Modern Atomic Theory - University of Idaho white Malone... · 2013. 6. 24. · Modern Atomic Theory . The beautiful color of fireworks comes from the particular elements used The

Modern Atomic Theory Modern Atomic Theory

The beautiful color of fireworks comes from the The beautiful color of fireworks comes from the particular elements usedparticular elements usedThe colors are characteristic of the elements and The colors are characteristic of the elements and can be used to understand their propertiescan be used to understand their properties

When atoms are excited (like in a flame) they emit When atoms are excited (like in a flame) they emit discrete wavelengths of light, not a continuumdiscrete wavelengths of light, not a continuumShown below is the visible light range at the top, the Shown below is the visible light range at the top, the emission spectrum of Na in the middle and the emission emission spectrum of Na in the middle and the emission spectrum of H at the bottomspectrum of H at the bottom

The atoms in the gas discharge tube are excited The atoms in the gas discharge tube are excited by electricityby electricityThe light produced is separated by the prismThe light produced is separated by the prism

Electromagnetic Electromagnetic energyenergyTravels at 3.0 Travels at 3.0 ×× 10101010

cm/scm/sWhite light is a White light is a continuous spectrum continuous spectrum of light (all energies of light (all energies represented) and can represented) and can be separatedbe separated

Light has intensity, which is termed amplitudeLight has intensity, which is termed amplitudeLight is characterized by its wavelength, which is given Light is characterized by its wavelength, which is given the symbol the symbol λλ (lambda)(lambda)Wavelength is the distance between two adjacent peaksWavelength is the distance between two adjacent peaks

Wavelength and energy have an inverse Wavelength and energy have an inverse relationship, as shown belowrelationship, as shown belowhh is Planck’s constant (6.626 is Planck’s constant (6.626 ×× 1010--3434 JJ··s)s)cc is the speed of lightis the speed of light

λλ=∝

λν λ ν ν

Infrared light is lower energy than visible lightInfrared light is lower energy than visible lightUltraviolet light is higher energy than visible light Ultraviolet light is higher energy than visible light (some of the ultraviolet range is of high enough (some of the ultraviolet range is of high enough energy to damage living tissue)energy to damage living tissue)XX--rays and gamma rays are even higher energy rays and gamma rays are even higher energy forms of light (there are especial hazards forms of light (there are especial hazards associated with this type of radiation)associated with this type of radiation)

Visible light comprises only a small fraction of the Visible light comprises only a small fraction of the electromagnetic spectrum (400electromagnetic spectrum (400--800 nm)800 nm)Ultraviolet and infrared light cannot be seen with the Ultraviolet and infrared light cannot be seen with the unaided eye, but are quite importantunaided eye, but are quite important

The HThe H22 in the gas discharge tube is excited by in the gas discharge tube is excited by electricity (discharge produces excited H atoms)electricity (discharge produces excited H atoms)The light produced is separated by the prismThe light produced is separated by the prism

A model is a description or analogy used A model is a description or analogy used to help visualize a phenomenon or entityto help visualize a phenomenon or entity

Bohr proposed a model that violated Bohr proposed a model that violated classical physics, but started a new way of classical physics, but started a new way of looking at atomic and molecular structurelooking at atomic and molecular structure

The central concept of this model is that The central concept of this model is that the properties of atoms and electrons do the properties of atoms and electrons do not behave in a continuous fashionnot behave in a continuous fashion

Electrons revolve around Electrons revolve around the nucleus in stable the nucleus in stable orbitsorbits

Each orbit is quantizedEach orbit is quantized�� the electron is a discrete the electron is a discrete

distance from the nucleusdistance from the nucleus�� the orbit has a discrete the orbit has a discrete

energyenergy

These orbits are referred These orbits are referred to as energy levelsto as energy levels

The energy levels are characterized by an The energy levels are characterized by an integral number called the principle quantum integral number called the principle quantum number, nnumber, nn has discrete values of 1, 2, 3,…n has discrete values of 1, 2, 3,…

Only one idea remains from the Bohr modelOnly one idea remains from the Bohr model

Quantized energy levels are like the steps of a Quantized energy levels are like the steps of a staircasestaircaseAn electron can have one energy or another, but An electron can have one energy or another, but not an energy in between (can you stand not an energy in between (can you stand between two steps?)between two steps?)

Ground stateGround state�� the lowest energy statethe lowest energy stateExcited stateExcited state�� all states that have higher energiesall states that have higher energies�� atoms and molecules have many excited atoms and molecules have many excited

states but only one ground statestates but only one ground state

Modern theory is a complex mathematical Modern theory is a complex mathematical descriptiondescriptionTakes into account that atomic sized things Takes into account that atomic sized things behave more like waves than particlesbehave more like waves than particles

We treat electrons as waves, since we We treat electrons as waves, since we cannot precisely determine the location of cannot precisely determine the location of the electronsthe electrons

Electrons are said to be described by Electrons are said to be described by orbitals, which are orbitals, which are regions in space regions in space where there is a significant probability where there is a significant probability of finding the electronof finding the electron

Wave mechanics reveals that the energy Wave mechanics reveals that the energy levels (sometimes called shells) have levels (sometimes called shells) have sublevelssublevelsThe sublevels are designated by lettersThe sublevels are designated by letters

ss, , pp, , dd, , ffThe sublevels increase in energy as The sublevels increase in energy as ss<<pp<<dd<<ffEach sublevel has one or more orbitalsEach sublevel has one or more orbitals

The orbital is best thought of as an electron cloudThe orbital is best thought of as an electron cloudThe picture below is the density diagram for an The picture below is the density diagram for an ssorbitalorbitalIt indicates the probability of an electron being It indicates the probability of an electron being located in a particular location near the nucleuslocated in a particular location near the nucleus

We often represent orbitals as spherical volume We often represent orbitals as spherical volume of probability (volume in which the electron is of probability (volume in which the electron is found 90% of the time)found 90% of the time)The orbitals of the The orbitals of the ss sublevel are shown belowsublevel are shown below

pp

There are three orbitals in the There are three orbitals in the pp sublevelsublevelOne is aligned along each of the Cartesian One is aligned along each of the Cartesian coordinates around the nucleuscoordinates around the nucleusOrientation is indicated by a right subscriptOrientation is indicated by a right subscript

dd

There are five orbitals in the dThere are five orbitals in the d--sublevelsublevelThey are important to understanding the They are important to understanding the behavior of the transition metalsbehavior of the transition metals

The shell is indicated by the principle quantum The shell is indicated by the principle quantum number number n (n (Bohr orbit ideaBohr orbit idea))

1 2 3

The shell is indicated by the principle The shell is indicated by the principle quantum number quantum number n (n (Bohr orbit ideaBohr orbit idea))The subshell is indicated by the letter (The subshell is indicated by the letter (s, p, s, p, d, f)d, f)

1 2 3

s s and ps, pand d

The shell is indicated by the principle The shell is indicated by the principle quantum number quantum number n (n (Bohr orbit ideaBohr orbit idea))The subshell is indicated by the letter (The subshell is indicated by the letter (s, p, s, p, d, f)d, f)The number of electrons in the subshell is The number of electrons in the subshell is indicated by a indicated by a right superscriptright superscript

For example, 4For example, 4pp33 (4th shell, p subshell, (4th shell, p subshell, three electrons)three electrons)

The electrons in the atom are indicated by an The electrons in the atom are indicated by an electron configurationelectron configurationWe use only as many subshells and shells as are We use only as many subshells and shells as are needed for the number of electronsneeded for the number of electronsThe number of available subshells depends on the The number of available subshells depends on the shell that is being filledshell that is being filled�� nn = 1 only has an = 1 only has an ss subshellsubshell ((only 1 type of s orbitalonly 1 type of s orbital))�� nn = 2 has = 2 has ss and and pp subshellssubshells ((3 p 3 p orbitalsorbitals, , ppxx, , ppyy,,ppzz))�� nn = 3 has = 3 has ss, , pp and and dd subshellssubshells(5 d orbitals,d(5 d orbitals,d11, d, d22, d, d33, d, d44, d, d55))

aufbauaufbau

The electrons are added to the atom The electrons are added to the atom beginning with the lowest energy levelbeginning with the lowest energy levelAs the sublevels and levels are filled, As the sublevels and levels are filled, electrons are added to the next higher electrons are added to the next higher energy levelenergy levelEach orbital can hold two electrons, so Each orbital can hold two electrons, so each sublevel can hold twice as many each sublevel can hold twice as many electrons as there are orbitalselectrons as there are orbitals

aufbau principleaufbau principle

electrons occupy the available orbitals in electrons occupy the available orbitals in the subshells of lowest energythe subshells of lowest energyThe electronic configurationThe electronic configuration of an element of an element is the assignment of all the electrons of the is the assignment of all the electrons of the atom into shells and subshellsatom into shells and subshells

The levels increase in The levels increase in energyenergyWithin the levels, the Within the levels, the sublevels also sublevels also increase (recall increase (recall ss<<pp<<dd<<ff))Recall that the Recall that the electrons go into the electrons go into the lowest orbital first (1s, lowest orbital first (1s, then 2s, etc.)then 2s, etc.)

Consider SConsider SSulfur has 16 electronsSulfur has 16 electronsElectronic configuration is thereforeElectronic configuration is therefore11ss2222ss2222pp6633ss2233pp44

dd and and ff subshells are used for heavier subshells are used for heavier elementselements

Chemically, we find that the electrons in the shell Chemically, we find that the electrons in the shell with the highest value of with the highest value of nn are the ones involved are the ones involved in chemical reactionsin chemical reactionsThis shell is termed the valence shellThis shell is termed the valence shellElectrons in shells with lower Electrons in shells with lower nn values are values are chemically unreactive because they are of such chemically unreactive because they are of such low energy. low energy. These shells are grouped together as the coreThese shells are grouped together as the core

We develop a shorthand for the electron We develop a shorthand for the electron configuration by noting that the core is really the configuration by noting that the core is really the same as the electron configuration for the noble same as the electron configuration for the noble gas that occurs earlier in the periodic tablegas that occurs earlier in the periodic table

E.g. for S (1E.g. for S (1ss2222ss2222pp6633ss2233pp44), the core is ), the core is 11ss2222ss2222pp66 which is the same as the electron which is the same as the electron configuration for configuration for NeNe

Abbreviated form is [Ne]3Abbreviated form is [Ne]3ss2233pp44

We note that the valence shell electron We note that the valence shell electron configuration has the same pattern for configuration has the same pattern for elements in the same groupelements in the same groupAll elements in the same column as S All elements in the same column as S have the valence electron configurationhave the valence electron configuration[core][core]nsns22npnp44

This is the secret to the structure of the This is the secret to the structure of the periodic tableperiodic table

Representative elements have the general Representative elements have the general electron configuration [NG]electron configuration [NG]nsnsxxnpnpyy (NG (NG represents noble gas)represents noble gas)

Each representative element has one Each representative element has one more electron in the valence shell than the more electron in the valence shell than the previous elementprevious element

[NG][NG]nsns11 -- alkali metalsalkali metals[NG][NG]nsns22 -- alkaline earthsalkaline earths......[NG][NG]nsns22npnp55 -- halogenshalogens[NG][NG]nsns22npnp66 -- noble gases noble gases

[NG][NG]nsns22(n(n--1)d 1)d yy -- Transition elementsTransition elements

[NG][NG]nsns22(n(n--1)d1)d11(n(n--2)f 2)f zz -- Inner transition Inner transition elementselements

Electrons act as if they are little magnets due to Electrons act as if they are little magnets due to a property called a property called spinspin, and they can have a , and they can have a spin spin up (up (↑↑ )) or or spin down (spin down (↓↓ ))

The The PauliPauli Exclusion PrincipleExclusion Principle -- two electrons two electrons can occupy an orbital if they have different spinscan occupy an orbital if they have different spins..

Practically, if two electrons are in the same Practically, if two electrons are in the same orbital, they have opposite spinsorbital, they have opposite spins

Hund’sHund’s RuleRule -- when filling a subshell, electrons when filling a subshell, electrons will avoid entering an orbital that already has an will avoid entering an orbital that already has an electron in it until there is no other alternativeelectron in it until there is no other alternative

Represent the orbitals of the subshells as Represent the orbitals of the subshells as boxes and electrons as arrowsboxes and electrons as arrows

We can think of the We can think of the levels as dorm roomslevels as dorm roomsA student will choose A student will choose the dorm that is the dorm that is closest to the campus closest to the campus and on the lowest and on the lowest floorfloor

The dorms correspond to the shellsThe dorms correspond to the shellsThe floors correspond to the subshellsThe floors correspond to the subshells

Atomic radius is the distance from the nucleus to Atomic radius is the distance from the nucleus to the outermost electronsthe outermost electrons�� Atomic radii decrease across a periodAtomic radii decrease across a period�� Atomic radii increase down a groupAtomic radii increase down a group

The decrease from left to right in a period results The decrease from left to right in a period results from adding electrons to the same outer shell from adding electrons to the same outer shell while protons are being added to the nucleuswhile protons are being added to the nucleus

All of the outer electrons are drawn closer to the All of the outer electrons are drawn closer to the nucleusnucleus

The energy required to remove an electron from The energy required to remove an electron from a gaseous atom to form a gaseous iona gaseous atom to form a gaseous ion1st ionization energy (IE1st ionization energy (IE11))

MM(g)(g) →→ MM++(g)(g) + e+ e--

Ionization energies increase across a periodIonization energies increase across a period((Increased nuclear charge and size)Increased nuclear charge and size)

Ionization energies decrease down a groupIonization energies decrease down a group(Increased distance from nucleus)(Increased distance from nucleus)

Smaller atoms have higher ionization energies Smaller atoms have higher ionization energies (small atoms hold their electrons more tightly)(small atoms hold their electrons more tightly)

Metals are characterized by the relative Metals are characterized by the relative ease that electrons are removed from ease that electrons are removed from themthemAtoms can lose more than one electronsAtoms can lose more than one electrons

MM++(g)(g) →→ MM2+2+

(g)(g) + e+ e-- IEIE22

Note that IENote that IE22 for Na, IEfor Na, IE33 for Mg, IEfor Mg, IE44 for Al for Al are all very large compared to the are all very large compared to the preceding (IE; see Table 5preceding (IE; see Table 5--3)3)

When atoms lose electrons, the remaining electrons are When atoms lose electrons, the remaining electrons are more strongly attracted to the nucleus, hence cations are more strongly attracted to the nucleus, hence cations are smaller than the atomsmaller than the atomAnions hold the electrons less tightly, so they are largerAnions hold the electrons less tightly, so they are larger

To remove an electron from an inner shell To remove an electron from an inner shell is energetically very costlyis energetically very costlyRemoving electrons from the valence shell Removing electrons from the valence shell requires quite a bit less energyrequires quite a bit less energyThis will have consequences for the This will have consequences for the formation of compoundsformation of compounds