Atom Structure

Atom StructureCookIB Chemistry IIThe AtomFirst Periodic Tables were much different than the one todayFirst Periodic table had 100 elementsElementsSimplest form of matter and cant be broken down into simpler componetsAtomsSmallest unit of an elementThere are 92 elements that occur naturallyDiscovery of the atomJ. J. Thompson Discovered that different metals produce a stream of negatively charged particles when a high voltage is applied across 2 electrodes= electronsThese were the same regardless of the metal.Atoms have no chargePlum pudding model: Negative ions scattered

Rutherfords ModelFired alpha particles at a piece of gold foil. So, he hypothesized that these particles should pass straight through or get struck in the positive sponge.The atomProtons and neutrons are present in the nucleus of an atom. Electrons are in orbits or energy levels around the nucleus.The relative masses and relative charges of the sub atomic particles are:

Relative MassRelative ChargeProton1+1Neutron10Electron5x10-4-1Atomic NumberAtomic Number (Z)=number of protons. It is the fundamental characteristic of and elementMass Number(A)=number of protons + neutronsIsotopes:Are atoms with the same atomic number, different mass number or the same number of protons, but different number of neutrons.Number of Protons=ZNumber of Electrons=Z-qNumber of neutrons=A-ZDaltons Atomic TheoryAll matter is composed of tiny indiviable particles called atomsAtoms cant be created or destroyedAtoms of the same element are alike in every wayAtoms of different elements are differentAtoms can combine together in small number to form molecules.What do atoms look like. Kind of like hard spheresIsotopesIsotopes differ in physical properties that depend on mass such as: density, rate of diffusion, etc.This difference is very significant for the isotopes of hydrogen as deuterium has the twice the mass of the more abundant . As isotopes have the same electron arrangement they have the same chemical properties.Examples of the uses of radioisotopes: C-14 in radiocarbon dating, CO-60 in radiotherapy and I-131 and I-125 as medical tracers.

IsotopesShow the same chemical properties, as a difference in the number of neutrons makes no difference to how they react and so they occupy the same place in the periodic tableChlorine exist as 2 isotopes:35Cl and 37Cl The average relative atomic mass of the isotopes is not 36 but 35.45. 35Cl is the more abundant isotope, in a sample of 100 chlorine atoms, there are 75 atoms of 35Cl and 25 atoms of 37Cl.How would you calculate the relative atomic mass?

TO WORK IT OUTUses of RadioisotopesThe stability of a nucleus depends on the balance between the number of protons and neutrons. When a nucleus contains either too many or too few neutrons, it is:RadioactiveAnd will change to a more stable nucleus by giving out radiationThere are several different forms of radiation based on ionization and penetration abilities:AlphaBetaGammaUsesCarbon-14 datingThe most stable isotope of carbon is 12C:Has 6 protons and 6 neutrons. 14C has8 neutrons , which is too many to be stable. It can reduce the neutron to proton ratio when a neutron changes to a proton and an electron.The protons stays in the nucleus but the electron is ejected from the atom as beta particles.146C 147N + o-1eCarbon 14The relative abundances of carbon-14 present in living plants is constant as the carbon continually replenishes from carbon present in CO2 in the atmosphere.When organisms die no carbon 14 is absorbed and the levels carbon 14 fall due to decay.As this process occurs at a regular rate, it can be used to date carbon containing materials. The rate of decay is measured in half lifeThis is the time taken for half the atoms to decayThe carbon-14 to carbon-12 ratio falls by 50% every 5730 years after the death of an organismThis is what archeologist use to date objects.Colbalt 60RadiotherapyRadiation TherapyIs the treatment of cancer and other diseases with ionizing raditation.Cancerous cells are abnormal cells which divide at rapid rates to produce tumors that invade surrounding tissue.The treatment damages the genetic material inside the cell by knocking off electrons and making it impossible for the cell to growThis therapy damages both cancer and normal cells, the normal cells are able to recover if the treatment is carefully controlled.Radiation TherpyCan treat localizedSolid tumorsCancers ofSkinTongueLarynxBrain BreastUnterine cervixBloodLeukemiaColbalt 60 is commonly used as it emits very penetrating gamma radiation when their protons and neutrons change their positions in the nucleus.Iodine 31Radioisotopes have the same chemical properties as any other atom of the same element, and so they play the same role in the bodyTheir positions, unlike other isotopes can be monitored by detecting radiation levels making them suitableRadioactivityUnstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are called radiation. When unstable nuclei decompose in nature, the process is referred to as natural radioactivity. When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivityType of RadiationAlpha Particles:Emitted by nuclei with too many protons to be stableComposed of 2 protons and 2 neutronsBeta ParticlesEmitted by nuclei with too many neutrons, are electron which have been ejected from the nucleus by neutron decayGamma ParticlesAre form of electromagnetic radiationMass SpectrometryMass spectrometry (MS) is an analytical technique that produces spectra (singular spectrum) of the masses of the atoms or molecules comprising a sample of material. The spectra are used to determine the elemental or isotopic signature of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of molecules, such as peptides and other chemical compounds. Mass spectrometry works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.[1]

Mass SpectrometryVaporization:The sample is turned into a gas using an electrical heater IonizationThe gas particles are bombarded with high-energy electrons which knock electrons which ionize them. Electrons are knocked off the particles leaving positive ions. AccelerationPositive ions are attracted to negatively charged plates. The positive ions are accelerated by an electric field. Deflection:The positive ions paths are altered with a magnetic field at right angles of each other. The amount of deflection is proportional to the charge mass ratio. Ions with smaller mass are deflected more than heavier ones. Lighter ions have less momentum and are deflected more than heavier ions. For a given field, only ions with a particular mass/charge ratio will make it to the detector. Detection The magnetic field strength is slowly increased. This changes the mass charge ration of ions that can reach the detector. A mass spectrum is produced. Mass charge ratio is detected and a signal is sent to a recorderRelative Atomic mass) of an element is the average mass of an atom according to the relative abundances of its isotopes, on a scale where the mass of one atom of is 12 For example for 35 17Cl which has two isotopes (75 %) and 3717Cl(25 %).

ElectromagneticThis type of radiation comes in different forms of different energyAll electromagnetic waves travel at the same speed (c)These waves can be distinguished by their different wavelengths ()\Different colors of visible light have different wavelengthsRed light has a lower wavelength than blueThe number of waves that which pass a particular point in 1 sec is called:Frequency (f)

Parts of a wave

Visible lightForms only a small part of the elctromagnetic spectrumInfrared waves have longer wavelengths than red light and ultraviolet waves have shorter wavelengths than violet.

Electromagnetic spectrum

Visible light spectrum

Line SpectraWhen white light is passed through hydrogen gas, an absorption spectrum is produced. This line spectrum with some colors of the continuous spectrum missingSee diagram on page 51Evidence of Bohr modelHydrogen atoms absorb and emit energy. This picture of the atom was considered with the electrons orbiting the nucleus in a circular energy level. Niels Bohr proposed that an electron moves into orbit or higher energy level further away from the nucleus when an atom absorbed energy.This is called the : Excited stateThis is produced It is unstableElectrons soon fall back to lowest state=Ground StateThe energy the electron gives out as it falls back into lower levels is calledElectromagnetic Radiation

PhotonThis energy is calledPacket of energy=photonPhotons are released for each electron transistion.The energy of the photon of light emitted is equal to the energy change in the atomEelectron=Ephoton

It is also related to the frequency of the radiation by plancks equationEelectron=hfPlancks Constant =h=6.63x10-34JsYou will use this equation to calculate the wavelength to break bonds. Page 7 of chemistry data bookletPlancks constantIn 1900, Max Planck was working on the problem of how the radiation an object emits is related to its temperature. He came up with a formula that agreed very closely with experimental data, but the formula only made sense if he assumed that the energy of a vibrating molecule was quantized--that is, it could only take on certain values. The energy would have to be proportional to the frequency of vibration, and it seemed to come in little "chunks" of the frequency multiplied by a certain constant. This constant came to be known as Planck's constant, or h, and it has the value

Hydrogen spectrumHydrogen atoms gives out energy when an electron falls from a higher to a lower energy level.Hydrogen produces visible light when the electron falls to the second energy level (n=2)The transition from to the first energy level corresponds to a higher energy change and are in the ultraviolet region of the spectrum.Infrared radiation is produced when an electron falls to the third energy levelHydrogen SpectrumLooking at figure 2.13 page 53 HL and 45 SL show how the energy levels inside the atom. The lines converge at higher energy levelsThis is due the energy levels inside the atom are closer together. When an electron is at its highest energy e=, it is no longer in the atom and the atom has been ionized. The energy needed to remove an electron from the ground state is calledIonization energyThe hydrogen spectrum:Series RegionElectron falls toLymanUVn = 1BalmerVisiblen = 2PaschenIRn = 3The ionization energy of hydrogen corresponds to the convergence limit of the Lyman series.

Building Atoms using the Bohr ModelAtoms react based the arrangement of sub atomic particles. We can now explore the structure of the atoms beyond hydrogen.

Energy LevelsEach energy level can hold a limited number of electrons. Ground StateElectrons are placed in the lowest energy level first, and when this becomes complete, electrons move to the second energy level, and so on.Helium has 42H, has 2 protons, 2 neutrons and 2 electrons. The protons and neutrons from the nucleus and the 2 electrons both occupy the lowest energy level.Electron arrangmentElementElectron ArrangementElementElectron arrangement1H2He1211 Na12Mg2,8,12,8,23Li2,113Al2,8,34Be2,214Si2,8,45B2,315P2,8,56C2,416S2,8,67N2,517Cl2,8,78O2,618Ar2,8,89F2,719K2,8,8,110Ne2,820Ca2,8,8,2Ionization of energyThe first ionization energy is the minimum energy required to remove one mole of electrons from a mole of gaseous atoms to form a mole of univalent cations in the gaseous state. It is the enthalpy change for the reaction: X (g) X + (g) + e.When an atom becomes ionized it loses an electron or proton. e-

Electron lost when it becomes positiveIonElectron ArrangementEnergy level from which the next electron is removed when ionizedAl2,8,3ThirdAl+ 2,8,2ThirdAl2+2,8,1ThirdAl3+2,8SecondElectron ConfigurationsBohr model has limitations. It doesnt explain levels after level 3.More energy is needed to remove electrons at higher ionization energy.More difficult to remove, so we have:-why we have sublevelsSee table on page 57 HL.Atomic Orbitalswe know that the first energy level is made up of 1s sub level. Due to Heisenberg Uncertainty principle we dont know the position of the electron..So we just say its in an orbitalAtomic orbital is a region around the atomic nucleus in which there is a 90% probability of finding electron.S orbitalsS orbitals at either level are spherical/circular1s and 2s2s are larger

P orbitalsP sub levels contain 3 p atomic orbitals of equal energy.They are dumbbell shape and are arranged at right angles

D and F orbitalsD orbitals are made up of 5 sublevelsF orbitals are made up of 7 sublevelsSee page 61 HL

Pauli Exclusion PrincipleNo more than 2 electrons can occupy an one orbital, and if two electrons are in the same orbital they must spin in opposite directions

Energy LevelsLevelSublevelMaximum number of electrons ins subshellMaximum number of electrons in leveln=44f

4d

4p

4f14 (7 f orbitals)

10 (5 d orbitals)

6 (3 p orbitals)

14 (7 f orbitals)Formula 2n2

n=4 so4 x 4=1616x 2=32

n=33d

3p

3s10 (5 d orbitals)

6 (3 p orbitals)

2 (1 s orbital)18n=22p

2s6 (3 p orbitals

2 (1 s orbital)8n=11s2 ( 1s orbital)2Energy LevelsAufbau Principle:Orbitals with lower energy are filled before those with higher energyHunds RuleEvery orbital in a sub level is singly occupied with electrons of same spin before any one orbital is doubly occupiedAufbau table

3d and 4s confusionElectrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible.

The diagram (not to scale) summarises the energies of the orbitals up to the 4p level.http://www.chemguide.co.uk/atoms/properties/3d4sproblem.html

Alpha RadiationAlpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus. Some characteristics of alpha radiation are:

Most alpha radiation is not able to penetrate human skin.

Alpha-emitting materials can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.

A variety of instruments has been designed to measure alpha radiation. Special training in the use of these instruments is essential for making accurate measurements.

A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.

Instruments cannot detect alpha radiation through even a thin layer of water, dust, paper, or other material, because alpha radiation is not penetrating.

Alpha radiation travels only a short distance (a few inches) in air, but is not an external hazard.

Alpha radiation is not able to penetrate clothing.

Examples of some alpha emitters: radium, radon, uranium, thorium.

Beta RadiationBeta radiation is a light, short-range particle and is actually an ejected electron. Some characteristics of beta radiation are:

Beta radiation may travel several feet in air and is moderately penetrating.

Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.

Beta-emitting contaminants may be harmful if deposited internally.

Most beta emitters can be detected with a survey instrument and a thin-window GM probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy, poorly penetrating radiation that may be difficult or impossible to detect. Examples of these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-35.

Clothing provides some protection against beta radiation.

Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.

Gamma Radiation.Gamma and X Radiation

Gamma radiation and x rays are highly penetrating electromagnetic radiation. Some characteristics of these radiations are:

Gamma radiation or x rays are able to travel many feet in air and many inches in human tissue. They readily penetrate most materials and are sometimes called "penetrating" radiation.

X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive sources and machines that emit gamma radiation and x rays respectively constitute mainly an external hazard to humans.

Gamma radiation and x rays are electromagnetic radiation like visible light, radiowaves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and x rays are the most energetic of these.

Dense materials are needed for shielding from gamma radiation. Clothing provides little shielding from penetrating radiation, but will prevent contamination of the skin by gamma-emitting radioactive materials.

Gamma radiation is easily detected by survey meters with a sodium iodide detector probe.

Gamma radiation and/or characteristic x rays frequently accompany the emission of alpha and beta radiation during radioactive decay.

Examples of some gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, and technetium-99m.

Hydrogen Spectrum