The nucleusThe tiny core of the atom is very dense. It consists of protons andneutrons and is orbited by electrons. A proton is about the samemass as a neutron but about 2000 times larger than an electron. Images: PHET build an atom simProton (aka atomic) number: 6 = 6 protonsNucleon (aka mass) number:213 = 6 protons + 7 neutrons
IsotopesIsotopes of an element are atoms which have the samenumber of protons but different numbers of neutrons. Images: PHET build an atom sim 3
Unstable nucleiCertain nuclei are unstable. They decay spontaneouslyand emit a particle (either alpha or beta) and thenucleus forms an isotope of a new element.Unstable = RadioactiveDecay = Disintegrate An unstable nucleusRandom = Spontaneous decays spontaneously A radioactive nucleus disintegrates randomly 4
Alpha, beta, gamma An alpha particle consists of 2 protons and 2 neutrons (charge = +2). It is the same as a helium nucleus. It leaves theImage: ionactive parent nucleus at a speed of 15 000 km/s. A beta particle consists of one electron (charge =-1; mass 1/8000 th of alpha). It leaves the nucleus at speeds of up to nearly 300 000 km/s. A gamma ray is NOT a particle. It is the most energetic electromagnetic radiation which is emitted by the nucleus at 300 thousand km/s,5the speed of light.
PenetrationPasses through ... Stopped by ... few cm of air paper, skin air, paper, skin few mm aluminiumfew mm aluminium, thick lead most things 6
Ionisation of atoms in theNuclear radiation can knock electrons out air or in your body.You can think of this as damage. Image: remnet causes a lot of ionisation in a short distance causes some ionisation and travels further When ionised, an atom becomes an causes very little ionisation ion pair and is difﬁcult to stop 7
Detecting radioactivityThe Geiger-Muller tube is one of many ways to do this. One alpha or beta particleentering the detector ionises the gas within which createsa voltage which is counted as a ‘click’ by the ratemeter. A photographic ﬁlm will darken when exposed to radiation 8 Image: darvill.clara.net
Background radiation Medical 12% The Earth We are constantlyCosmic rays 10% 14% exposed to low- Other level radiation in Food 2% our environment. 12% Most of these sources are Radon gas naturally occurring. 50% 9
Uses of radioactivity Thickness control Radiotherapy Find Leak out tracer about them Medical tracersThe smoke detector too 10
Carbon things After death, no All living datingThere is a ﬁxed contain the same more C-14 is proportion of fraction of C-14 as absorbed and itcarbon 14 in the the atmosphere due decays with a atmosphere. to the food chain. half-life of 5730 years. The fraction of C-14 remaining in old organic material can be used to calculate its age. Images: Answersingenesis NN takes no responsibility for the 11 views expressed by its sources
Hazards of radioactivity Find out about them 12
Radioactive decay Unstable nuclei decay randomly. They changeinto other elements. The decay is exponential which means that the number of nuclei alwaysreduces by half in a given time. The rate of decay falls as well, but it never gets mathematically to zero. 13
Half-life The activity of a sample is measured in Bq (Bequerels =decays per second). The activity decays exponentially with time, just like the number of nuclei.A sample of 24 millionnuclei has a half-life of 3 Solutionsyears. After how many over theyears will there be 3 pagemillion nuclei left?What is the half-life ofa sample whose activityfalls from 640 Bq to40Bq in 30 minutes? 14
Nuclear equations9 years7.5 min Parent Daughter nucleus nucleus A X−4 4 Alpha can be XZ ⇒ Y Z−2 + α 2 also written as He A X 0 Beta can be XZ ⇒ Y Z+1 + β also written −1 as e The particles in the nucleus are not changed by gamma decay. 15
Nuclear transformations The daughter nucleus is 2 elements lower than the parent on the Periodic table. The daughter nucleus is one element higher than the parent on the Periodic table. The daughter nucleus is identical to the parent nucleus.