# EDEXCEL IGCSE PHYSICS 7-1 Atoms and Radioactivity Edexcel IGCSE Physics pages 199 to 208 December 4 th 2010 All content applies for Triple & Double Science.

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EDEXCEL IGCSE PHYSICS 7-1 Atoms and Radioactivity Edexcel IGCSE Physics pages 199 to 208 December 4 th 2010 All content applies for Triple & Double Science THIS POWERPOINT IS NOT DUE FOR COMPLETION UNTIL JULY 2012

Edexcel IGCSE Specification Section 7: Radioactivity and particles b) Radioactivity describe the structure of an atom in terms of protons, neutrons and electrons and use symbols such as 14 6 C to describe particular nuclei understand the terms atomic (proton) number, mass (nucleon) number and isotope understand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random process describe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating power describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiation understand how to complete balanced nuclear equations

Atomic structure An atom consists of a small central nucleus composed of protons and neutrons surrounded by electrons. An atom will always have the same number of electrons as protons. A Lithium atom protons neutrons electrons

Atomic and mass number The atomic number of an atom is equal to the number of protons in its nucleus. The mass number of an atom is equal to the number of protons plus neutrons in its nucleus. This Lithium atom has: atomic number = 3 mass number = 7 protons = 3 neutrons = 4 electrons = 3

Properties of protons, neutrons and electrons Position in the atom Relative mass Relative electric charge PROTON NEUTRON ELECTRON nucleus outside nucleus 1 1 0.005 + 1 - 1 0

Nuclear notation C 14 6 Number of protons (Atomic number) Chemical symbol An isotope of carbon consists of 6 protons and 8 neutrons. This can be written as: OR: carbon 14 Number of protons PLUS neutrons (Mass number)

Isotopes The atoms of an element always have the same number of protons. Isotopes are atoms of the same element with different numbers of neutrons. The three isotopes of hydrogen neutrons hydrogen 1 hydrogen 3 (tritium) hydrogen 2 (deuterium) Note: The number after ‘hydrogen’ is the mass number of the isotope.

Question 1 U 235 92 An isotope of uranium (chemical symbol U) consists of 92 protons and 143 neutrons. Give the two different ways of notating this isotope. uranium 235 The mass number of the Uranium isotope: = 92 + 143 = 235 AND

Question 2 Determine the number of protons and neutrons in the isotopes notated below: N 13 7 (a) protons = 7 neutrons = 6 Co 60 27 (b) p = 27 n = 33 Au 197 79 (c) p = 79 n = 118 Pu 239 94 (d) p = 94 n = 145 Note: Apart from the smallest atoms, most nuclei have more neutrons than protons.

Radioactivity The atoms of some substances are unstable and they give out radiation from their nuclei all the time, whatever is done to them. These substances are said to be radioactive. The first three types of radiation discovered were alpha particles, beta particles and gamma rays. Henri Becquerel discovered radioactivity in 1896

Alpha, beta and gamma radiation An alpha particle is the same as a helium nucleus. It consists of two protons and two neutrons. A beta particle is a high speed electron. It has come from the nucleus where a neutron has decayed into an electron and proton. Gamma rays are very high frequency electromagnetic waves. They are produced when an unstable nucleus loses energy.

The penetrating power of alpha, beta and gamma radiation Paper or a few cm of air stops alpha particles 1cm or 1m of air of aluminium stops beta particles Several cm of lead or 1m of concrete is needed to stop gamma rays

Choose appropriate words to fill in the gaps below: Atoms consist of a very small _______, containing protons and neutrons, surrounded by _______. Atoms of the same element will always have the same number of _______ but different ________ of the same element will have different numbers of _________. The atoms of some substances are unstable and _________. They may give off alpha or ______ particles or gamma rays. Gamma rays are the most penetrating type of radiation, _____ is the least. isotopes radioactivenucleus protonselectronsalpha neutrons WORD SELECTION: beta isotopes radioactive nucleus protons electrons alpha neutrons beta

Ionisation Ionisation occurs when an atom loses one or more of its electrons. The atom becomes a positive ion. Alpha particles cause intense ionisation due to their large mass double positive charge. Beta particles cause moderate ionisation. Gamma rays only cause weak ionisation because they are uncharged. Lithium atom (uncharged) Lithium ion (positively charged)

S Deflection by magnetic fields Alpha and beta particles are deflected in opposite directions due to their opposite charges. Due to their much larger mass alpha particles are deflected far less than beta. Gamma rays are not deflected because they are not charged. Magnetic south pole placed behind the rays

Deflection by electric fields Alpha and beta particles are deflected in opposite directions due to their opposite charges. Due to their much larger mass alpha particles are deflected far less than beta. Gamma rays are not deflected because they are not charged. Electric field produced by positively and negatively charged plates +++ ---

Choose appropriate words to fill in the gaps below: Magnetic and ________ fields deflect alpha and beta particles in ________ directions due to their opposite ________. Beta particles deflect more because their ______ is about 8000 times ______ than alpha particles. Gamma rays, being _________, are not deflected by either type of field. Radioactivity causes __________ which can cause living cells to undergo genetic _________ leading on to possibly cancerous growth. It is therefore important to minimalise exposure especially to ______ particles which cause the most intense ionisation. massunchargedelectricoppositechargesmutationionisation WORD SELECTION: lessalpha mass uncharged electric opposite charges mutation ionisation less alpha

Alpha decay Alpha particles consist of two protons plus two neutrons. They are emitted by some of the isotopes of the heaviest elements.

Example: The decay of Uranium 238 U 238 92 Th 234 90 α 4 2 + Uranium 238 decays to Thorium 234 plus an alpha particle. Notes: 1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2) 2. The alpha particle can also be notated as: He 4 2

Question Show the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92). Pu 239 94 U 235 92 α 4 2 +

Beta decay Beta particles consist of high speed electrons. They are emitted by isotopes that have too many neutrons. One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle.

Example: The decay of Carbon 14 C 14 6 N 7 β-β- 0 + Carbon 14 decays to Nitrogen 14 plus a beta particle. Notes: 1. The beta particle, being negatively charged, has an effective atomic number of minus one. 2. The beta particle can also be notated as: e 0

Question Show the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg). Na 25 11 Mg 25 12 β-β- 0 +

Gamma decay Gamma decay is the emission of electromagnetic radiation from an unstable nucleus Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle. Example: Cobalt 90 Co 90 27 γ 0 0 + Co 90 27 Cobalt 90 with excess ENERGY decays to Cobalt 90 with less ENERGY plus gamma radiation.

Changing elements Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number. Decay typeAtomic numberMass number alphaDOWN by 2DOWN by 4 betaUP by 1NO CHANGE gammaNO CHANGE

Complete the decay equations below: Fe 59 26 Co 59 27 β-β- 0 + Ra 224 88 Rn 220 86 α 4 2 + N 16 7 O 8 β-β- 0 + (a) (c) (b)

Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!) Pb 202 82 Hg 198 80 α 4 2 + Pt 194 78 Au 194 79 β-β- 0 + ElementSymZ PlatinumPt78 GoldAu79 MercuryHg80 ThalliumTl81 LeadPb82 BismuthBi83 Hg 198 80 Pt 194 78 α 4 2 + There are other correct solutions

Choose appropriate words to fill in the gaps below: When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______. Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged. Background radiation is mainly due to natural sources of _________ radiation such as from ________ gas that seeps out from rocks in the ground. ionisingradonmasstwofourone WORD SELECTION: neutrons ionisingradon mass twofour one neutrons