Atomic Theory Atoms are building blocks of elements Similar atoms in each element Different from atoms of other elements Two or more different atoms bond in simple ratios to form compounds LecturePLUS Timberlake 1
Subatomic ParticlesParticle Symbol Charge MassElectron e- 1- 1/1840Proton p+ + 1Neutron n 0 1Charge measure in terms of electronic charge: e=1.6x10^-19Mass is measure in unified atomic mass: units(u)1 u is 1/12 of the mass of a carbon-12 atom. LecturePLUS Timberlake 2
Location of Subatomic Particles 10-13 cmelectrons protons neutrons nucleus 10-8 cm LecturePLUS Timberlake 3
Atomic NumberCounts the number of protons in an atom LecturePLUS Timberlake 4
Atomic Symbols Show the mass number and atomic number Give the symbol of the element mass number 23 Na sodium-23 atomic number 11 LecturePLUS Timberlake 5
Number of Electrons An atom is neutral The net charge is zero Number of protons = Number of electrons Atomic number = Number of electrons LecturePLUS Timberlake 6
Subatomic Particles in Some Atoms16 31 65 O P Zn 8 15 308 p+ 15 p+ 30 p+8n 16 n 35 n8 e- 15 e- 30 e- LecturePLUS Timberlake 7
Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37 LecturePLUS Timberlake 8
Learning Check Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6#p _______ _______ _______#n _______ _______ _______#e _______ _______ _______ LecturePLUS Timberlake 9
Nuclear Density• Mass of proton m(p)=1.67 x 10^-27 kg• Radius of proton r = 0.80 x 10^-15 m• Find the density=
IsotopesAtoms made of protons, neutrons and electrons.Neutrons+ Protons: Nucleus• Electron + Proton # influences-> Chemical Properties• Protons+ Neutron # influences-> Physical PropertiesIsotopesAtoms with same number of protons but different number of protons are Isotopes.
Radioactivity• A stable nuclei emits radiations. Radiation was discovered by Henri Becquerel in 1896. These radiations are not influenced by any outside conditions such as temperature or pressure.• It is a spontaneous process, but it occurs at fixed intervals. Hence a proportion of sample decay will occur in that time interval.
Three particles present in these Radiations:• Alpha, Beta And Gamma Alpha Beta Gamma
• We know that nuclei consists of protons and neutrons, if the balance between these two types of particles is too far to one side, the nucleus may emit alpha or B radiations.• Gamma emitted after alpha or B radiation to release excess energy.
A third conserved quantity• In radioactive decay, both nucleon number A and proton number Z are conserved.• On comparison b/w mass before and after decay, we witness a decrease in mass.• This is because energy lost caries mass- recall E=mc^2, so we witness a net decrease in energy.
Properties of Ionizing radiation• On moving past atoms, alpha or B collide/rub with them and this as a result causes to knock or drag away electrons from an atom.• This process is called Ionization.• Now, Energy is lost during ionization. Size: Motion: Ionization Ability:• Alpha: more mass &charge | slow | Most• Beta: lighter | faster | Moderate• Gamma: no charge | Least
Lesson Contents1. Physical properties of a, b and g2. Penetrating power of a, b and g3. N v Z graphs4. Decay laws
Alpha RadiationAlpha particles contain two protonsand two neutrons
Alpha Radiationa has the same constitution as ahelium nucleusAlpha particles may be written asThey have a double positive chargeand a mass of 4 u
Alpha• Since most effective ionization therefore, most energy is lost during travel, hence cover least distance and in a cloud chamber, most prominent tracks are formed.
Beta-minus RadiationBeta-minus particles are electrons
Beta-minus Radiationb- is produced when a neutron decaysBeta-minus particles may be writtenasThey have a negative charge and amass of 1/1800 u
Beta-minus Radiationb- is produced when a neutron decays + EnergyThe surplus mass is released askinetic energy in the b- and as anantineutrino
Gamma RadiationGamma rays are a form of electro-magnetic radiation
Gamma Radiationg release is often associated with aor b decayGamma rays remove energy froman unstable nucleus
Type of Nature of the Nuclear Penetrating power, and what will block it Ionising power - the ability to remove electronsradiation radiation Symbol (more dense material, more radiation is from atoms to form positive ionsemitted & (higher only) (higher only) absorbed BUT smaller mass or charge of symbol particle, more penetrating) a helium nucleus of 2 Low penetration, biggest mass and Very high ionising power, the biggest mass and protons and 2 charge, stopped by a few cm of air or charge of the three radiations, the biggest neutrons, mass = 4, thin sheet of paper punch! charge = +2Alpha high kinetic energy Moderate penetration, middle values of Moderate ionising power, with a smaller mass electrons, mass = charge and mass, most stopped by a few and charge than the alpha particle 1/1850, charge = -1 mm of metals like aluminium Beta very high frequency Very highly penetrating, smallest mass The lowest ionising power of the three, gamma electromagnetic and charge, most stopped by a thick radiation carries no electric charge and has radiation, mass = 0, layer of steel or concrete, but even a few virtually no mass, so not much of a punch charge = 0 cm of dense lead doesnt stop all of it! when colliding with an atomGamma
Randomness and Decay:Spontaneous:• Decays not effected by the presence of other nuclei.• No chemical reaction, external factors like pressure and temperature effect the decay.• Random:• Impossible to predict the time• Each nucleus has an equal probability to decay