Trends & the Periodic Table

Trends more than 20 properties change in predictable way based location of elements on PT more than 20 properties change in predictable way based location of elements on PT some properties: some properties: – Density – melting point/boiling point – atomic radius – ionization energy – electronegativity

Atomic Radius Atomic radius: defined as ½ distance between neighboring nuclei in molecule or crystal Atomic radius: defined as ½ distance between neighboring nuclei in molecule or crystal “size” varies a bit from substance to substance “size” varies a bit from substance to substance

X-ray diffraction pinpoints nuclei – then measures distance between them Cannot measure electron cloud

Trends: Atoms get larger as go down column: ↑ # principal energy levels Atoms get smaller as move across series: ↑ PPP “proton pulling power”

Fr Cs Rb K Na3 2-1Li2 1H1 ConfigurationElementPeriod Going down column 1: increasing # energy levels as go down - makes sense that atoms get larger in size

previousprevious | index | nextindexnext Li: Group 1 Period 2 Cs: Group 1 Period 6

2-8NeVIIIA or FVIIA or OVIA or NVA or CIVA or BIIIA or BeIIA or 2 2-1LiIA or 1 ConfigurationElementFamily Going across row 2: size atoms actually get a bit smaller as go across row left to right - what’s going on?

What do you remember about charge? opposites attract opposites attract like charges repel like charges repel largest influence on atomic size in order: largest influence on atomic size in order: # principal energy levels # principal energy levels “proton pulling power (PPP)” “proton pulling power (PPP)”

Effective nuclear charge Charge actually felt by valence electrons Charge actually felt by valence electrons = Atomic Number minus # inner shell electrons = Atomic Number minus # inner shell electrons Not same as nuclear charge or # protons in nucleus Not same as nuclear charge or # protons in nucleus Charge felt by valence electrons is attenuated (shielded) by inner shell electrons Charge felt by valence electrons is attenuated (shielded) by inner shell electrons

previousprevious | index | nextindexnext H and He: only elements whose valence electrons feel full nuclear charge (pull)

Li’s valence e - feels effective nuclear charge of +1 previousprevious | index | nextindexnext

previousprevious | index | nextindexnext Calculate “effective nuclear charge” # protons minus # inner electrons # protons minus # inner electrons

as go across row size tends to decrease a bit because of greater PPP “proton pulling power” previousprevious | index | nextindexnext

size  as you go  column size  as you go  column size  as you go  row size  as you go  row previousprevious | index | nextindexnext

Ionization Energy = amount energy required to remove valence electron from atom in gas phase = amount energy required to remove valence electron from atom in gas phase 1st ionization energy = energy required to remove most loosely held valence electron (e - farthest from nucleus) 1st ionization energy = energy required to remove most loosely held valence electron (e - farthest from nucleus)

Trends in ionization energy What do you think happens to the ionization energy as go down column of PT? What do you think happens to the ionization energy as go down column of PT? As go across row? As go across row? decreases increases

Valence electrons in atoms: effective nuclear charge of +1Valence electrons in atoms: effective nuclear charge of +1 Cs valence electron lot farther away from nucleus than LiCs valence electron lot farther away from nucleus than Li electrostatic attraction much weaker so easier to steal electron away from Cselectrostatic attraction much weaker so easier to steal electron away from Cs previousprevious | index | nextindexnext

easier to steal electron from Li than from Neeasier to steal electron from Li than from Ne Li: smaller effective nuclear charge - valence electron farther away from nucleusLi: smaller effective nuclear charge - valence electron farther away from nucleus Li has less “proton pulling power” than NeLi has less “proton pulling power” than Ne previousprevious | index | nextindexnext

Trends in ionization energy Ionization energy decreases as go down a column Ionization energy decreases as go down a column – gets easier to remove valence electron Ionization energy increases as go across a row Ionization energy increases as go across a row – more difficult to remove valence electron

Periodic properties: Graph shows a repetitive pattern (Note: Doesn’t have to be a straight line)

Electronegativity ability of atom to attract electrons in bond ability of atom to attract electrons in bond noble gases tend not to form bonds, so don’t have electronegativity values noble gases tend not to form bonds, so don’t have electronegativity values Unit = Pauling Unit = Pauling Fluorine: most electronegative element Fluorine: most electronegative element = 4.0 Paulings = 4.0 Paulings

Trends in electronegativity Related to PPP Related to PPP Increases as go across row Increases as go across row Decreases as go down column Decreases as go down column Remember: F most electronegative element! Remember: F most electronegative element!

Reactivity of Metals Metals are losers! Metals are losers! judge reactivity of metals by how easily electrons judge reactivity of metals by how easily give up electrons most active metals: Fr (then Cs) most active metals: Fr (then Cs) For metals, reactivity increases as ionization energy goes down For metals, reactivity increases as ionization energy goes down

Trends for Reactivity of Metals or Metallic Character Increases as go down column Increases as go down column – easier to lose electrons! Decreases as go across row Decreases as go across row – more difficult to lose electrons!

Reactivity of Non-metals Non-metals are winners! Non-metals are winners! judge reactivity of non-metals by how easily electrons judge reactivity of non-metals by how easily gain electrons F: most active non-metal F: most active non-metal For non-metals: For non-metals: – reactivity ↑ as electronegativity ↑

Trend for Reactivity of Non-metals: Depends on PPP Increases as go across row Increases as go across row Decreases as go down column Decreases as go down column – (shielded by more inner-shell electrons)

Ionic Size Relative to Parent Atom Depends if (+) ion or (-) ion Depends if (+) ion or (-) ion How do you make a positive ion? How do you make a positive ion? How do you make a negative ion? How do you make a negative ion? Remove electrons Add electrons

How do you know if an atom gains or loses electrons? Think back to the Lewis structures of ions Think back to the Lewis structures of ions Atoms form ions to get a valence of 8 Atoms form ions to get a valence of 8 (or 2 for H) (or 2 for H) Metals tend to have 1, 2, or 3 valence electrons Metals tend to have 1, 2, or 3 valence electrons – It’s easier to lose them Nonmetals tend to have 5, 6, or 7 valence electrons Nonmetals tend to have 5, 6, or 7 valence electrons – It’s easier to add some Noble gases already have 8 so they don’t form ions very easily Noble gases already have 8 so they don’t form ions very easily

Positive ions (cations) Formed by loss of electrons Formed by loss of electrons Cations always smaller than parent atom Cations always smaller than parent atom

Negative ions or (anions) Formed by gain of electrons Formed by gain of electrons Anions always larger than parent atom Anions always larger than parent atom

Allotropes Different forms of element in same phase Different forms of element in same phase – different structures and properties

O 2 and O 3 - both gas phase O 2 (oxygen) - necessary for life O 3 (ozone) - toxic to life

Graphite, diamond: both carbon in solid form both carbon in solid form

Graphite and Diamond