Understanding Periodic Understanding Periodic TrendsTrends

CHAPTER 6CHAPTER 6

Timeline1829 J.W. Dobereiner Classification system in which elements are

grouped in triads 3 elements with similar chemical properties Didn’t work for all of the elements

1869-Russian Chemist, Dmitri Mendeleev: Designed a Periodic Table 1869 Elements are listed in order of increasing atomic

mass

Timeline continued

1913-British Scientist, Henry Moseley: Designed the Modern Periodic Table Elements are listed in order of increasing atomic # Based on Periodic Law: physical & chemical

properties of elements are periodic functions

of their atomic #s

PERIODICITYThere are general trends in the properties of atoms and their ions.

These trends can be explained using the periodic table and the electron configurations of the atoms.

Atomic Radii

D

I

atomHalf the distance between the nuclei in a molecule consisting of identical atoms

Increases as you move down a groupe- s are added to higher energy levelsShielding effect: inner e-s shield the outer e-s from the attractive force of the nucleus

Decreases as you move across a periodIncreasing # of protons attract valence electrons moreStronger nuclear charge

Atomic Radii

D

I

Atomic RadiiPractice

Which has a larger atomic radii?

Li or Ne?

LiMore protons, so Ne is smaller

Ionization Energy

I

D

positive ion

electronI.E.

Li 2 1 2

Energy needed to remove loosely held electron from outer energy level

Decreases as you move down a groupShielding effect: inner e-s shield the outer e-s from the attractive force of the nucleus

Bigger atom so it is easier to remove valence e-s Nucleus can’t “hold” onto e-s that are far away

Increases as you move acrossMore valence e-s, more energy needed to remove e-s

e

Ionization Energy in kJ/mol

D

I

Ionization Energy

Which has more ionization energy?

He or Xe? (kJ/mol)

He Xe2372 kJ/mol 1170 kJ/mol

Practice

Ionic Size: Cation(+) vs. Anion(-)

Anions decrease as you move across a period

I

Cation (D)

Anion (D)

Increases as you move down a group

• more energy levels greater the size Cations decrease as you move across a period

Ionic Size: Cation(+) vs. Anion(-)

Cations lose electrons and get smaller

Anions gain electrons and get bigger

Ionic SizePractice

Which has a larger ionic size?

Li or Cs?

Cs

ElectronegativityThe ability of an atom to attract e-s to itself when it is chemically combined with another element.

Decreases as you move down a group Bigger the atom, the harder it is for the nucleus to attract e-s

Increases as you move across a periodMore valence e-s, easier to gain e-s than lose

(MAGIC 8)Noble gases are omitted because they don’t form

many compounds I

D

Electronegativity

I

D

ElectronegativityPractice

Which is more electronegative?

Li or F?

F

More valence e-s

PERIODICITY

Ato

mic

Rad

ii I

ncre

ases

Atomic Radii DecreasesIonization Energy IncreasesElectronegativity IncreasesShielding is Constant

Ionic Size (cation)Decreases

Ionic Size (anion)Decreases

PERIODICITY

Shi

eldi

ng I

ncre

ases

Ele

ctro

nega

tivi

ty D

ecre

ases

Ioni

zati

on E

nerg

y D

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ases