1 Mr. ShieldsRegents Chemistry U08 L04 2 Size Trends Atomic Radii follows two trends: 1) Radii increases going down a group 1) Radii decreases going.

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Mr. Shields Regents Chemistry U08 L04

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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?

44How can we account for this trend?

Atomic Radii Trends

Atomic Radii

Decreasing radii

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easi

ng

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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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Variation in atomic and Ionic Radii

What’s going onHere?

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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

2020

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The ionic compound MgO

2-8 2-8Electron Config

If we were to look at individual atoms Mg wouldActually be larger than Oxygen!

O atom

Mg atom