Atomic structureNucleusThe mass number gives theThis contains protons andnumber of nucleons, that is theneutrons, called collectivelynumber of protons+neutrons.nucleonsNIsotopes are atoms with the samePPatomic number but different massNNPNnumbers. All the atoms of anelement have the same atomicNPNPnumber and it is this that makesNPthem all atoms of a particularThe atomic number gives theelement.number of protons in theThe masses of atoms of particularnucleus. It also gives theisotopes, called the relative isotopicnumber of electrons in themass, are expressed on a relativeneutral atom and the positionscale on which the mass of an atomof the element in the periodicof the isotope carbon-12 has 121tableunits exactly
1 Atomic structure Nucleus This contains protons and neutrons, called collectively nucleons. The mass number gives the number of nucleons, that is the number of protons+neutrons. The atomic number gives the number of protons in the nucleus. It also gives the number of electrons in the neutral atom and the position of the element in the periodic table Isotopes are atoms with the same atomic number but different mass numbers. All the atoms of an element have the same atomic number and it is this that makes them all atoms of a particular element. The masses of atoms of particular isotopes, called the relative isotopic mass, are expressed on a relative scale on which the mass of an atom of the isotope carbon–12 has 12 units exactly
Atomic structureRelative atomic massAll elements exist in several isotopic forms and it is useful to have anaverage value for the masses of the atoms of each element. This iscalled the relative atomic mass and is defined as the weighted mean ofthe masses of the naturally occurring isotopes of the element expressedon the carbon-12 scale. These are found using a mass spectrometer.IsotopeRelativeisotopemassRelativeabundancein naturalchlorine35CI3575%37CI3725%Relative atomic mass = 35X 0.75 + 37X 0.25 = 35.52
2 Atomic structure All elements exist in several isotopic forms and it is useful to have an average value for the masses of the atoms of each element. This is called the relative atomic mass and is defined as the weighted mean of the masses of the naturally occurring isotopes of the element expressed on the carbon-12 scale. These are found using a mass spectrometer. Isotope Relative isotope mass Relative abundance in natural chlorine 35Cl 35 75% 37Cl 37 25% Relative atomic mass = 35×0.75 + 37×0.25 = 35.5 Relative atomic mass
Atomic structureRADIOACTIVITYSome isotopes are stable, but others, often with unevennumbers of protons and/or neutrons are unstable. Thisinstability increases with atomic number, resulting fromthe growing repulsion between increasing numbers ofprotons. When an unstable isotope decays it gives offradiation known as radioactivity. This can be in one ofthree forms as this table shows.3
3 Atomic structure Some isotopes are stable, but others, often with uneven numbers of protons and/or neutrons are unstable. This instability increases with atomic number, resulting from the growing repulsion between increasing numbers of protons. When an unstable isotope decays it gives off radiation known as radioactivity. This can be in one of three forms as this table shows. RADIOACTIVITY
Atomic structureName ofradiationMadeofBehaviorinelectricfieldPenetratingpowerAlphahelium nucleideflected slightlystopped bypaperBetaelectronsdeflected a lotin other direction stopped by mm ofleadGammaelectromagneticradiation similartoX-rayspenetrates cm ofleadelectricfieldα,alpha particlescardleadβ,betaparticlesy,gammaraysleadAll radioactive decay is a first order rate process4
4 Atomic structure All radioactive decay is a first order rate process Name of radiation Made of Behavior in electric field Penetrating power Alpha helium nuclei deflected slightly stopped by paper Beta electrons deflected a lot in other direction stopped by mm of lead Gamma electromagnetic radiation similar to X-rays penetrates cm of lead
Atomic structureUses of isotopesIsotopes are often used as tracers. Very small tracers of anisotope can be detected and hence followed through a process.biology e.g.32P to study nutrient uptake in plantsmedicine e.g. 131I to study thyroid(甲状腺) functionindustry e.g. 57Fe to study wear and lubrication in enginesgenerating power e.g. 235U in fission reactors; 3H in fusiongeography e.g. 57Fe to study river flowarchaeology e.g. 14C in carbon dating5
5 Atomic structure Isotopes are often used as tracers. Very small tracers of an isotope can be detected and hence followed through a process. Uses of isotopes biology e.g. 32P to study nutrient uptake in plants medicine e.g. 131I to study thyroid(甲状腺) function industry e.g. 57Fe to study wear and lubrication in engines generating power e.g. 235U in fission reactors; 3H in fusion geography e.g. 57Fe to study river flow archaeology e.g. 14C in carbon dating