Atomic Mass. Atomic mass Most of the mass of an atom is in the nucleus. Most of the mass of an atom is in the nucleus. The nucleus is where all of the.

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Presentation transcript:

Atomic Mass

Atomic mass Most of the mass of an atom is in the nucleus. Most of the mass of an atom is in the nucleus. The nucleus is where all of the protons and neutrons are found. The nucleus is where all of the protons and neutrons are found. Electrons are not included in the mass because they are much smaller than protons and neutrons. Electrons are not included in the mass because they are much smaller than protons and neutrons.

Atomic Mass Units Atomic mass units (amu) are used to measure the masses of atoms. Atomic mass units (amu) are used to measure the masses of atoms. Each proton and neutron has mass equal to 1 amu. Each proton and neutron has mass equal to 1 amu. The amu is defined as 1/12 the mass of a carbon atom. The amu is defined as 1/12 the mass of a carbon atom.

Identifying elements Atoms of different elements have different numbers of protons. Atoms of different elements have different numbers of protons. Each atom has a unique number of protons, called its atomic number. Each atom has a unique number of protons, called its atomic number. For example, all carbon atoms have an atomic number of 6, which means that they have 6 protons. For example, all carbon atoms have an atomic number of 6, which means that they have 6 protons. All nitrogen atoms have an atomic number of 7 and have 7 protons. All nitrogen atoms have an atomic number of 7 and have 7 protons.

Mass number The mass number of an atom is the sum of the number of protons and neutrons that are found in the nucleus. The mass number of an atom is the sum of the number of protons and neutrons that are found in the nucleus. For example, the mass number of a boron atom that has 5 protons and 6 neutrons = 11. For example, the mass number of a boron atom that has 5 protons and 6 neutrons = 11. You can also use the mass number to find the number of protons. You can also use the mass number to find the number of protons. Neutrons = mass – atomic number Neutrons = mass – atomic number If we know the mass number of a boron atom is 11, we subtract 11-5 = 6 neutrons If we know the mass number of a boron atom is 11, we subtract 11-5 = 6 neutrons

Isotopes An isotope is when atoms of the same element have different numbers of neutrons. An isotope is when atoms of the same element have different numbers of neutrons. For example, some boron atoms have 6 neutrons and some have 5. For example, some boron atoms have 6 neutrons and some have 5. Radioactive isotopes can be used to find the ages of fossils by using their half-life. Radioactive isotopes can be used to find the ages of fossils by using their half-life. Half-life is the amount of time it takes for half of the radioactive isotope to decay. Half-life is the amount of time it takes for half of the radioactive isotope to decay.

Identifying isotopes We identify isotopes using a naming system. We identify isotopes using a naming system. Because isotopes have different numbers of neutrons, they also have different mass numbers. Because isotopes have different numbers of neutrons, they also have different mass numbers. To identify an isotope, we write the name of the element followed by its mass number. To identify an isotope, we write the name of the element followed by its mass number. Example, boron-11 is a boron atom with 5 protons and 6 neutrons. Example, boron-11 is a boron atom with 5 protons and 6 neutrons. Boron-10 is a boron atom with 5 protons and 5 neutrons. Boron-10 is a boron atom with 5 protons and 5 neutrons.

Average Atomic Mass Average atomic mass is the weighted average masses of all of the isotopes for that element. Average atomic mass is the weighted average masses of all of the isotopes for that element. For example, we know that boron has 2 isotopes, boron-10 and boron-11. For example, we know that boron has 2 isotopes, boron-10 and boron-11. To find the average atomic mass, we have to know how many of each isotope there are and then add their weighted averages together. To find the average atomic mass, we have to know how many of each isotope there are and then add their weighted averages together. The atomic masses on the periodic table are decimal numbers because they are an average, not just the mass of one atom. The atomic masses on the periodic table are decimal numbers because they are an average, not just the mass of one atom. If we round off the atomic mass to the nearest whole number, that is the mass of the most abundant isotope. If we round off the atomic mass to the nearest whole number, that is the mass of the most abundant isotope.