Back To CourseLife Science: Middle School
35 chapters | 241 lessons
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Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.
Everything in the world is made of atoms. You can't see an individual atom because it's too small, but when you get a bunch of atoms together, they can make some amazing things. For example, your body is made of lots of different types of atoms, as are the trees in your backyard and the car you drive.
And what accounts for all the different substances and materials found on Earth are chemical bonds. These are attractions that hold atoms together. Things you encounter every day depend on these attractions. Table salt is created from sodium and chlorine atoms, and water is made from hydrogen and oxygen. Even 'pure' materials, like some minerals, depend on chemical bonds to hold these atoms together.
Not all chemical bonds are created equal, though. Some are stronger than others, while some only occur between certain types of atoms. In this lesson, we'll look at two very important types of chemical bonds: covalent and ionic.
Covalent bonds occur when atoms share electrons and are the strongest type of chemical bond. Covalent bonds are like holding hands with your best friend as you go through a haunted house. You hold on very tightly because you and your friend are very close and trust each other a lot! Alone, you may be frightened, but together you are a force to be reckoned with because the 'bond' between your hands is so strong.
Different atoms can form a different number of covalent bonds. It all depends on how many electrons that atom needs in order to fill its outer shell. Say that you need to fill your car with people in order to use the carpool lane. You have five extra seats in your car, so you need to find five more people in order to use that special lane on the highway. But your neighbor has a smaller car that only has three extra seats. They only need to find three other people in order to be 'full.' Atoms are the same way: hydrogen can only form one bond because it only has one electron in its outer shell. But carbon's outer shell has four available electrons so it can form four covalent bonds.
Now imagine that instead of co-workers in your car, it's full of young children. And instead of nicely sharing the backseat, the kids are arguing over who has the most space. The kid in the middle seat might try to encroach on one of the window seats, and when this happens they probably get pushed back toward the middle.
This more accurately represents how covalent bonds work than a car full of quiet, well-behaved adults. This is because the atoms that are part of the molecule are constantly 'arguing' over those shared electrons in the outer shell. It's a bit like an ongoing game of Tug-of-War where one atom pulls on the electrons and the other pulls right back.
The strength with which each atom pulls on shared electrons is called its electronegativity. When the pull between atoms is unequal we have a polar covalent bond. We call it polar because the electrons are not evenly shared, and one end (or pole) of the molecule pulls a little harder than the other. This creates an unequal charge in the molecule -- the end that is pulling harder (the more electronegative end) is slightly negative while the other end is slightly positive.
In some molecules, though, the pull between atoms is equal, and in this case, we have a non-polar covalent bond. Methane is a non-polar molecule made of carbon and hydrogen, and these two atoms know how to share their electrons fairly. But put hydrogen with oxygen to get water and it's a different story, because now we have a very polar molecule that doesn't know how to ride in a car like nicely behaved methane. It's in this way that we can see how it all depends on the electronegativity of the atoms involved.
Unfortunately, not all atoms know how to share. Sometimes, one atom pulls so hard that it completely strips an electron off the other. But even this strong pull results in a chemical bond, called an ionic bond. This type of bond is when electrons are transferred between atoms of opposite charge.
Remember that table salt from before? The sodium and chlorine atoms in this molecule are held together by ionic bonds. Sodium has only one electron in its outer shell, but chlorine has a whopping 7. While sodium would need 7 more electrons to fill its outer shell, chlorine only needs one, and it gladly takes it from something like sodium.
But that greediness comes with a price! Because sodium 'donated' its electron to chlorine, it gets to hang out there whether chlorine likes it or not. Electrons have a negative charge, so when sodium donated its atom to chlorine, it brought its net charge to +1. Chlorine, on the other hand, brought its net charge to -1 when it took that negative electron from sodium. But because each has an opposite and equal charge, as a pair, they are a neutral molecule.
We call this type of bond 'ionic' because it's made of ions. These are atoms with a net positive or negative charge. This charge comes from a difference between the number of protons and electrons in the atom. Initially, sodium had a net charge of 0 because it had the same number of protons as electrons. But when chlorine took the electron away, it took away a charge of -1 with it, leaving the sodium atom with one more proton than electron and, therefore, one more positive charge overall. Chlorine, on the other hand, gained an additional electron and therefore an additional negative charge.
Ionic bonds are not always found in a 1:1 ratio like this. The number of each atom in the molecule depends on the number of electrons each atom has in its outer shell. For example, let's put chlorine with calcium instead of sodium. Calcium has two electrons in its outer shell, so instead of just one chlorine, it pairs with two. Each chlorine takes one atom from the calcium to fill its outer shell, giving us a 2:1 ratio of chlorine to calcium for this molecule.
The strength of ionic bonds varies wildly with the environment the molecule is in. You would have a difficult time trying to break apart a dry crystal of salt, but water does it almost effortlessly! This is one reason that many prescription drugs are salts (ionic molecules) -- they break apart easily in water but last for a long time as dry solids on the shelf.
Atoms are so small that you can't see individual ones. But when they get together they make up everything that we see on Earth. The attractions that hold atoms together are called chemical bonds. Two very important types of chemical bonds are covalent bonds, which occur when atoms share electrons, and ionic bonds, which occur when electrons are transferred between atoms of opposite charge.
Covalent bonds may be polar or non-polar, depending on the electronegativity of the atoms making up the molecule. Ionic bonds are made of ions, which are atoms with a net positive or negative charge. This alteration in charge comes from one atom in the molecule donating an electron to the other atom through that bond. Covalent bonds are the strongest type of chemical bond, but depending on the environment, ionic bonds may prove to be difficult to break as well!
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Back To CourseLife Science: Middle School
35 chapters | 241 lessons