, .

Anderson Junior CollegeGhemistry Lectures 2004

ATOMIC STRUCTURE

ObjectivesCandidates should be able to:(a) identify and describe protons, neutrons and electrons in terms of their relative

charges and relative masses.(b) deduce the behaviour of beams of protons, neutrons and electrons in both

electric and magnetic fields.(c) describe the distribution of mass and charges within an atom.(d) deduce the numberof protons, neutrons and electrons present in both atoms

and ions given proton and nucleon numbers (and charge).(e) (i) describe the contribution of protons and rieutrons to atomic nuclei in terms

of proton number and nucleon number.(ii) distinguish between isotopes on basis of different numbers of neutrons

present.(f) describe the number and relative energies of the s, p and d orbitals for the

principal quantum numbers 1 ,2 and 3 and also the 4s and 4p orbitals.(S) describe the shapes of s and p orbitals.(h) state the electronic configuration of atoms and ions given the proton number

(and charge).(i) (i) explain the factors influencing the ionisation energies of elements (see

Data Booklet);(ii) explain the trends in ionisation energies across a period and down a

group of the Periodic Table.(j) deduce the electronic configurations of elements from successive ionisation

energy data.(k) interpret successive ionisation energy data of an element in terms of the position

of that element within the Periodic Table.

References1 Chemistry, The Central Science (8h edition). Brown, LeMay, Burster. (Chapter

2,6 )2 Chemistry (2no edition). Chris Conoley and Phil Hills. (Chapter 3)3 Chemistry. Ann & Patrick Fullick. (Chapter 1.4)2 Chemistry In Context (5h edition). Hitl and Holman (Chapters 5, 6)4 Longman A-Level Course in Chemistry. JGR Briggs. (Chapter 2)5 A-Level Chemistry (4' edition). E.N. Ramsden. (Chapter 2)6 Chemistry (3'edition). Olmsted & Williams. (Chapters 6, 7)

chai/atomicstructure/2004

Atoms. smallest electrically neutral components of an element

. possess all the chemical properties of the element

o an atom composes of 3 fundamental subatomic particles:

o the quantity 1 .602x1O-ts C is called the etectronic charge

. atOmS haVe no net electrical charge

atoms are extremely small [1x10-10m (1A) < diameter < 5x'10-10m (5A) ]atoms have extremely small masses

a unit called atomic mass unit ( amu ) is used to express such small masses ingram

1 amu = 1.66054x10-2ag

protons and neutrons are collectively called --g!gg

all the mass of the atom is concentrated in the n^tl?.rr

electrons reside in the rest ofthe space a'ov"a the nucleus

electrons are attracted to the protons in the nucleus by ete*.alot. kuo s

the protons and neutrons in nucleus are drawn very close together by extreme forces

these forces are only effective over a very short range because they do not pull theother electrons into the nucleus

+1 .602 x 10-

-1.602 x 10-

5.486x 10

chai/atomicstructure/2004

o many more subatomic particles are now known to exist (eg. the positrons' theneutrinos, the hyperons, etc.), but these are not generally considered in chemistry

Behaviour of protons. neutrons and electrons in:

(a) an electric field

beam of protons,neutrons. electrons

(b) a maqnetic field

beam of protons,neutrons and electrons

p\ DnHt It\o' ftd tdaPaT

Atomic Numbers. Mass Numbers and lsotopes. 'nuclide' applies to a nucleus wilh a specified number of protons and neutrons

. Symbol of a nuclide: A

Xz

&,1&r.

Atomic Number

Mass Number

l=

A=

= number of protons in the nucleus= proton number

= number of protons and neutrons in the nucleus= nucleon number

chai/atomicstruchrrey'2004

. afl atoms of an element have the 3A* . proton number in the nucleus

. the specific number of protons is different for different elements

lsotooes. isotopes are atoms of an element that have.the s4w number of protons (same

atomic number) but &f&ra,'t number of neutrons (different mass number). the different mass numbers are known as isotoplc masses

. isotopes same

Example 1

CVUii ral properties but different rt qs:ra t properties.

Pick out(i) a pair of isotoPes A on) E(ii) a pair of ions which came from the isotopes s cbr a(iii) an ion with a -3 charge D

Example 2

chai/atomicstructurey'2004

Electronic Structure of an Atom. knowledge of the modem atomic structure is the result of quantum theory. major development of the quantum theory:

Theory Postulates Failure of theory

1. Rutherford'sModel

. electrons rotating around thenucleus at various distances

. electrons are prevented fromfalling into the nucleus bycentrifugal force

By the laws of electromagnetics. moving electrons would radiate

energy continuously anddecrease its velocity

. electrons would eventually fallinto the nucleus

. atom would collaose

. atom would produce acontinuous spectrum whichcontradict the line spectrumobserved

2. Bohr'sTheory

. electrons move in fixed,stationary orbits of fixed anddefinite energies around thenucleus.

. electron in any of thestationary states does notradiate or absorb energy

e energy is emitted whenelectron moves from a higherenergy state to a lower energystate

o energy is absorbed whenelectron moves from a lowerenergy state to a higherenergy state

. electrons move in circularorbits around the nucleus

. electron has quantized angularmomentum

. can account for the spectrallines of atomic soectrum ofhvdroqen

r contradicted Heisenberg'sUncertainty Principle which saysthat it is imoossible to determineprecisely both the position andvelocity (and hence momentum)of a particle at the same time.

. for complex atoms, each line in aline spectrum is made up of manyfine lines close together (highresolution)

J.

Schrodinge/squantummechanics

. electrons show dual naturei.e. they have properties ofboth particles and waves

. mathematical expressionscalled Schrodinger waveequations describe motion ofan electron in terms of itsenergy

. wave equations state theprobability of finding theelectron at any particular place

. cannot tell exactly position ofan electron at any particular

chai/atomicsFucture/2004

moment. an orbital is the region in I

space where an electron is Ilikely to be found I

. orbitals have distinctive Ishapes and symmetries andeach denotes an allowedenergy state for the electron

. orbitals are described by 4quantum numbers

Quantum Numbers. each electron in an atom is designated a set of &.r quantum nurnbers

1 Principal Quantum number (n). electrons in an.atom are arranged in a series of shells

. each shell is given a number known as the principal quantum number n

. n has integral values 1,2,3,4,...and determines the energy of the electron

. the shell nearest the nucleus (n = 1 ) has the lo "'(rt energy

. shells with higher n have kqrhv energies

. each principal quantum shell is capable of holding 2n2 electrons.

eg. n = l,total numberof electrons = '

n = 2, total number of electrons = 8

r each principal quantum shell consists of different types of subshells

2 Subsidiarv Quantum Number (l). each subshell can be denoted by a subsidiary quantum number, , = 0,1,2,3,.. (n'1)r the value of I for subshell is generally designated by the letters s, p, d, f

r different subshells within a specific quantum shell are associated with differentenergies, in ascending order:

s(p

:,.

3a

each type of subshell contains one or more orbitals ,

MaEnetic Quantum Numbereach orbital in a subshell can be denoted by a magnetic quantum number, mr,ranging from J to +lthe number of orbitals is determined by the type of subshell

orbitals that have the same energy are known aseg. the 3 'p'orbitals are degenerate,

the 5 'd' orbitals are degenerate,the 7 'f orbitals are degenerate.

Spin Quantum Numbereach orbital can contain a maximum ofspin

efectrons which are of owAh

an electron in an atom behaves like tiny magnet which spins on its axis, either in aclockwise (usuafly represented by ll or anti-clockwise direction (f )the spin of each electron can be denoted by the quantum number, ffig= +lh 67 'y2

4c4t2144 orbitals

4a

tPJ

oIz

sPJI

I

chai/atomicstructure/2004

1 s 1 s 2 Total 2

2s 1 s 2

Total B

p JPt 2

o

2

P. 2

2

s I s 2

Total 18p ?

P" 26

Pv 2

9. 2

d

2

10do 2

dvz 2

df-f 2

dl 2

Summarv

Electrons in Orbitals. an ofiital is a 3dimensional volume of space where there is a high probability (more

than 95%) of finding an electron.Orbital Enerqvo all the orbitals in a particular subshell are at the sard energy level

. as the principal quantum number o inrrraleg , the energy gap betweensuccessive shells gets s'"Cf,,

o as a result, an orbital in an inner shell may be associated with a higher energy levelthan an orbital in the ner:t shell out

eg. Relative energy: 4s < 3d < 4p

chai/atomicstructure/2004

3d

3p

2p

.r-)

1s

Orbital Shape. an electron spreads itself out around the nucleus as a cloud (electron cloud)o the shapes of orbitals reflect the electron density of this electrcn doud

. regions of high eleclron density have a high probability of finding the electlon

t s orbital. scl*,i taltq symmetrical about the nucleus

equal probability of finding an electron at a distance rfrom the nucleus in alldirections

s orlritals in the second and third shells onwards have regions calbd 3l,'l-,where the electrcn density is zero

1s

cetalh spac^ f-f.r] *raeLr+mu car.rc+ h S.-1.

chai/atomicstructun/2004

the number of nodes increases with increasing value for the principal quantumnumber n

the size of the orbital iv'(.toa as n increases: the electron is more andmore likely to be located farther from the nucleus

2 p orbitals. 3 degenerate p orbitals; approximately Juv' Llr{tl shaped; mutually

perpendicular to each other

each orbital consists of two lobes with the atomic nucleus lying between them

they are labelled t. , f , , Pr , according to their respective axis

each p orbital has a node at the nucleus of the atom

the probability of finding the electron is higher further away from the nucleus

3 d orbital. five d degenerate orbitals,

shapes: 4 have shapes like 3-dimensionsl cl,o*v - lea,lrst hbs shape like a p orbital, and a 'doughnut' of electrons density in the xyplane

P,PvPt

dxydl,'

l0 chai/atomicstructure/2004

dr'-f

d"r, dn, d,e lie in the aq , Y* andlobes oointed between the axes

x z coordinate planes respectively, with the

df-f lie in the 1 'l plane and has its lobes point along the x and y axes

dz'? has its major lobes pointing along the z axis

dt

Orbital sizeIn any particular atom

Electrons Confiquartion. electronic configuration refers to the

various orbitals in the atom or ionN*vjh"+o, * CV.+'v"t

orbitals get largerasthe value of n increases eg.n=3orbitals > n=2 orbitals

all orbitals with the same principal quantum number are similar in size

each orbital becomes smaller as nuclear charge increases.eg 2s orbital steadily decreases in size across the second period of the Periodic

Table from Li (z=3) to Ne (z=10)

among the

. normally applies to atoms or ions in the ground state ( i.e. lowest possible energylevel available to it )

o electrons are arranged according to a set of rules:

1 The Aufbau principle f 0"1 rrr* - rt r.truprcl.. when formulating the electronic configuration of a multi-electron atom, protons are

added to the nucleus and electrons are added to orbitals in the order of

o this means that electrons are accommodated in the orbitals ofenergy first

l l

hr^xrt

chai/atomicstructure/2004

2a

a

Pauli Exclusion Principleno two electrons in any atom may have the same set of quantum numbers

this implies that only 2 electrons can occupy the same orbital, and in order to doso, they must have opprclt+, spln"

Hund's Rules35rr,'413when filling degenerate orbitals, electrons occupy these orbitals

and with o arallr r spin before any pairing occurs

o electrons tend to avoid being in the same orbital due to

Writing the electronic confi gurationo several ways of denoting the electronic configuration of an atom or ion of an element

include:

1 using boxeseg. 6C

| 1Vlt l

l'tv It l

| 4Vll l

l r

l1vlt l

1J

f-tT1Tl

Wry42?

l lv lt l wT+q

3P

eg. roS

t2 chai,/atomicstrucbre/2004

enerqv levgl diaqramseg.rsAl

1s

s. D. d. f notation

es. 4hton

,, st

rc l

Shorthand notation

b')5 '1, n?s" r7elr-)g) 2?'3t '

| 3')s t ,f r 3s- 3p' ts '

4v

3s

2s4l/4y

1

1v

3p

2p

1vc

c

I9

4v

,

ft'l rsl f^rr1 as-sP t'

Noble gas configurationit rert l . ,St l lo 2P

I

*&rcctronic atomsliooso atoms or ions that have the same number of electrons are said to be isoelectronic

eg' No.r ls 12s'2,pg

roNe ls"2s'?rq?- ls, 1s ' 2pt

Excited state elec'tron confiquratlon.o ground state configurafion is the xo}l ttrul! anangement of electrcns

r when an atom absorbs l'tght or is bombarded by energetic electrons, it can gainenerqv to reach an excited state

'v'zt'vv*

Ca

h" ls"2s' lp L 3e'3rs

w,

I . ' chai/atomicstruoture./2004

. electrons can move to a higher energy state and assume a new electronicconfiguration

o excited atoms areconfiguration

r. .l+alol' and spontaneously retum to the ground state

o gnly otr ground state exists but there can be

eg. g.t,b eE ig .

rv' 4r4 excited states

atrt.a a+ &Fo{..1Dt+*a

#_-_-(ne t"," 4o..fa ror,,rl1 )

* q{.- at ercitJ cf a*1

".4s+etta )

3ps zn "

oX. r la ( ?o. . r f +. ,L ) ls? !1 1p03s,N4 [erc,* tr t . r {a+,) tsr )3 ' : , ,

-d g! ls " 2s,

Electronic Confiquration of Elements in the Periodic Table

t- l" t

f ia l ' r '

Periodic Table showing the grouping of elements accoiding to the type of orbital beingfilled with electrons

In the periodic table,. elements areranged in order of their a*o*t1* 1Oalo.) - numbers

. elements in the'same Period have electrons occupying the same o,rb' "r"l-tshell

. elements in the same Group have the srh, number of valence electronsand thus, ia'n'Q electronic configuration in the valence (outermost)orbitals in their atoms

Tlo. t ( , I1s

2s 2p3s 3p4s 3d 405s 4d 5D6s 5d 6o-7s 6d

ya

l4 chai/atomicstructure/2004

Group Valence shell electronic configuration

il^9

ill . t'n?ts

IV. - l - t rP

ns nfVI

ng'nP'vt l

ng" f5

vill .bnS - nP

Electronic configurations of first row transition elements:. similar electronic configurations with pr,vt'? lls filled d orbitals

o all have 4s2 3d'1 x = 1 to 10 ) except for Cr and Cu.Element Atomic No. Electronic Configuration

Sc 21 fAr l +s2 3Jr

Ti 22 It" l qs-gJ'

23rA"l 4s ' i l l

Cr 24 EA' l +s 'gJ 5Mn 25

EAv) I s7 3)eFe 26 fA ' l +: 'sJ(Co 27 ga"f +s* 3JaNi 28 f l "J+. '3JtCu ZJ f* .J ' ts ' 3J'"Zn an fa ' ]+"a3Jro

l5 chai,/atomicslructur/2004

lonisation Energv. Definition

The first ionisation energy of an element is the energy needed to remove one moleof electron from one mole of gaseous atoms of the element to form one mole ofsingly charged gaseous cations.

X(S)-tXt(S) * t

o ionisation energy increases with

shielding increases with an increase

. increase in shielding results in

energy

in effective nuclear charge

first ionisation

tnca OS

r effective nuclear charge is the actual charge experienced by the electrons, differentfrom nuclear charge (atomic number)

Factors affecting the effective nuclear charse1 nuclear charqe. increases with the number of ?,o+o * (indicated by the atomic number). the larger the nuclear charge, the s+-s.^{v +W a*.r|'q., between the

nucleus and the electron (provided other factors remain the same)Shieldins or screeninq effect of inner electronseach electron in a multi-electron atom is simultaneously attracted to the nucleus andrepelled by the other electrons

shielding is the phenomenon where attraction between any given electron and thenucleus is by the electron-electron repulsion

electrons

2a

in the number of

& t n1^st

shells

in effective nuclear charoe6 rrr,rtt""t '! rL

^+'aa.h\l aPtrls?r"shields than those with

s > p >J > f

relative shieldinq and effective nuclear charqe:-

quantum shells of smaller n are morehigher n

eN,tw

for a given shell n, the screening ability decreases in the order:

hence, for a given shell n, effective nuclear charge for:

s electrons > I .Lr*'*lr ; { ekr{'r^r > + ekc +'e,ng

lo chai/atomicstruchrre/2004

First l.E./ kJ mol'l

2000

1-- I tz 3 z2p3

0. 1,2 2t : zpf

1500

I s ' L r t

ls ' )s-Pr1000

t ' 2s ' 3P 6 3r '500

504010 30

Atomic number

across a period- nuclear charge

down a group- nuclear charge

ir,o trfaJr3

- screening effect remains rtl*i(|3 qxhnn4d=> effective nuclear charge 'th f /f aJJ

- hence, ionisation energy 'r^(?l'l I cross a period for successiveelements

- noble gase has the highest first ionisation energy and Group I element has thelowest

He

Ne

Kr

Li

ira r.{.4:

11 chai/atomicstructure./2004

- screening effect increases *9n i n Otta "v

hd V 1' c \.' 'Y

=> effective nuclear charge )Q c N a,*t

- hence, ionisation energy At t nat/J

for the d and f block elements-

increase in nuclear charge is "lf'

eg:Ca (g) -+ Ca- (g) + e-Ca* (g) -+ Ca2'19; + s-cat* (g) -+ ca3* (g) + e

l.E. kJmotl59011504540

DeducinE the Electronic Gonfiquration from successive ionisation enercv data

. . example using successive ionisation energies for a potassium atom:

lg (1.E. / kJ mol'')

{lu d"J.' +lushltt +o aecl&J,,{h l,rh" +haen qn aqutnl

0

DeductionsThere are 4

Number of electronsionised

a

1 quantum shells. a large in ionisation energy is observed when electrons

are removed from an rv'vt'l/ ouantum shell closer to the nucleus

r 3 such sharp increase can be seen from the plot, conesponding to the removalof the 2nd. 1Oth and the 18th electron.

2

3

4

5

This electron is found in the n = 'l shell. it has the L*.ti ionisation energy, eatieJ f to remove

These 8 electrons are in the n = 3 shell.

These 8 electrons are in the n = 2 shell.

These electrons are cl"wlt to the nucleus, i.e. n = shell. these 2 electrons have the htlh.rr ionisation energies

Hence, the electronic configuration of potassiumis ls "2r '+( b'3Pt +J'

I9 chai/atomicstrucrure/2004

' :

Ex.1The first eight successive ionisation energies bran element are as follows (in kJmol-l)

790 600 3200 4400 16100 19800 23800 23800

Which group in the Periodic Table does this element belong? a..g tt/O v61 laeg ivocalr n^ x,E twv*A rrk, rtn^. v'!C

hrro +tn a.n 4 rtcalrrrc nn o, tba.a,t f oLtl , 4r*^t

++1t 61+.,r funJ

gbcfon ,

ih ^ufry' .{ tlu e.io&a Tarl. ,

Write down its outer electronic configuration.

Ex.2

!al^3 hP

Deduce the eleclronic configuration of the element with successive ionisation energiesas shown. Hence determine its group in the Perbdic Table.

lsE

No. of electrons removed

E[ulrnr cI Ef.rcr f I

.sfiell s

stJ I

*oil"sfra,l'

lt" tsL 2pc )E'

llun art 3

I eltct.o6

t ebcthh!

). elcCfi.ort

1^