1 What is an atomic number and an atomic mass?

2 Elements and Atomic Number The atoms of different elements have different numbers of protons. The atoms of different elements have different numbers of protons. You have learned that protons are particles that have a positive charge (+) and are found within the nucleus of an atom. You have learned that protons are particles that have a positive charge (+) and are found within the nucleus of an atom. The number of protons found in the nucleus of an atom is called the atomic number. The number of protons found in the nucleus of an atom is called the atomic number. Each element has a different atomic number because the atoms of each element have a different number of protons in their nuclei. Each element has a different atomic number because the atoms of each element have a different number of protons in their nuclei.

3 Atomic Number The element with the smallest atomic number is hydrogen. The element with the smallest atomic number is hydrogen. Hydrogen has an atomic number of 1. Hydrogen has an atomic number of 1. This means that an atom of hydrogen has only one proton in its nucleus. This means that an atom of hydrogen has only one proton in its nucleus. Oxygen has an atomic number of 8 because there are 8 protons in the nucleus of an atom of oxygen. Oxygen has an atomic number of 8 because there are 8 protons in the nucleus of an atom of oxygen. You can see that the number of protons in the nuclei of each element is the same as the atomic number of each element. You can see that the number of protons in the nuclei of each element is the same as the atomic number of each element.

4 The nuclei of helium, beryllium, and neon

5 Atomic numbers of some common elements. Atomic numbers of some common elements.

6 Importance of Atomic Number The atomic number of an element is very important because it identifies that element. The atomic number of an element is very important because it identifies that element. No two elements have the same atomic number. No two elements have the same atomic number. In fact, the number of protons in the nucleus of an atom tells you what that element is. In fact, the number of protons in the nucleus of an atom tells you what that element is.

7 Atomic Number and Electrons Electrons that move outside the nucleus have a negative charge. Electrons that move outside the nucleus have a negative charge. If you know the atomic number of an element, you also know the number of electrons in an atom of that element - they are both the same. If you know the atomic number of an element, you also know the number of electrons in an atom of that element - they are both the same. They are the same because an atom has neither a positive nor a negative charge. It is neutral. They are the same because an atom has neither a positive nor a negative charge. It is neutral. In order for an atom to be neutral, the positive charges of the protons and the negative charges of the electrons must cancel each other out. In order for an atom to be neutral, the positive charges of the protons and the negative charges of the electrons must cancel each other out. So the number of electrons must be the same as the number of protons. So the number of electrons must be the same as the number of protons.

8 Atomic Mass Unit The mass of an atom is very small. The mass of an atom is very small. In order to measure the mass of an atom, scientists have developed a special unit. In order to measure the mass of an atom, scientists have developed a special unit. This unit is called the Atomic Mass Unit, or amu. This unit is called the Atomic Mass Unit, or amu. One amu is equal to the mass of one proton. One amu is equal to the mass of one proton. Neutrons and protons have almost the same mass. Neutrons and protons have almost the same mass. The mass of an electron is equal to 1/1,836 amu. The mass of an electron is equal to 1/1,836 amu. Because electrons are so small, only the masses of protons and neutrons are used to find the mass of an atom. Because electrons are so small, only the masses of protons and neutrons are used to find the mass of an atom.

9 Atomic Mass The total mass of the protons and neutrons in an atom is called the atomic mass. The total mass of the protons and neutrons in an atom is called the atomic mass. Atomic mass is measured in atomic mass units (amu). Atomic mass is measured in atomic mass units (amu). Because the atoms of an element can have different numbers of neutrons, scientists often give the average atomic mass for an element. Because the atoms of an element can have different numbers of neutrons, scientists often give the average atomic mass for an element.

10 Mass Number The total number of protons and neutrons in an atom is called the mass number. The total number of protons and neutrons in an atom is called the mass number. You can find the mass number for any element. You can find the mass number for any element. The mass number can be found by rounding the average atomic mass to the nearest whole number. The mass number can be found by rounding the average atomic mass to the nearest whole number. For example, the average atomic mass of lithium is amu. For example, the average atomic mass of lithium is amu. The mass number of lithium is 7 (6.941 rounds to 7). The mass number of lithium is 7 (6.941 rounds to 7).

11 To find the mass number, round the average atomic mass to the nearest whole number.

12 Finding Neutrons If you know the atomic number and the mass number of an element, then you can find the number of neutrons in an atom of the most common form of that element. You can use the following formula: If you know the atomic number and the mass number of an element, then you can find the number of neutrons in an atom of the most common form of that element. You can use the following formula: Mass number Mass number – Atomic number – Atomic number number of neutrons number of neutrons

13 You can find the number of neutrons in an atom of the most common form of lithium.

14 Checking Concepts 1. The atomic number is the number of _________ in the nucleus of an atom. 2. No two elements have the same _______. 3. The number of protons in an atom is equal to the number of _______ in the atom. 4. The mass of a neutron is the same as the mass of a _________. 5. Because they are so small, _________ are not counted when measuring the mass of an atom. 6. The mass number tells the total number of ________ and neutrons in the nucleus of an atom.

15 Answers 1. Protons 2. Atomic number 3. Electrons 4. Proton 5. Electrons 6. Protons