Atoms are the building blocks of the universe, but how do these building blocks stick together to form molecules? Everyday Einstein explains bond behavior.
My kids love building with Lego blocks. I’m always amazed at how many different ways they can come up with to combine the different blocks into something cool, something that is much more than the sum of its parts.
In last week’s episode, What Are Atoms?, I compared the atom to a building block, but how far does that analogy hold? Can atoms be combined into something that is more than the sum of their parts? Yes my friends, they most certainly can.
The World’s Smallest Shell Game
Last week I mentioned that despite some unpredictable behavior, electrons spend their time in specific areas around the atom’s nucleus called atomic orbitals. There are several layers of these orbitals. Each layer, or shell, corresponds to a certain level of energy. The electrons contained within the shells closest to the nucleus have less energy than those contained in the outer shells.
Usually the electrons in the outermost shell, or highest energy level, are the ones that are involved with forming bonds with other atoms. These friendly electrons are called valence electrons.
Valence electrons are hot commodities in the world of chemical interaction, and depending on the atoms involved, the valence electrons of one atom can be shared, stolen, chased after, or just slightly coveted by another atom. It’s like the atomic version of your favorite soap opera.
Bond, Ionic Bond
There are three main types of bonds between atoms: ionic bonds, covalent bonds, and polar bonds (sometimes called hydrogen bonds).
To understand ionic bonds, you should know that atoms are a little funny about their valence electrons. It takes eight valance electrons to make a full set. And like most persnickety collectors, atoms prefer to have either a full set of valance electrons, or none at all. The exceptions to this are hydrogen and helium, which are a bit odd in that they can only hold 2 valence electrons each. Though one important difference between collecting valence electrons and collecting say, Pokémon cards, is that there aren’t any special-edition holographic electrons. All valence electrons are essentially the same.
Let’s consider for a moment the atoms chlorine (atomic symbol Cl) and sodium (atomic symbol Na). Chlorine has 7 valence electrons. It’s so close to a full set, it can just taste it. Sodium on the other hand, has only 1 valence electron.
When chlorine and sodium get together, you can almost imagine chlorine covertly eyeing sodium’s one lone valence electron and thinking, “It won’t miss it. Probably doesn’t even care…” Suddenly in the blink of an eye, chlorine has swiped sodium’s valence electron and is a happy (though hopefully guilt-ridden) camper.
Now as you probably remember from the episode on radiation, when an atom loses an electron, it becomes a positive ion. Likewise, when an atom gains an extra electron, it becomes a negative ion. So now we have a negative chlorine ion (or anion) and a positive sodium ion (or cation). As in the case with people, opposites attract and these two ions are attracted to one another, forming an ionic bond.
Can’t We All Just Get Along?
Sometimes atoms start their molecular relationships with a bit more honesty. Take for example two oxygen atoms. Each one of them has 6 valence electrons. Rather than trying to steal from one another, the oxygen atoms decide to share valence electrons in order to complete their sets.
Each oxygen atom shares two electrons with the other. The electrons spend a relatively equal amount of time with each atom, resulting in both oxygen atoms feeling like they have a complete set of valence electrons.
The bonds formed by sharing valence electrons between atoms are called covalent bonds. And like all relationships built on trust and sharing rather than theft, covalent bonds are stronger than ionic bonds. Also like most relationships, the more electrons shared between atoms, the stronger the bonds become. Atoms can share one electron each to form a single bond, two electrons each to form a double bond, or three electrons each to form a triple bond. (Three is the maximum).
You’re So Greedy
Unfortunately, as in real life, not all bonded atoms share equally. You probably know that one oxygen atom and two hydrogen atoms join together to form a water molecule, H2O. The oxygen atom shares one electron with each hydrogen atom, which in turn share one electron each with hydrogen. Oxygen ends up with 8 valence electrons and each hydrogen atom ends up with 2. On the surface everyone seems happy.
However, while oxygen shares equally with some atoms, such as other oxygen atoms, it doesn’t always play fair with others. For example, oxygen has stronger electronegativity than hydrogen, so when oxygen bonds with hydrogen, it holds onto the shared electrons longer.
Since the electrons spend so much more time around oxygen, it gets a sort of partial negative charge. Likewise, since hydrogen spends so much extra time without the shared electrons, it has a partial positive charge.
The partial charges created by this unequal sharing cause the atoms to try and make other friends. When two water molecules drift by each other, the partial positive charge of a hydrogen atom on one molecule pulls it towards the partial negative charge of the oxygen atom on the other molecule. These tenuous relationships are called polar bonds or hydrogen bonds and are responsible for most of the interesting properties of water, something that we’ll discuss in a later episode.
So now you know about the three main types of bonds that can form between atoms: ionic bonds, covalent bonds, and polar or hydrogen bonds. Next time, we’ll talk about a few ways that you can tell which kind of bond two atoms will make.
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Atoms image courtesy of Shutterstock.