 In this video, I will differentiate between ionic, covalent, and hydrogen bonds, and provide examples of polar and non-polar molecules that are common in the body. An element is a pure substance that is distinguished from all other matter by the fact that it cannot be created or broken down by ordinary chemical means. All of the elements are on the periodic table of the elements. The smallest quantity of matter that still retains the properties of an element is known as an atom. Here we can see the relative proportions of elements that make up the human body. You can see that 65% of the human body is oxygen, 18% carbon, 10% hydrogen, and 3% nitrogen. And then the remaining portion of the body consists of a smaller amount of calcium, phosphorus, sodium, chloride, magnesium, and even smaller amounts of boron, chromium, cobalt, copper, fluorine, iodine, iron, manganese, molydenum, selenium, silicon, tin, vanadium, and zinc. The structure of an atom consists of a central region called the nucleus where there are subatomic particles called protons that have a positive charge and subatomic particles called neutrons that have no charge. Surrounding the nucleus are subatomic particles called electrons that have a negative charge. The charge of an electron is equal and opposite to the charge of a proton. The illustration here shows us two different ways of visualizing the distribution of subatomic particles in an atom. In the planetary model, the idea is that the nucleus is in the center containing the protons and neutrons, and the electrons are in orbitals surrounding the nucleus similar to how the planets are orbiting around the sun. On the right here we see the electron cloud model. In the electron cloud model, the electrons do not orbit, but instead are spread out into a cloud of negative charge. The electron cloud model may do a better job of explaining modern observations, however it's still useful for us to think about the location of electrons as occupying orbitals. Here we can see the periodic table of the elements. Elements are organized on the table based on the number of protons in the nucleus. Each hydrogen has just one proton. There is also one electron to balance that proton in a typical hydrogen atom. Helium has two protons, and so you'll notice that the number in the top left known as the atomic number of hydrogen is one and helium is two. So in helium there's two protons, but there's also two neutrons and two electrons. Carbon, we can see, has an atomic number of six. So carbon has six protons and six electrons. Typically, carbon also has six neutrons. So the atomic mass of carbon is 12 atomic mass units, because each neutron has an atomic mass unit of one, and each proton has an atomic mass unit of one, and electrons have a negligibly very small amount of mass, and so we can consider it essentially zero. A couple other major atoms we'll talk about, of course, include oxygen, where we can see oxygen has eight as its atomic number, so there are eight protons in the nucleus of an oxygen atom. There's also typically eight neutrons, so the mass of an oxygen atom is 16 atomic mass units. Now if there are eight protons, how many electrons should be in an oxygen atom? Of course, eight electrons would equally balance the eight protons. So similarly we could take a look at the example of nitrogen, where you can see nitrogen has seven protons, it would have seven electrons to balance that, and there are seven neutrons giving it an atomic mass of 14. While often the number of protons and neutrons is equal in an atom, there are different isotopes of an element, that is, atoms that can have more or less neutrons, if they have the same number of protons, they are still considered the same element. The most common form of hydrogen is protium, where there's just one proton and there are no neutrons, so one proton and one electron forms a protium atom. But there are two heavier isotopes of hydrogen, deuterium and tritium. Tritium has one proton, one neutron and one electron giving it a mass of two atomic mass units, and tritium has an atomic mass of three, because there are two neutrons in addition to one proton and one electron. The fact that there are different isotopes of an element has been useful to enable a form of medical imaging known as the positron emission tomography scan, commonly just known as a PET scan. In a PET scan, a heavy isotope of an element is used to make a synthetic form of a molecule, and then this synthetic molecule is either injected or ingested, then the PET scan can visualize the location of the labeled molecule in the body. This technique is useful for the diagnosis of cancer, but has a wide range of applications in the medical field. The planetary model of atomic structure is useful for helping us conceptualize how atoms can form chemical bonds. In the top left here, we see an illustration of the hydrogen atoms planetary structure, where there is one electron in the outer orbital, also known as the outer shell. The outer shell will be most stable if the atom can have the maximum number of electrons that will fit in that shell. The first shell surrounding the nucleus of any atom can hold a maximum of two electrons. So the hydrogen atom would be more stable if it could accept an electron from another atom, or share an electron with another atom. Similarly, if the hydrogen ion were to lose that one electron that it started with, that would also make the hydrogen atom stable. However, the one proton in the nucleus of the hydrogen atom would not be balanced by a negatively charged electron. And so the resulting atom would be positively charged, what we call a cation, a positively charged ion. So moving to the right, the next example we see is the helium atom structure. And there are two electrons in the outer shell of helium. So the outer shell is completely filled with that two electrons, and those two electrons balance the two protons in its nucleus, so the helium atom is very stable and is not likely to form chemical bonds with other atoms. In the bottom left here we see the structure of carbon that has six electrons. The innermost shell is completely filled with two electrons, but the second shell would be completely filled with eight electrons, and only has four electrons. This means that the carbon atom would be more stable if it was able to accept or share four more electrons to fill its outer shell. In the bottom right here we see the example of neon, so neon has ten electrons. There's two filling the inner shell and eight filling the outer shell. And so because the outer shell is filled, there are eight electrons in the outer shell and it could not accept any more electrons without moving to a third shell, neon is very stable and is not likely to form chemical bonds with other atoms. Chemical bonds hold atoms together to form molecules. Chemical bonds involve the electrons being either shared between two atoms in a molecule, or electrons may be lost from one atom and taken by another atom to create ions with opposite charges and then these oppositely charged ions can be attracted to form a bond. We'll start with the example of ionic bonding. Looking at the familiar molecule sodium chloride or table salt, sodium will have 11 protons and 11 electrons equally balanced at 11 protons. That gives two electrons in the innermost shell, eight electrons in the middle shell and one electron in the outer most shell. But the outer most shell could contain eight electrons total. If we look at the chlorine we will see that there are 17 protons and 17 electrons, two electrons in the innermost shell, eight electrons in the middle shell and seven electrons in the outer most shell. But the outer most shell could contain eight electrons and that would make the resulting atom more stable. When sodium and chlorine are placed together, chlorine will take the electron from the outer most shell of sodium in order to fill its outer most shell. The resulting ions are more stable because their outer most shells are completely filled with electrons. The net positive charge of sodium is attracted to the net negative charge of the chloride ion and this attraction between the positive and negative charges creates what's known as an ionic bond. Ionic bonding holds the sodium ions and chloride ions together in the crystal structure forming a cube shape typical of a salt crystal. This ionic bonding is a very strong attraction between the complete positive and negative charge of the sodium ion and the chloride ion, an atom or molecule with a negative charge is known as an anion, so the chloride ion is an anion. The sodium ion with a positive charge is known as a cation and cations and anions are attracted to each other to form ionic bonds. A covalent bond occurs when electrons are shared between atoms and a nonpolar covalent bond is when the electrons are shared equally between atoms. If the two atoms are the same, typically this would form a nonpolar covalent bond. We have an example on top here of two hydrogen atoms forming a nonpolar covalent bond, each sharing one of the electrons from the other to create a stable outer shell that's filled with just two electrons. Next we can see the example of two oxygen atoms bonded together to form an oxygen molecule because there are six electrons in the outer shell of oxygen before the bond, two nonpolar covalent bonds are needed to fill the outer shell of each oxygen atom. This creates a double nonpolar covalent bond where four electrons total are shared between two atoms. On the bottom we can see an example where two different atoms form a nonpolar covalent bond. Carbon and oxygen will bond to form a nonpolar covalent bond because the attraction of the nucleus for electrons is roughly equal for carbon and oxygen. However, carbon only has four electrons in its outer shell and will need to share four more electrons to fill its shell with eight total electrons. Each oxygen atom will contribute two electrons forming a double bond, so the carbon is double bonded with each oxygen atom forming two double bonds, but each of those double bonds is a nonpolar covalent bond because the electrons are shared equally between the carbon and oxygen atoms. In contrast to a nonpolar covalent bond, a polar covalent bond occurs when one atom attracts electrons more strongly than another atom. Although the electrons are shared between the atoms to form a covalent bond, the electrons spend more time next to one atom creating a partial charge. The example we see here is oxygen and hydrogen forming a polar covalent bond. Oxygen with its eight protons attracts electrons more forcefully than hydrogen with its one proton, therefore a partial negative charge forms on the oxygen and a partial positive charge forms on the hydrogen. The molecule we're looking at in this illustration is a water molecule where there are two polar covalent bonds, one between each hydrogen with the oxygen atom. Each of the hydrogen atoms in the water molecule has a partial positive charge and the oxygen atom in the water molecule has a partial negative charge. When we have polar covalent bonding, the partial charges that result can then participate in another type of chemical bonding known as hydrogen bonds. So the bonds between hydrogen and oxygen atoms within a water molecule are polar covalent bonds, however there are hydrogen bonds between different water molecules where the oxygen has a partial negative charge that is attracted to the partial positive charge of the hydrogen in a nearby water molecule. And so while the intramolecular bonds in water are polar covalent bonds, the intermolecular bonds in water are hydrogen bonds. We will see polar covalent bonds are formed whenever oxygen binds to hydrogen, similarly polar covalent bonds form when nitrogen binds to hydrogen or when sulfur binds to hydrogen and the resulting partial charges will make the molecules, polar molecules, that can participate in hydrogen bonds. Here we have some examples of polar molecules on the left and non-polar molecules on the right. Of course water is a polar molecule because the polar covalent bonds between oxygen and hydrogen form partial positive and negative charges. Similarly glucose is a polar molecule because the polar covalent bonds form between oxygen and hydrogen atoms. On the bottom left here we can see the example of urea. Urea is also a polar molecule because there are polar covalent bonds between the nitrogen and hydrogen atoms in a urea molecule. On the right here we can see non-polar molecules. Oxygen gas is one example because the two oxygen atoms are bonded together with non-polar covalent double bond. The oxygen molecule does not have any partial positive or negative charges, the electrons are evenly distributed throughout the molecule. Similarly carbon dioxide has only non-polar covalent bonds and therefore an equal distribution of electron density throughout the molecule creates a non-polar molecule. On the bottom right here we see the example of a triglyceride or a neutral fat. This is the chemical that we commonly refer to as fat, whether it's a solid fat like the animal fat you see around the edge of your stake or if it's the fat that forms a liquid form such as vegetable oil, either way the molecules are triglycerides which have long chains of carbon bound to hydrogen and then those chains are bonded to a backbone of carbon through oxygen. The backbone is a glycerol molecule which itself before having bonded to the fatty acid chains would have been a partially polar molecule, similarly the fatty acid chains before bonding to glycerol would have had a polar region but once the glycerol is bound to three of the fatty acid chains all of the structure is now a non-polar molecule. Now non-polar molecules like fat will not mix easily and dissolve in a polar solvent like water. You are probably familiar with this from making salad dressing where the oil does not easily mix with vinegar. Vinegar is primarily water and acetic acid which are polar molecules that cannot mix with and dissolve the non-polar molecules of the fat in the oil. A few other major examples of polar molecules we'll see a lot in this class include nucleotides and nucleic acids. We can see on the top left here the structure of a nucleotide, the nucleotide has a negative charged region where the phosphate groups are anions but there is also some polar covalent bonds between oxygen and hydrogen and the sugar unit of the nucleotide as well as some polar covalent bonds where nitrogen is bound to hydrogen in the nitrogenous base region of the nucleotide. And so a nucleotide is a polar molecule that can dissolve well in a polar solvent like water and nucleotides are linked together through covalent bonds to form long chains known as nucleic acids. The illustration on the bottom left represents the oxyribonucleic acid or DNA. DNA is an important molecule as it serves as the genetic instructions within cells located within the nucleus and nucleic acids because they're made out of nucleotides are polar molecules and they do dissolve well in a polar solvent such as water to form a solution. Amino acids we can see in the top right an example of a generic structure of an amino acid where there is nitrogen bound to hydrogen through polar covalent bonds and there's oxygen bound to hydrogen through a polar covalent bond. And so the amino acid is also a polar molecule and as a polar molecule it can dissolve in water or other polar solvents. Amino acids are joined together to form long chains when covalent bonds form between individual amino acids they form a long polymer known as a polypeptide and then one or more polypeptides form the major functional molecules found in all cells known as proteins. So proteins are very important molecules that we will discuss a lot as we go through the class and proteins have lots of amino acids linked together to form the protein although some proteins have more polar regions and less polar regions generally proteins have polar regions facing outward into the surrounding environment to dissolve into the solvent surrounding them the watery solution of the cell or the blood or extracellular fluid so generally we consider proteins to also be polar molecules although there can be non-polar regions of proteins and we'll see that some proteins are found embedded within non-polar molecules in places like the plasma membrane that forms the outer boundary around a cell the non-polar regions of the protein will be able to mix well with non-polar molecules whereas the polar regions of the proteins will mix well with polar molecules like water.