 In this video, we are going to talk more about atoms and specifically how to differentiate an atom of one element from an atom of a different element. And if there are several ways we can do that, the first way is by using the atomic number. The atomic number equals the number of protons that are in the nucleus of that specific atom. The atomic number is listed on the periodic table, and I have a few elements off the periodic table listed here. The atomic number is the whole number that is listed right above the symbol for the element. So for carbon, the atomic number is 6. For nitrogen, the atomic number is 7. And for oxygen, the atomic number is 8. That tells you how many protons are in the nucleus of that atom. Every atom of carbon has six protons. Every atom of nitrogen has seven, and every atom of oxygen has eight. They will always be the same. However, different atoms have different numbers of protons in the nucleus. Now, the atoms are also neutral. So that means there is no charge. Protons, as you recall, are positively charged subatomic particles. So that means we have to have the same number of negatively charged particles to balance it out. So the electrons are the negatively charged subatomic particle. So we're going to have the same number of protons and neutrons in every neutral atom. So our atomic number not only tells us the number of protons, it also tells us the number of electrons in that atom as well. So carbon has six protons and six electrons, while oxygen has six protons, sorry, eight protons and eight electrons. Another number we can use to differentiate between atoms is the mass number. The mass number is not found on the periodic table. It is a counted number, and it is the sum of the protons and the neutrons that are in the nucleus. So you would have to be given the number of neutrons for that specific atom to determine its mass number, the number of protons you would find on the periodic table. Another way that you, another question you may be asked is to determine how many neutrons are in an atom. In that case, if you knew the mass number, you could subtract the atomic number or the number of protons, and that would leave you with the number of neutrons. Again, number of neutrons and the mass number cannot be found on the periodic table. One of those numbers would have to be provided for you in the question. An atomic symbol is used to represent a specific atom. It gives you both the mass number and the atomic number. It is set up differently than the symbols on the periodic table. For the atomic symbol, the symbol of the element is written, then the mass number is written as a superscript, and the atomic number is written as a subscript. Okay? Similarly, the mass number is larger than the atomic number, however there are cases when they are the same. So if you are given an atomic symbol like this, you can determine the number of protons, electrons and neutrons that are in that specific atom. So the atomic number represents the number of protons, so there are six protons. The atomic number also represents the number of electrons. The number of neutrons, again, can be determined by subtracting the atomic number from the mass number. So 13 minus 6 gives us 7 neutrons. This is a more descriptive way to express the symbol of the element because it provides mass number and atomic number, so you can get the numbers of all the protons, neutrons and electrons for that specific atom. Okay? Another way you might see these atomic symbols represented are abbreviated ways. You may see them written with only the mass number because if it is 13c, then the atomic number has to be 6 because carbon always has atomic number 6. And then you may also see it written this way, a c-13. If you see a symbol of an element followed by a number, that number is always the mass number. Then isotopes. Isotopes are atoms of the same element that have different mass numbers. As we saw on the periodic table, the atomic number is always the same for carbon, always 6. Okay? However, the mass number for carbon can be different and I've put four different isotopes of carbon on the board. Mass number of 11, 12, 13 and 14, these are all different isotopes. Notice the atomic numbers are the same for all four of them. From these atomic symbols, we can determine the number of protons, electrons and neutrons. Protons and electrons are going to be the same for every single one of these isotopes of carbon because the atomic number is the same, so it's 6. The number of neutrons is what differs. So if you subtract 6 from 11, you have 5 neutrons. In this isotope you have 6 neutrons, in this one 7, in this one 8. So the only thing that is different in different isotopes of the same element is the mass number or number of neutrons. The next number we're going to talk about is the atomic mass. Atomic mass is listed on the periodic table underneath the symbol of each element. As you can see, it's a decimal number. For carbon, the atomic mass is 12.01, for nitrogen it's 14.01, for oxygen it's 16.00. This number represents a weighted average of the mass numbers of all the naturally occurring isotopes for that specific element. Since it is an average, it's a decimal number. So you will find this on the periodic table and it represents that weighted average of all the mass numbers of the isotopes. So just a quick review. On the periodic table you will find the atomic number, which represents the number of protons and number of electrons in an atom. On the periodic table you'll find the atomic mass, which is the weighted average. The mass number represents the number of protons and neutrons in an element. So with those different numbers, you can identify or differentiate between atoms of the same element and atoms of different elements.