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Transcript
Nuclear Chemistry Nuclear Forces • THE POSITIVE PROTONS TEND TO REPEL EACH OTHER THROUGH ELECTROSTATIC REPULSION. • BUT THE PRESENCE OF THE NEUTRONS PROVIDES A ‘NUCLEAR FORCE’, OR STRONG FORCE WHICH HOLDS THE NUCLEUS TOGETHER AND STABILIZES IT. Neutron Proton nucleons Nuclear Forces The nuclear force is only strong when the nucleons are close together. As the number of protons increases, the electrostatic repulsion increases and the nuclear force weakens. So, even more neutrons are needed to stabilize the nucleus. Nuclear Forces A stable neutron - proton ratio is 1:1. As the number of protons increases, so does the number of neutrons. This keeps the nucleus stable. But, this increases the neutron – proton ratio. Nuclear Forces A neutron-proton ratio of 1.5 : 1 is at the limit of stability. Nuclear Forces •Atoms with more than 83 protons cannot reach stability even with their larger numbers of neutrons. •All elements beyond bismuth on the periodic table are unstable and undergo some sort of ‘decay’ in order to become stable. RADIOACTIVE DECAY Radioactive Decay Unstable nuclei are those with a high neutron:proton ratio; this will result in decay or a change in the nucleus in order to become stable. Transmutation: a change in the identity of an element as a result of a change in the number of protons. Radioactive Decay Atoms will decay by ejecting nucleons, or altering the nucleons into different particles by releasing one or more of the following: Alpha Beta rays rays Gamma rays Positron Emission Electron Capture Alpha Particles Helium Nucleus 2+ charge Alpha particles consist of two protons and two neutrons, and are emitted during some kinds of radioactive decay. Remember that protons determine the identity of the element : if an alpha particle is emitted, the identity of the element changes. The atomic number will be reduced by two, and the mass number reduced by four. Alpha Particles Alpha rays are streams of alpha particles given off during nuclear decay. rays are relatively slow and easy to stop – a piece of paper will stop alpha particles. Alpha They will travel only a few centimeters before stopping even if they do not encounter any matter. The positive charge of the alpha particle attracts electrons nearby and the particle becomes a harmless helium atom. Beta Particles A Beta particle is an electron created and emitted when a neutron is transformed* into a proton and an electron during radioactive decay. This action adds a proton and thus changes the identity of the atom. The mass number stays the same. *The proton and electron are not ‘inside’ the neutron. They are created at the time of the action. Beta Particles Beta rays travel faster than alpha rays and can penetrate paper, but are generally stopped by thin sheets of metal such as aluminum. Their negative charge causes them to interact with other atoms which slows their speed. - Gamma Rays Gamma rays are photons of electromagnetic radiation with high frequency and energy. Gamma rays are given off when the nucleons undergo an abrupt energy difference. rays have no mass – they are pure energy. Gamma Gamma Rays Because they have no charge and are high energy, gamma rays travel far and penetrate further than alpha or beta rays. Thick concrete or lead is needed to stop gamma rays. Gamma Rays The release of gamma rays alone do not affect the identity of the atom since they have no mass and no charge. But, gamma radiation may be released along with release of an alpha or beta particle. Beta Particle Gamma Ray Positron Emission • A positron is a particle that has the same mass as that of an electron, but has a positive charge. • A positron is emitted from the nucleus as a proton is converted into a neutron. • The atomic number decreases by one but the mass number stays the same. 1 p 1 1 n 0 0 B +1 Positron Electron Capture • The nucleus can ‘capture’ one of its own inner-orbital electrons if the atom is unstable due to too many protons. • The electron will combine with a proton in the nucleus and form a neutron. • The atomic number decreases by one but the mass number stays the same. 0 -1 e 1 1 p 1 0 n