, cohesion is also playing a role, along with a pressure gradient, but it is definitely involved. Adhesion occurs when a water molecule is attracted to another molecule or surface (other than water, that is) because of its charge or polarity.
Water is Less Dense as a Solid than as a Liquid
Water is Less Dense as a Solid than as a Liquid
Ice floats... duh, right? Well, that's actually kind of amazing. Most substances actually get denser when you cool them down. If I take a rock and put it in the freezer, its particles are going to slow down. When they slow down, the shape of the rock actually contracts (shrinks) because of the slower motion. So it still has the same number of particles, but now in a smaller volume. That means it is denser because density = mass/volume. If volume goes down, density goes up.
But water is different. Water gets less dense as it freezes. Very few substances found on Earth do this, so that is one reason water is such a staple to our planet and all living things. We don't need to worry too much about the nitty, gritty details of this property of water for this course, but just know that all of those hydrogen bonds basically create a crystal lattice. That crystal consists of water molecules moving farther apart from one another than when they were warmer. Thus, volume increases, meaning density decreases. This means less dense ice floats atop denser liquid water.
The Universal Solvent
The Universal Solvent
Water is really, really good at dissolving things. Remember from chemistry that a solvent is a dissolver. A solute is what gets dissolved. Think about salt water. It's just water that contains some salt. The salt dissolved in the water. So the water is the solvent and the salt is the solute. These terms are going to be VERY important when we get to cell membranes, so just keep them in mind.
Why is water so good at dissolving things? You guessed it... hydrogen bonding and polarity (noticing a trend here?). Those partial charges allow the water to be attracted to something like salt. The water molecules surround the solute and break it up when possible. This is why salt is in its crystal form at first, but then with time (especially if you stir or agitate it), the crystals disappear. Did the salt go away entirely? No, it's just now so spread out that you can't even see it with your naked eye.
Carbon - What Makes It Special
Carbon - What Makes It Special
Basically, carbon is incredibly versatile. It can form 4 bonds because of its half-full (or half-empty depending on your worldview) valence shell. This also means that it can form single, double, or triple covalent bonds. And it isn't too attached to its electrons, so it might form nonpolar or polar covalent bonds depending on how hard its neighbor's play tug-of-war.
But here's the real magic: not only can carbon bond with 4 atoms, like in methane (CH4), it can also bond with itself. So it can form really long chains, rings, and just about any shape you can imagine by bonding with lots of other carbons in conjunction with other atoms. Some of these shapes will become very important when we talk about macromolecules next. Below are some examples of what these shapes may look like at a molecular level, but keep in mind - this is just the tip of the iceberg. There are technically infinite shapes that organic (carbon-containing) molecules can take. Don't bother memorizing the names or shapes of these structures, just take a quick glance at them and note some similarities and differences.
Functional Groups
Functional Groups
Functional groups are basically what gives molecules their properties. Having certain atoms in certain configurations affects what a molecule can do, or how it behaves. Think about water: having oxygen (super greedy for electrons) bonded to hydrogen (not too attached to its electrons) resulted in a polar bond. That polarity resulted in water behaving in such amazing ways.
Other molecules are similarly determined by their functional groups. You do not need to memorize these functional groups, but you will be encountering them often, and so it is good to expose yourself to them as often as possible. This is not at all a complete, comprehensive list of functional groups. This is just a sample.
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