1 mr. shieldsregents chemistry u08 l04 2 size trends atomic radii follows two trends: 1) radii...
TRANSCRIPT
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Size TrendsSize Trends
Atomic Radii follows two trends:
1) Radii increases going down a group
1) Radii decreases going across a period
But how do we measure Atomic Radii?
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Atomic RadiiAtomic Radii
Atomic Radius is measured as ½ the distance betweenAdjacent nuclei in a molecule.
Why not just measure ½ the diameter of the atom?
Hint: What’s the definition of an atomic orbital?
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Atomic RadiiAtomic Radii
The trend down a group may be easier to explain first
As we move down a group what happens to the principalEnergy level ?
As the principal energy level increases, electrons movefurther and further from the nucleus.
Nucleus n=1 2 3 4
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Atomic Radii in GroupsAtomic Radii in Groups
As we move down group 1 each successive S orbital electronIs further from the nucleus and thus Atomic radii increases
nn ElementElement Radii (pm)Radii (pm) changechange
22 LiLi 155155 --
33 NaNa 190190 3535
44 KK 235235 4545
55 RbRb 248248 1313
66 CsCs 267267 1919
77 FrFr 270270 33
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Atomic Radii in groupsAtomic Radii in groups
Nuclear Nuclear chargecharge
Radii (pm)Radii (pm) changechange
Li (Li (33)) 155155 --
Na (Na (1111)) 190190 3535
K (K (1919)) 235235 4545
Rb (Rb (3737)) 248248 1313
Cs (Cs (5555)) 267267 1919
Fr (Fr (8787)) 270270 33
Since the s orbital is further from the nucleus the radii of theAtom increases. But …
The nuclear charge is also increasing since Atomic Number is increasing.
Increasing nuclear chargeDiminishes the rate ofChange of increasingAtomic Radii Down a Group.
Electrons are pulled Toward the nucleus more strongly
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Atomic Radii Atomic Radii acrossacross periods periods
We’ve seen how atomic radii increases going down a groupBut what happens when we go across a period?
We’ll, in fact atomic radii decreases. But why?
We can begin to understand what is happening if we look atBoth the Atomic number and what principle energy level electrons are being added to.
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Atomic Radii across periodsAtomic Radii across periods
As we move across a period Atomic numbers increase
- Pos. Nuclear charge also increases so would expect the electrons to be pulled closer to the nucleus. So this could explain decreasing Atomic radius
- BUT … this same thing happens as we move down a group. And for groups Atomic Radii increases as we add more electrons? So why does radius increase in groups but not across a period?
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Atomic radii across periodsAtomic radii across periodsThe difference is that when we go across a period electrons doNot fill higher energy levels. They either occupy lower energyLevels or the same energy level
Whereas when going down a group electrons occupy successively Higher principle energy levels. For example …
1 2 3 13n=3 2-8-1 2-8-2 2-8-3n=4 2-8-8-1 2-8-8-2 2-8-9-2 2-8-18-3 n=5 2-8-18-8-1 - - -
group
But why doesn’t atomic radii remain about the same acrossa group?
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Atomic radii across periodsAtomic radii across periodsAs atomic number increases across a row additional electronsare added to the same (or lower) energy level. The effectiveNuclear charge may also be increasing and electrons are pulled inMore strongly towards the larger more positive nucleus.
Unlike when moving down a group, there are no new principal energy levels being added to counteract the effect of increasing nuclear charge and increasing effective nuclear charge.
+16 e-+11 e-
Na: Effective nuclear charge = +1 S: Eff. Nuclear Charge = +6
2-8-62-8-1
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Nuclear charge (K) = 19Effective Nuc. Chg. (Grp I) = 1
Nuclear charge (Br) = 35Effective Nuc. Chg. (Grp VII)= +7
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Ionic RadiiIonic Radii
We’ve now seen how atomic radii changes in Periods & Groups
But what happens when Atoms either gain or lose electrons to form ions?
How does Ionic Radii vary down Groups & across Rows?
Remember …
The representative elements of groups 1, 2, 13 and 14 give up electrons to form +1, +2, +3 and +4 ions respectively
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Positive IonsPositive Ions
When atoms lose all their valence electrons they lose the outermost quantum level (n).
Consider Aluminums electron configuration. What is it?
2-8-3 (principle energy levels 1, 2 and 3 are occupied)
What is the electron config after Al loses its 3 valence electrons?
2-8 (only principle energy levels 1 and 2 are occupied)
What is the charge on Aluminum?
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Positive IonsPositive Ions
The loss of the outermost valence shell has two effects:
1)The atoms radius shrinks because it loses it’s outermost principle quantum level
AND …
2) The Nucleus now has more positive charge than the total negative charge from electrons. The larger effective nuclear charge will now pull electrons in closer to the nucleus
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Positive IonsPositive IonsNotice that even though the ionic electron config is the sameionic radius gets progressively smaller moving across the period.
NaNa Mg Mg AlAl
Atomic No.Atomic No. 1111 1212 1313
Ionic chargeIonic charge +1+1 +2+2 +3+3
Atomic Radius Atomic Radius (pm)(pm)
190190 160160 143143
Ionic Radius (pm)Ionic Radius (pm) 9999 6565 5050
Ionic Elec. ConfigIonic Elec. Config 2-82-8 2-82-8 2-82-8This happens because the positive eff. nuclear charge seen by the same number of electrons increases as we move across a the row
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Negative IonsNegative IonsLet’s next look at the non-metals, for example Chlorine
Non-metals form ions by gaining electrons
Cl 2-8-7 2-8-8 Cl- (negative ion)
When we add electrons the effective nuclear charge perelectron decreases AND there is increases electron repulsion
So … you would expect the ionic radius to increase and it does
Cl atomic radius = 99 nmCl- ionic radius = 181 nm
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Negative IonsNegative IonsMoving down groups the principal energy level increases
- This is true for all atoms, Anions (-) & Cations (+)
Li 2-1 Li+ 2 F 2-7 F- 2-8Na 2-8-1 Na+ 2-8 Cl 2-8-7 Cl- 2-8-8K 2-8-8-1 K+ 2-8-8 Br 2-8-18-7 Br- 2-8-18-8
So atom & ionic size increases going down Groups
Going across periods Ionic size first decreases then jumps up When Oxidation states change from positive to negative.
- after the jump up the downward trend in size continues