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Atoms and Isotopes What are atoms, isotopes, and radioactive decay?

The Harnessed Atom 3 Atoms and Isotopes Today’s Topics How is energy stored in an atom? How is energy stored in an atom? Atoms and subatomic particles Atoms and subatomic particles Elements (Periodic Table) Elements (Periodic Table) Isotopes (Chart of Nuclides) Isotopes (Chart of Nuclides) Nuclear forces, stability, and binding energy Nuclear forces, stability, and binding energy Radioactive decay Radioactive decay

The Harnessed Atom 4 Atoms and Isotopes Atoms and Subatomic Particles Atoms are the smallest unit of a chemical element that has all the chemical properties of that element. Atoms are the smallest unit of a chemical element that has all the chemical properties of that element. Made up of: Made up of: Protons– positive charge Protons– positive charge Neutrons—no charge Neutrons—no charge Electrons—negative charge Electrons—negative charge

The Harnessed Atom 5 Atoms and Isotopes The Periodic Table of the Elements

The Harnessed Atom 6 Atoms and Isotopes Reading Atomic Notations Z is the atomic (proton) number Z is the atomic (proton) number N is the neutron number N is the neutron number A is the mass number (N+Z) A is the mass number (N+Z) X is the chemical element symbol X is the chemical element symbol

The Harnessed Atom 7 Atoms and Isotopes Isotopes Atoms of one element may have different number of neutrons: the different possible versions of each element are called isotopes. Atoms of one element may have different number of neutrons: the different possible versions of each element are called isotopes. Isotopes of one element all have the same number of protons (atomic number, Z) but different numbers of neutrons (thus different atomic weights, A). Isotopes of one element all have the same number of protons (atomic number, Z) but different numbers of neutrons (thus different atomic weights, A). Every element has several isotopes Every element has several isotopes All isotopes are shown on the chart of the nuclides. All isotopes are shown on the chart of the nuclides.

The Harnessed Atom 8 Atoms and Isotopes Isotopes of the element sodium– 11 protons Isotopes of the element oxygen– 8 protons

The Harnessed Atom 9 Atoms and Isotopes Nuclear Forces Weak Nuclear Forces Weak Nuclear Forces Particles with like charges repel. Particles with like charges repel. This causes electrons to orbit around the nucleus. This causes electrons to orbit around the nucleus.

The Harnessed Atom 10 Atoms and Isotopes Nuclear Forces Strong Nuclear Forces Strong Nuclear Forces Particles in the nucleus actually are held together by an even stronger attractive force. Particles in the nucleus actually are held together by an even stronger attractive force. Acts only at very short distances (about m)—beyond this distance, the strong nuclear force is negligible. Acts only at very short distances (about m)—beyond this distance, the strong nuclear force is negligible.

The Harnessed Atom 11 Atoms and Isotopes Nuclear Forces Two protons more than m will repel each other by their like charges. Two protons more than m will repel each other by their like charges. Inside a nucleus, the distances are small enough that the strong nuclear force overcomes the weak repulsive force, holding the protons and neutrons together. Inside a nucleus, the distances are small enough that the strong nuclear force overcomes the weak repulsive force, holding the protons and neutrons together.

The Harnessed Atom 12 Atoms and Isotopes Nuclear Forces Big Idea of Science: There are only four known forces in nature: 1. Gravity 2. Electromagnetism 3. Weak Nuclear Force 4. Strong Nuclear Force

The Harnessed Atom 13 Atoms and Isotopes Think about it… How might a higher number of neutrons change the balance between the repulsive and attractive forces in a nucleus? How might a higher number of neutrons change the balance between the repulsive and attractive forces in a nucleus? How might a lower number of neutrons affect this same balance? How might a lower number of neutrons affect this same balance?

The Harnessed Atom 14 Atoms and Isotopes Nuclear Stability The stability of an atom is the balance of the repulsive and attractive forces within the nucleus (strong and weak force in equilibrium). The stability of an atom is the balance of the repulsive and attractive forces within the nucleus (strong and weak force in equilibrium).

The Harnessed Atom 15 Atoms and Isotopes Nuclear Stability If the attractive strong forces prevail, the nucleus is stable. If the attractive strong forces prevail, the nucleus is stable. If the repulsive weak forces outweigh the attraction of the strong forces, the nucleus is unstable. If the repulsive weak forces outweigh the attraction of the strong forces, the nucleus is unstable.

The Harnessed Atom 16 Atoms and Isotopes Nuclear Stability For elements with low atomic numbers, atoms are stable when their neutron to proton ratio is close to one (1:1). For elements with low atomic numbers, atoms are stable when their neutron to proton ratio is close to one (1:1). As atomic number increases, stable atoms have ratios greater than one (1:1.5). As atomic number increases, stable atoms have ratios greater than one (1:1.5). This is because at higher atomic numbers, more neutrons are needed to counteract the repulsive forces between the protons. This is because at higher atomic numbers, more neutrons are needed to counteract the repulsive forces between the protons.

The Harnessed Atom 17 Atoms and Isotopes Nuclear Stability The shaded cluster is the “band of stability.” The shaded cluster is the “band of stability.” The solid line represents a neutron- to-proton ratio of 1:1. The solid line represents a neutron- to-proton ratio of 1:1. Nuclei to the right of the band of stability don’t have enough neutrons to remain stable. Nuclei to the right of the band of stability don’t have enough neutrons to remain stable. Nuclei to the left of the band have too many neutrons to remain stable. Nuclei to the left of the band have too many neutrons to remain stable.

The Harnessed Atom 18 Atoms and Isotopes Nuclear Binding Energy The energy stored in the bonds within an atom The energy stored in the bonds within an atom Released when an atom breaks apart Released when an atom breaks apart Represented by the equation: Represented by the equation: E b (MeV) = (Zm p + Nm n – M A ) x MeV/amu Where: 1 amu = 1.66 x grams [amu = atomic mass unit] 1 amu = 1.66 x grams [amu = atomic mass unit] E b = binding energy E b = binding energy Zm= mass of the protons in amu Zm= mass of the protons in amu Nm= mass of neutrons Nm= mass of neutrons M A = mass of the atom M A = mass of the atom MeV= millions of electron volts, a unit of measure used to represent the energy in nuclear equations MeV= millions of electron volts, a unit of measure used to represent the energy in nuclear equations

The Harnessed Atom 19 Atoms and Isotopes Radioactive Decay Unstable atoms will spontaneously transform until they reach a stable configuration. Unstable atoms will spontaneously transform until they reach a stable configuration. These transformations are accompanied by releases of energy. These transformations are accompanied by releases of energy.

The Harnessed Atom 20 Atoms and Isotopes Radioactive Decay This energy, given off in waves from an atom, is known as radiation. This energy, given off in waves from an atom, is known as radiation. Substances that give off radiation are called radioactive. Substances that give off radiation are called radioactive. The process of isotopes emitting particles and energy to become more stable is called radioactive decay. The process of isotopes emitting particles and energy to become more stable is called radioactive decay.

The Harnessed Atom 21 Atoms and Isotopes Radioactive Decay Main types of radioactive decay: Main types of radioactive decay: Alpha emission Alpha emission Beta emission Beta emission Positron emission Positron emission Gamma emission Gamma emission

The Harnessed Atom 22 Atoms and Isotopes Radioactive Decay Alpha emission ( α ) Nucleus emits an alpha particle—two protons and two neutrons Equivalent to a helium nucleus (He). Alpha Decay Animation

The Harnessed Atom 23 Atoms and Isotopes Radioactive Decay Beta Emission ( β ) Nucleus emits an electron, and a neutron is converted to a proton. Nucleus emits an electron, and a neutron is converted to a proton. Beta Decay Animations:

The Harnessed Atom 24 Atoms and Isotopes Radioactive Decay Positron Emission Nucleus emits a positron (identical to an electron in mass, but has a positive charge) Nucleus emits a positron (identical to an electron in mass, but has a positive charge) Positron is formed when a proton converts to a neutron. Positron is formed when a proton converts to a neutron.

The Harnessed Atom 25 Atoms and Isotopes Radioactive Decay Gamma emission ( γ ) Nuclei seeking lower energy states emit electromagnetic radiation, which is in the gamma ray region of the electromagnetic spectrum. Nuclei seeking lower energy states emit electromagnetic radiation, which is in the gamma ray region of the electromagnetic spectrum. Rays are emitted in conjunction with another type of decay (alpha or beta). Rays are emitted in conjunction with another type of decay (alpha or beta). Gamma Decay Additional animations:

The Harnessed Atom 26 Atoms and Isotopes Radioactive Decay Chains

The Harnessed Atom 27 Atoms and Isotopes Half Life The amount of time it takes for half of the atoms of a given isotope to decay to another form is known as its half-life. The amount of time it takes for half of the atoms of a given isotope to decay to another form is known as its half-life. The value can be from fractions of a second to billions of years. The value can be from fractions of a second to billions of years.

The Harnessed Atom 28 Atoms and Isotopes Half Life Half-life values are constant. Half-life values are constant. There is no way to speed up or slow down this natural process. There is no way to speed up or slow down this natural process. Cannot predict when a specific atom will decay. Cannot predict when a specific atom will decay. Can predict the number of atoms that will decay in a certain time period. Can predict the number of atoms that will decay in a certain time period.

The Harnessed Atom 29 Atoms and Isotopes Half Life of Uranium-235

The Harnessed Atom 30 Atoms and Isotopes Words to Know… Alpha decay Alpha decay Atom Atom Beta decay Beta decay Chart of Nuclides Chart of Nuclides Coulomb’s force Coulomb’s force Electron Electron Electron capture Electron capture Element Element Gamma decay Gamma decay Half-Life Half-Life Isotope Isotope Neutron Nuclear force Nucleus Nuclide Periodic Table Positron Proton Radiation Radioactive Decay Stability

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