chap02 atomic bonding

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Chapter 2Atomic Structure and Atomic Bonding

We will first look at structure on the atomic level. We begin this by looking at the structureof the atom and then at atomic bonding.

Atomic bonding describes the interactions between the atoms in a material, and morespecifically, the interactions between their electrons. The type of atomic bonding in amaterial will significantly affect the properties of a material.

For example, graphite and diamond, both made of Carbon, have very different propertiesattributable to different types of atomic bonding. iamond is one of the hardest substancesin the world, and graphite is used as a dry lubricant because of its ability to have ad!acentlayers slide over each other.

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Chemistry Review Basic Vocabulary We begin with the basic vocabulary of the individual atom. element " Fundamental chemical species, represented in the periodic table. All materialsare made of elements or combinations of these elements. atom " The smallest building block of an element, consisting of a central nucleus of protonsand neutrons with electrons orbiting the nucleus.

nucleus " The central portion of the atom containing the protons and neutrons. protons " #ositively charged particles of $.%& x %$ '%( C and a mass of %.&& x %$ ')* g.

neutrons " +eutral particles having approximately the same mass as protons.

electrons " +egatively charged particles with a mass of $. %% x %$ ')- g. The charge is thesame magnitude as that on the proton. The electrons reside in orbits around the nucleus.

Note: The mass of the electron is on the order of 1000 times smaller than protons and neutrons. Hence the mass of the atom resides primarily in thenucleus. However, the volume is determined by the electron orbitals.

If the nucleus were the size of a pin head, the closest electron would beappro imately a football field!s len"th away. The vast ma#ority of the atomis space$ In this picture, if you were a pinhead standin" at the "oalpost, theelectron would be at the far end of the field. %hat is in&between' Thin(about this$ %hat does it say about the nature of the material world'

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atomic number, Z " The number of protons in an atom. For an electrically neutral andcomplete atom this is e ual to the number of electrons. The atomic number is the basis ofchemical identification for an element. atomic mass, A " The number of protons plus the number of neutrons in an atom.

isotopes " Atoms of the same atomic number that have different numbers of neutrons. atomic weight " This is not a weight/ 0t is the wei"hted average of the atomic masses forall isotopes of an element. This is why the atomic weight of carbon is %).$%% and not%).$$. 12ost carbon is C %) , however about %.%3 of naturally occurring carbon is C %45 amu " atomic mass unit approximately e ual to the mass of a proton or a neutron. 6trictlyspeaking, it is defined to be %7%) of the atomic mass of C %) ' the isotope of carbon with &protons and & neutrons. Avogadro s number , + A" &.$) x %$ )4 " This is the number of protons in % gram. 8ence %g 9

&.$) x %$)4

amu. gram!atom " &.$) x %$ )4 atoms of a element. This is the same as the element:s atomicweight expressed in grams. mole " &.$) x %$ )4 molecules of a compound. This is the same as the compound:s molecularweight expressed in grams.

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"uantum #echanics

"uantum #echanics is a set of principles and laws that govern all physical systems;however, it is most relevant for systems of atomic and subatomic entities. 0t was developedbecause classical mechanics failed to explain certain phenomenon.

Classical mechanics focused solely on the particle'like behavior of electrons whereasuantum mechanics addresses the fact that matter has wave'like properties in addition to

particle'like properties. Furthermore, waves have particle'like properties in addition totheir wave'like properties. This is called the wave!particle duality of matter andradiation.

Another difference between classical mechanics and uantum mechanics is that uantummechanics predicts probabilities in situations where classical mechanics predictscertainties.

$he Bohr #odel o% an atom 1or sometimes called the planetary model of the atom5 is anearly outgrowth of uantum mechanics. This model represents electrons as particlesrevolving around the nucleus much like planets around a sun.

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http://en.wikipedia.org/wiki/Wave-particle_dualityhttp://en.wikipedia.org/wiki/Wave-particle_duality


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The electrons can only take on certain discrete values of energy. These energy levels correspond to the fixed binding energy between the electron and its nucleus. Any valueother than these permitted values is forbidden.

As brilliant as this is called anorbital , sometimes referred to as an energy state . There are different types of orbitals,each with a distinctive shape.

The electron cloud model is a newer, better version of the


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Subshells and orbitalsThe first subshell of any shell is called the s subshell which can only hold one orbital.6o a fully'filled s subshell will contain only two electrons.The shape of this orbital is spherical.

The next subshell is called the p subshell which can hold three orbitals.A fully'filled p subshell will contain six electrons.The shapes of these three p orbitals are like dumbbells, arranged at right angles to eachother. @r a teardrop petal shape with the point towards the nucleus.

The next subshell is called the d subshell which can hold five orbitals.A fully'filled d subshell will contain ten electrons.The shape of the five d orbitals are similar to the p orbital shape, but with more petalslike a clover leaf. They can also have ring shapes around the base of the petals.

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The next subshell is called the f subshell which can hold seven orbitals.A fully'filled f subshell will contain fourteen electrons.These are the shapes of the seven f orbitals. They tend to look similar to d orbitals, butwith even more petals .

Further subshells are designated alphabeticallyB g, h, i, etc.

&soteric note'An atomic orbital is actually a mathematical function that describes the wave'like behaviorof an electron in an atom. This function can be used to calculate the probability of findingany electron of an atom in any specific region around the atom s nucleus. The term orbitalhas become known as either the mathematical function or the region generated with thefunction. 6pecifically, atomic orbitals are the possible uantum states of an individualelectron in the electron cloud around a single atom, as described by the function.

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http://en.wikipedia.org/wiki/Mathematical_functionhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Quantum_statehttp://en.wikipedia.org/wiki/Electron_cloudhttp://en.wikipedia.org/wiki/Mathematical_functionhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Quantum_statehttp://en.wikipedia.org/wiki/Electron_cloud


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"uantum (umbers

The electrons in the orbitals surrounding an atom can be characteriDed by a uni ue set offour parameters called )uantum numbers* The four uantum numbers, n, l, m and s canbe used to describe any electron in a stable atom. These numbers determine the shell1energy level5, the subshell and the orbital of the electron along with something called thespin which indicates whether the electron is spinning clockwise or counter'clockwise.

+rinciple "uantum (umber, nThe first of these uantum numbers is the principle )uantum number, n, which designatesthe main shell or main energy level . The values of n are the integers from % to -.They value of n can also be designated by lettersB

n9% corresponds to n9E n9) corresponds to n9

n94 corresponds to n92 n9* corresponds to n9+ n9G corresponds to n9@ n9& corresponds to n9# n9- corresponds to n9H

8igher values of n mean more energy for the electron and the corresponding radius of theelectron cloud or orbital is further away from the nucleus.

This is the only uantum number that the


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$wo rules about how electrons %ill orbitals +auli &-clusion +rinciple " ?ach electron state 1orbital5 can hold no more than twoelectrons and they must have different spins. 0n other words, no two electrons in an atomcan have the same four uantum numbers.

.und s Rule " ?lectrons will avoid sharing the same state. i.e. every orbital in a subshell issingly occupied with one electron before any one orbital is doubly occupied, and allelectrons in singly occupied orbitals have the same spin. /n summary'

?lectrons in an atom reside in discrete energy levels represented by a set of *uantum numbers 1n, l, m, s5.

The


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Allowable Values o% "uantum (umbers

mainuantum

numbern

% ) 4 *

6ubsidiaryuantum

numberl

s s p s p d s p d f

magneticuantum

numberm l

$ $ '% $ I% $ '% $ I% ') '% $ I% I) $ '% $ I% ') '% $ I% I) '4 ') '% $ I% I) I4

electronspinnumberm s

?ach magnetic uantum number has ) spin numbersB I%7) and '%7) 1sometimes !ust designated as up and down5

Allowable Values o% "uantum (umbers 0cont*1

mainuantum

numbern

G

6ubsidiaryuantum

numberl

s p d f g

magneticuantum

numberm l

$ '% $ I% ') '% $ I% I) '4 ') '% $ I% I) I4 '* '4 ') '% $ I% I) I4 I*

electronspinnumberm s

?ach magnetic uantum number has ) spin numbersB I%7) and '%7) 1sometimes !ust designated as up and down5

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&lectron Con%igurationThe configuration of electrons in an atom is the manner in which electron states 1orbitals5are occupied. 0n general electrons will fill up the lower energy orbitals first.

The energy of the orbitals increase with both the principle uantum number, n, and thesubsidiary uantum number, l, however, there is some overlap as you can see in thisdiagram.

energy is increasing in this direction

8ere is another diagram showing the order of increasing energy for orbitals.8owever, it also gives you an easy way to determine the order.

An electron con%iguration is a code that describes how many electrons are in each energylevel of an atom and how the electrons are arranged in the orbitals within each energylevel.

For example the electron configuration of +a which has %% electrons is%s) )s ) )p &4s%

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-p&d

Gf -s

&pGd

*f &s

Gp*d

Gs*p

4d*s

4p4s

)p)s

%s


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Chemistry Review #ore Vocabulary Concepts photon "a discrete bundle 1or )uantum 5 of electromagnetic energy.

@ther facts about photonsB they can be destroyed7created when radiation is absorbed7emitted they are elementary particle s with Dero mass and Dero rest energy they are electrically neutral they are always in motion with constant velocity of light in free space they can have particle'like interactions 1i.e. collisions5 with electrons and other

particles. fun factB they are one of the rare particles that are identical to their antiparticle,

the antiphoton.

?lectrons are able to move from one energy level to another by emission or absorption of auantum 1discrete amount5 of energy, in the form of a photon . The amount of energy can

be calculated by #lanck:s e uationB 9 h 9 h c7

whereB h 9 #lanck:s Constant, &.&4x%$ '4* Ks = fre uency of photon c 9 speed of light, ). x %$ ( m7s 9 wavelength of photon

This e uation gives the amount of energy 1in the form of electromagnetic radiation5needed for an electron to transition from one energy level to another. 0f the electrontransitions to a higher energy level, i.e. farther from the nucleus, energy is absorbed, if

the electron transitions to a lower energy level, energy is released.

ioni ation energy " The energy necessary to remove an electron from its orbital to aninfinite distance from the nucleus of the atom. ground state ' The state in which all the electrons of an atom are in the lowest energylevels. valence electrons " ?lectrons in the outermost orbitals of atoms that take part in bonding.

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http://physics.about.com/od/atomsparticles/a/particles.htmhttp://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Quantumhttp://en.wikipedia.org/wiki/Photonhttp://physics.about.com/od/atomsparticles/a/particles.htmhttp://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Quantumhttp://en.wikipedia.org/wiki/Photon


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hybrid sp n orbitals " 6ometimes the s and p orbitals combine. n 9 number of p orbitalsinvolved in the combination. The driving force behind this is a lower energy state forvalence electrons.

For example, we would expect C %) to have the electron configurationB %s ) )s ) )p ) .


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$he +eriodic $ableThe periodic table is a brilliant arrangement of the elements according to their electronconfigurations.

periods " The rows of the periodic table. The first element in a period of the #eriodic Tableintroduces a new principle 1main5 energy level. groups " The columns of the periodic table. These elements have similar electronconfigurations, and hence, have similar properties. stable electron con%iguration " one in which the outermost orbitals of an atom arecompletely filled.

inert gasses " Lroup $ elements. These elements have very high chemical stability due tothe fact that their outermost orbitals are completely filled, i.e. they have stable electronconfigurations.

.alogens " Lroup M00 elements. These elements have one less electron than needed for astable electron configuration.

Al3ali metals " Lroup 0A elements. These elements have one more electron than needed fora stable electron configuration.

Al3ali earth metals " Lroup 00A elements. These elements have two more electrons thanneeded for a stable electron configuration.

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http://chemistry.about.com/od/chemistryglossary/a/perioddefinitio.htmhttp://chemistry.about.com/library/blper5.htmhttp://chemistry.about.com/od/chemistryglossary/a/perioddefinitio.htmhttp://chemistry.about.com/library/blper5.htm


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$ransition #etals " ?lements in the period between 000< and 00


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The potential energy between the two atoms is given byB ? 9 F da or F 9 d?7da

When F + 9 $, the configuration is stable and the atoms are at their e)uilibrium separationa o. At this point ? is at a minimum value, &o, called the bond energy for the two atomsinvolved in the bond.

When there are more than two atoms in a bond, the picture is more complex, however,

there is still a bond energy. This energy will depend on the type of atomic bonding.#any material properties depend on the bond energy curve'

2aterials with large &o will typically have high melting temperatures.

2aterials with steeper slopes around a o will be =stiff>. 6hallower slopes meanmore flexible materials.

2aterials with deep and narrow troughs will have low coefficient of thermalexpansion and relatively small dimensional alterations for changes intemperature.

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+rimary Atomic Bonds

There are three types of primary atomic bonds.%. 0onic). Covalent4. 2etallic

These bonds involve the transfer or the sharing of valence electrons. The bonding type willbe determined by the electron configuration of these valence electrons.

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/onic Bonding0onic bonding is between a metal and a nonmetal. Alternatively, one can say it is a bondbetween electropositive and an electronegative elements.

?lectrons are transferred to produce a more stable electron configuration in each of thebonding atoms. The result is cations and anions. The force of attraction is Columbic in nature for ionic bonding.

The bond length is a o 9 r ion I r cation where r ion and r cation are the radii corresponding to theaverage electron density in the outermost electron orbital. The bond is non'directional, i.e. the magnitude of the bond force is the same in alldirections around an ion.

The fact that this type of bonding is a non'directional bond means that it tends tomaximiDe packing efficiency. This leads to large coordination numbers.1The coordination number , C+, of an atom is the number of nearest neighbors at e ualdistances.5 ? o is relatively large giving high melting temperatures to these materials.

2aterials with this type of bonding tend to be hard and brittle. Also electrically andthermally insulative.

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+ote that the ionic radii differ from the atomic radii more than one would expectB

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Coordination (umbers predicted %rom Radius RatiosFor non'directional bonds, C+s are predicted by the radius ratio which is the ratio of thesmaller ion to the larger ion in the bond, r7N.

6ince ionic bonding tends to maximiDe packing efficiency, the C+ is limited only by therelative siDe of the ions and by the need to maintain charge neutrality for the solid.

Considering only the radius ratio 1and ignoring the need to maintain charge neutrality fornow5 we can predict the C+ for atoms in ionic bonds.

We need to consider many larger ions will =fit> around the smaller one. 16ince this is amore restrictive case.5

Consider a C+ of %. What is the minimum r7NPAnswerB $/ 1The smaller ion can have essentially a Dero radius.5

For a C+ of ), what is the minimum r7NPAnswerB $/ 1The smaller ion can have essentially a Dero radius.5

For a C+ of 4, what is the minimum r7NP

0t is restricted by the geometry. With some geometry and trig calculations you should beable to conclude that r7N Q $.%GG giving a minimum radius ratio of $.%GG for a C+ of 4.

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$able o% C( and corresponding range %or r5R 8ere is a table giving the Coordination number corresponding to different ranges of radiusratios. Note that the lower bound of the range is the radius ratio for the CN !he u""er boundis #ust the lower bound on the ne$t CN

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Covalent BondingCovalent bonding is between elements whose electronegativities are not too different. ?lectrons are shared so that each atom attains a stable electron configuration.

A covalent bond is an instance of a pair of electrons being shared by two atoms. 1Forexample in chlorine gas.5

Atoms can share more than one pair of electrons. 0f they share two, we have a doublecovalent bond. 1For example in the ethylene molecule.5 Three makes for a triple covalentbond. Carbon has four covalent bonds between each atom.

The number of covalent bonds possible is ( " +: where +: is the number of valenceelectrons. This is a highly directional bond, i.e. it exists between specific atoms and may exist only inthe direction between one atom and another that participates in the electron sharing.

The directionality means that this bonding does not maximiDe packing efficiency and so C+tend to be low. For example iamond has C+ 9 *, not %) as would be predicted by r7N 9 %.This is due to the highly direction nature of the bond angle. There are * e ually spacedsp4 orbitals around the carbon atom make for a bond angle of %$ o. &o can be strong or weak.

2ost covalent bonding has some ionic character and visa'versa. The greater the differencebetween the electronegativities, the more ionic the bond will be. The percentage of ioniccharacter is given by this e uationB

3 ionic character 9 1% ' e "$.)G1xA ' x


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#etallic BondingMalence electrons are not bonded to any particular atom but belong to the whole solid.These electrons are termed =delocaliDed> or =free>. They are also referred to as a =sea>or =cloud>. They act as glue for the positive ion cores.

The metallic bond is non'directional. Therefore it tends to maximiDe packing efficiency.C+s are determined primarily by efficient packing considerations. This is why most metalshave C+ 9 %) 1as predicted by r7N 9%5. &o can be strong or weak. The free electrons are the basis for high electrical and thermal conduction values. 0n general, the fewer the valence electrons per atom involved in the bond, the lower thebond energy and melting temperature. 2etals are ductile because they can slide without completely disrupting the structure.

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Secondary Bonding6econdary bonding is also called van der 6aals bonds* The primary bonding types areconsidered chemical bonds . 6econdary bonding is considered to be or physical bonds .

6econdary bonding arises from molecular 1electric5 dipoles and their columbic attractionfor each other.

9 d 1 % d &units are deb'e = 3 34 $ 10 (3 Coulo)b()eters* This is much weaker than primary atomic bonding although the bonding force and bondingenergy curves have the same basic shapes.

6econdary bonds may be betweenB%. induced dipoles and indu+ed di"oles &also +alled temporary or fluctuating

dipoles)). induced dipoles and permanent dipoles 0polar molecules14. permanent dipoles and permanent dipoles

$he attraction o% inert gasses is an e-ample o% number 7*

0n fact it is these temporary dipole moments that cause inert gases to deviate from beingtrue ideal gasses. 1An ideal gas has absolutely no inter'atomic attractive forces.5

.ydrogen bonding is a special case of number 4.

When 8ydrogen is covalently bonded, there is a =naked> proton that strongly attracts thenegative end of an ad!acent molecule. This is a relatively strong force.


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8ydrogen bonding, 8F and water are li uids at ambient temperatures instead of gasses.ife as we know it would not be possible without this 8ydrogen bonding. 16ometimes

called a .ydrogen Bridge .5

A molecule is considered to be a group of atoms bonded together by strong primary bonds.6o ionic and metallic solids are sometimes considered to be one large macromolecule.

8owever, covalent substances are not because the large molecules have secondary bondingbetween them.

#i-ed Bonding ,s so often in life- things are not alwa's "ure .an' ato)i+ bonds ha/e )i$ed +hara+ter

/onic!Covalent #i-ed Bonding6ometimes covalent bonding has ionic character or visa'versa. The amount of ioniccharacter in a bond can be calculated byB 3 ionic character 9 1% ' e "$.)G1xA ' x

chap02 atomic bonding

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