Notes

00:00:00TEACHER: Throughout history, people have wondered what matter is made of, and how types of matter differ. The Greek philosopher, Empedocles, you see in this image here, proposed the idea that all matter was made up four things that he called elements-- air, earth, fire, and water. He believed that all other matter was a combination of
00:00:21these four things. Well, people believed his idea for over 2,000 years. Although this idea is not exactly correct, it paved the way for several future scientific investigations. In this lesson, we will talk about what scientists know now about what Empedocles first proposed.
Section 2
00:00:00TEACHER: In order to address your lesson question, we will explore elements. To do this, you will examine the properties of an element, describe the isotopes of an element, and finally, explain how ions form. Next, let's look at some vocabulary words that you'll need to know for this lesson.
00:00:18Don't forget to type these four important vocabulary words into your eNotes and check the glossary for the definitions. The words are, atomic number, compound, element, and valence electrons. Recall that atoms are the smallest part of matter. Atoms cannot be separated into smaller parts by ordinary means like shaking or cutting.
00:00:44The atom has two main parts. The central part of the atom is called the nucleus. Right here. And the area around the nucleus is where you find the electron cloud. The nucleus of an atom contains protons, which are positively charged particles, and neutrons, neutral
00:01:08particles, so I'll put 0. In the electron cloud, you will find electrons, negatively charged particles that orbit the nucleus. So these two electrons that orbit the nucleus.
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00:00:00TEACHER: Previously you reviewed the parts of an atom, the smallest unit of matter. Atoms are the basic particles that make up elements. In this lesson, we will define elements and examine their properties.
Section 2
00:00:00TEACHER: In the late 1600s, early chemists discovered that matter was made up of many more than just four elements-- air, earth, fire, and water-- which was proposed by Empedocles. Now scientists know that there are more than 100 different elements that make up all matter in the universe. An element is a pure substance that is made
00:00:19only one type of atom. This is why elements are considered pure substances. It doesn't matter how many atoms make up the element-- it could be two or 2000, as long as they're all of the same type. For example, gold or aluminum. An element cannot be broken down into a simpler form. This gold nugget is made of only gold atoms.
00:00:46No matter how many times that you break it into smaller pieces, you would still only get gold atoms, you will never get any other kind of atom from it. Atoms in one element are the same, so gold is made only of gold atoms, but the atoms of another element are unique to that element. So for example, aluminum is made up of aluminum atoms, not gold atoms.
Section 4
00:00:00TEACHER: The symbols that you see in this image represent different elements. Note that each element has its own symbol. Not only that, these symbols are universal, meaning no matter where you are in the world, they are used. It's also important to understand that they are one, two, or three letters long.
00:00:18If it has one letter, the letter is capitalized. If it has two or three letters, then the first letter is capitalized, but the second one is not. Sometimes, the letters are similar to the name of the elements, so, for example, if you see carbon, it has one letter, and that letter is capitalized. For titanium, has two letters. The first letter is capitalized, but the second
00:00:43one is not. Now, other symbols may seem a bit unusual when compared to the name of the element. This is because the letters have to do with the Latin names of the elements. So if you see iron, the symbol is not "Ir" or "I." It's Fe. And for mercury, it's Hg. Note that a three-letter example is not shown here
00:01:09because they are rare and have not yet been formally named. Each atom has a specific number of protons in the nucleus. This number is called the atomic number. This is how elements can be distinguished from one another. An atom of an element may have just a few protons, so for example, helium has two protons.
00:01:36Remember that there are the same number of electrons in an atom as there are protons, so if helium has two protons, how many electrons does it have? It has two electrons. Some atoms may have several more protons, like calcium has 20 protons. So if it has 20 protons, how many electrons does it have? 20 electrons.
00:02:00Now, some atoms even have more protons and electrons than the ones that you saw here, with helium and calcium. Look at platinum, has 78 protons and electrons, and you know what? There are atoms that have even more protons and electrons than that. Electrons are constantly and quickly
00:02:26moving about the nucleus. So the space that electrons occupy is called the electron cloud. Each electron occupies an energy state, so electrons farther from the nucleus occupy a higher energy state. The electron cloud is divided into energy levels. So the first level that's closest to the nucleus has the lowest energy, and so does the energy state of the
00:02:55electrons in it. Each energy level can hold a certain number of electrons. So, as you can see, the first level, here, has one, two electrons. And then you've got second level, here, that has one, two, three, four, five, six, seven, eight, and so forth. When energy is added to the atom, electrons may get excited and jump to a higher energy level.
00:03:32Now, valence electrons are found usually in the highest energy level. They have the highest energy and are held loosely. Valence electrons are used for bonding and determine how one atom bonds with another.
Section 6
00:00:00TEACHER: A limited number of elements make up the Earth. Two or more elements may combine to form compounds. However, only about seven elements and a small percentage of others make up the Earth, yet we see a huge variety of living and non-living things in the world around us. For example, living things include plants,
00:00:21insects, and mammals. And non-living things include something like buildings and rocks. So where do all of these things come from if there are only a limited number of elements that make up the Earth? There are actually over 100 elements total. These elements combine to form compounds that make up all
00:00:41these things we see around us. For example, sodium chloride, which you know as table salt. So chemical symbol for sodium is Na. And then you got Cl, which is chlorine, to form sodium chloride, and this is table salt. What about another example?
00:01:08Water. You've got H2O, which is the chemical symbol for water. So H is the symbol for hydrogen, and then O is for oxygen, and you get water. Now, if the atoms combine chemically, it forms a compound, which is also considered a pure substance. Both examples, table salt and water, are compounds that can
00:01:39be isolated from each other as pure substances.
Section 8
00:00:00TEACHER: Recall learning about the properties of elements. One property that distinguishes one element from another is the number of protons each atom of the element contains. Now, remember the number of protons is also the same as the number of electrons. So as you see in this picture here, you have blue dots that
00:00:20represent protons for positively charged particles and these green dots are electrons, negatively charged particles. Now, you may not be able to tell from this picture, but the number of blue dots is equal to the number of green dots. There's still one subatomic particle we haven't talked about yet, neutrons.
00:00:40In this lesson, we will talk about the neutrons in an atom of a particular element.
Section 9
00:00:00TEACHER: Here are three additional vocabulary words that you'll need to know for the remainder of the lesson. Don't forget to type these important words into your eNotes and check the glossary for definitions. The words are ion, isotope, and mass number. While the number of protons and electrons in an atom of a particular element remains the same, the number of neutrons
00:00:23may differ. So for example, all carbon atoms have six protons. However, some carbon atoms have six neutrons and others have eight neutrons. Carbon atoms with six and eight neutrons are called carbon isotopes. There are the same number of protons, which is six, in each carbon atom.
00:00:43So on the left image, you have an atom that has six neutrons represented by red dots. One, two, three, four, five, six. Now the one on the right, carbon-14, has how many neutrons? Let's count. One, two, three, four, five, six, seven, eight. Both of these are isotopes of carbon.
00:01:06What's an isotope? Remember that an isotope has the same number of protons but a different number of neutrons than other atoms of the same element. Some elements may have many isotopes. Others only have a few. Isotopes of an element are given specific names to tell them apart.
00:01:29Isotopes are identified by their mass number. What is a mass number? Remember, it's the total number of protons and neutrons in the nucleus of an atom. Let's look at the isotopes of carbon again. The most common isotope of carbon is shown on the left, carbon-12. It has a mass number of 12.
00:01:50That's why it's called carbon-12, right here. You can find the number of neutrons in the isotope by subtracting the number of protons from the mass number. So as we discussed earlier, carbon has how many protons? Six. So you would subtract 12 minus 6, which equals 6. So carbon-12 has six protons and six neutrons. Now let's look at carbon-14.
00:02:20So carbon-14 has a mass number of what? 14. So how many neutrons does carbon-14 have? You already know that carbon has six protons. So you would subtract 14 minus 6, which equals 8. So carbon-14 has six protons and eight neutrons.
Section 11
00:00:00TEACHER: Recall that you learned how isotopes differ based on the number of neutrons. The number of electrons in an atom of an element may also differ. In this lesson, we'll discuss what happens when the number of electrons in an atom changes.
Section 12
00:00:00TEACHER: The number of protons in an atom is equal to the number of electrons. If you look at this image here, we have protons represented by blue dots and electrons represented by green dots. Remember that protons have positive charge and electrons have a negative charge.
00:00:19So these charges are balanced and the atom has no overall charge. But what if one of these electrons was removed? Bam. Now the charges are not balanced. You have more positive charges, two in this case, than negative charges, one in this case. Now the charges are not balanced, so the atom will
00:00:42have an overall charge that is positive. This atom is then called an ion. Ions form when one or more electrons are lost or gained. So let's say that we gain an electron. Well, now you have two positive charges and three negative charges. So, the atom will have an overall charge that is negative.
00:01:12So remember once again, ions form when one or more electrons are lost or gained. Next we'll talk more about positive and negative ions. Remember that a positive ion is formed when one or more electrons are lost from the atom. So if you look at this image, we have sodium atom. Remember the chemical symbol for sodium? It's Na.
00:01:35Well, sodium has 11 protons and 11 electrons. Since these numbers are equal, there's no overall charge. However, if the atom loses an electron, well, now you have 11 protons and 10 electrons. The charges are no longer balanced. Well, remember that now we have a sodium ion, sodium plus one, then atom. Since there is one more proton than electron, there is a
00:02:05positive charge on the sodium ion. Now, there can be a loss of more than one electron. For example, in calcium. Let's say calcium loses two electrons. Well, now it becomes calcium plus 2 ion. What if aluminum loses three electrons? It will now form aluminum plus 3 ion. A negative ion is formed when an atom
00:02:35gains one or more electrons. So, if you look at the chlorine atom, remember the chemical symbol for chlorine is Cl. Remember that chlorine has 17 protons and electrons. Well, these numbers are equal, so now there is no overall charge. If the atom gains an electron however, now you have 17 protons and 18 electrons.
00:03:01The charges are no longer balanced, so now we have chlorine negative 1 ion. Since there is one more electron than proton, there is a positive change on the chlorine ion. Note that there can be a gain of more than one electron. So, for example, oxygen-- remember the symbol for oxygen is O-- gains two electrons, it forms the ion oxygen negative 2.