Download Elements, Periodic Trends and Lewis Dot Diagrams

LS50 2015 Lecture 09: Elements, Periodic Trends, and Lewis Dot Diagrams Cassandra Extavour Dpts. OEB & MCB Learning goals for today By the end of this lecture, you should be able to: •  Describe and explain some basic properJes of an element based on periodic trends •  Know how to figure out the number of valence electrons for an atom •  Understand electronegaJvity; use this to predict formaJon of compounds in general terms •  State the octet rule –  name and define the types of bonds that atoms can engage in to saJsfy it –  recognize the three major types of violaJons of this rule •  Draw Lewis dot structures of atoms, ionic compounds, and covalent compounds The Periodic Table of the Elements Early discovery of elements •  First materials used by humans: likely wood, stone, bone, hide •  Anthropology helps us learn about the history of discovery of elements: –  Copper 4000 BC? –  Gold –  Bronze (copper-­‐Jn alloy), Iron 2000 BC (Bronze Age) –  Steel (iron-­‐carbon alloy) 1200 BC (Iron Age) •  Aristotle (350 BC): 5 elements Brief History of the Periodic Table I •  Early 1800s: less than 25 elements had been idenJfied •  First decades of 1800: explosion in element discoveryà 55 by 1830 –  This is a lot! How many more are there? Is the number finite? Are they organized in any way? • 
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A`empts at organizaJon: Döbereiner 1829: bromine seems “intermediate” to chlorine and iodine! –  He found a few more examples like that and called them triads –  But most elements did not fit into a triad Atomic weights were a big focus (although there was disagreement for a long Jme as to what that really was) de Chancourtois 1862: cylindrical graph (spiral plot) – 
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Newlands 1864: law of octaves – 
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First to try arranging elements in order of atomic weight Some periodicity was revealed in this way Organized elements in order of atomic weight with seven groups in each column à first element of each column has somewhat similar chemistry But it had some inconsistencies De Chancourtois’ spiral plot (1862) • 
He called it the “telluric helix” because tellurium was in the middle Newland’s law of octaves (1864) Brief History of the Periodic Table II •  Meyer 1870 considered atomic volumes –  This led to a new insight: periodicity Brief History of the Periodic Table II •  Mendeleev 1869 had a be`er* idea: three main insights –  Valence was given priority over atomic weight –  Gaps are not mistakes, they are predicJons –  The properJes of these “missing” elements can be predicted from the elements below and above the gaps (shout out to Döbereiner) •  Approx 1850: spectroscope à many of Mendeleev’s predicJons about “missing” elements were proven right •  * “be7er” here means “a be7er fit for the available data” Mendeleev’s original periodic table (1869) The Periodic Table at 0ºC h`p://www.ptable.com/ The Periodic Table at body temperature h`p://www.ptable.com/ The Periodic Table •  How do you interpret the informaJon wri`en in the periodic Charge number: table? An ion would have charge value wri`en here. If there is nothing there, assume it’s a neutral atom Atomic number (Z): Number of protons (stays the same for all isotopes and ions of an element) Atomic symbol: Abbreviated element name Element name These have some cool etymologies Atomic weight: Weighted average of atomic mass numbers* of all known isotopes A Z Energy levels (n values) with electrons in them in a neutral atom. For K we go up to n = 4. Remember electron configuraJon: 1s22s22p63s23p64s1 *Atomic mass (A): Number of protons plus number of neutrons: the la`er varies for different isotopes The Periodic Table The known element with highest atomic number occupies 7 energy levels (up to n = 7 = Q shell) and up to the f subshell (l = 4): [Rn]5f146d107s27p6 Metals •  Tend to lose electrons and become caJons •  Shiny, malleable, ducJle, good conductors Non-­‐metals •  Tend to gain electrons and become anions •  Dull, bri`le, poor conductors Metalloids Can lose or gain electrons : no great way to predict what they will do (although Al tends to be mostly metallic and B tends to be mostly non-­‐metallic) Valence electrons •  These are the electrons that can parJcipate in forming chemical bonds! •  à These are the ones we care about most in chemistry •  For main group elements (not transiJon metals), you can use the periodic table to figure out how many valence electrons a given element has •  # valence electrons corresponds to group number Valence electrons •  For main group elements, the number of valence electrons = the CAS group number (roman numeral system) •  (groups are referred to in many different ways – see Table in Lecture 09 roadmap) 1 2 3 4 5 6 7 8 IA IIA IIIA IVA VA VIA VIIA VIIIA Lewis dot structures • 
These are schemaJc representaJons of –  the number of valence electrons in an atom or molecule –  “Where” these electrons are relaJve to the atoms in the molecule –  What their role is in the bonds of the molecule • 
Show atoms as atomic symbols surrounded by valence electrons (dots) • 
Electron pairs between atoms indicate bond formaJon: 2 electrons = single bond • 
Octet rule: the most stable arrangement of electrons is one where all atoms have a “noble gas configuraJon” i.e. eight electrons C
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Lone Pair (6 x) C
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Group 14 (IVA) Group 17 (VIIA) F
Bonding Pair Valence electrons in ionic bonds Valence electrons in covalent bonds Group 1 (IA) Group 16 (VIA) Periodic trends in more detail –  The elements in a given group (column) have similar properJes because their outer electron (valence electron) configuraHons are similar to each other Periodic trends in more detail –  The elements in a given group (column) have similar properJes because their outer electron (valence electron) configuraHons are similar to each other –  There are also trends across a period (row) –  As long as we arrange elements according to atomic number (assuming neutral atoms, so that # protons = # electrons), the physical and chemical properJes of the elements vary periodically –  We’ll look at four major trends that help us understand how atoms behave in chemical bonds: •  Atomic radius •  IonizaJon energy •  ElectronegaJvity •  Electron affinity Periodic trends – atomic radius •  Atomic size depends on –  n (the larger the value of n, the larger the size of the shell) –  effecJve nuclear charge (ENC) = the posiJve charge an electron experiences from the nucleus (affected by shielding effects for polyelectronic atoms) •  Decreases across a period from L to R (ENC goes up with increasing proton number) •  Increases down a group (n increases this way too) Periodic trends – atomic radius Periodic trends – ionizaJon energy •  This is the minimum energy (in kJ/mol) needed to remove the highest energy electron from an atom (creaJng a caJon) •  Increases across a period from L to R (gets harder to remove electrons L to R across a period as the electrons get closer to the nucleus – remember atomic radius) •  Decreases down a group (gets easier to remove electrons going down a group as the electrons spend more Jme on average further from the nucleus) •  If electrons are removed successively, call the energies required first, second third etc. ionizaJon energies Periodic trends – ionizaJon energy Periodic trends – electronegaJvity •  This is the net ability of an atom to take an electron from another atom •  Increases across a period from L to R (the elements get be`er at stealing electrons) •  Decreases down a group (the elements get worse at stealing electrons) •  ElectronegaJvity is a dimensionless quanJty measured in “Pauling units” on a relaJve scale from around 1 to around 4 •  The noble gases have no electronegaJvity (but it is sJll possible for some of them to form compounds with Fl and O, albeit relaJvely unstable ones) Periodic trends – electronegaJvity Periodic trends – electron affinity •  This is the energy change (in kJ/mol) required to add an electron to a neutral atom (creaJng an anion) •  Increases from bo`om lex to top right (gets harder to add electrons in this direcJon) •  This is clearly very related to electronegaJvity (see the previous two slides): –  atoms that are highly electronegaJve have high electron affinity –  The main conceptual difference is that they are measuring different things: •  ElectronegaJvity is a measurement of the “power” of an atom to a`ract an electron •  Electron affinity is a measurement of the amount of energy needed for that to happen Periodic trends – electron affinity No=ce that the trends on this table are basically the same as the ones shown on the electronega=vity representa=on two slides back Lewis dot structures – covalent compounds •  Don’t forget that atoms can share more than one electron pair: – 
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Single bond = one electron pair shared = bond order 1
Double bond – two electron pairs shared = bond order 2
Triple bond = three electron pairs shared = bond order 3
Quadruple bond = four electron pairs shared = bond order 4 •  FracJonal bond orders are also possible (resonance is coming up – see Lecture 10) Lewis dot structures – covalent compounds CO2 carbon dioxide O
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O C O
Octet ViolaJon CO double bond 16 e-­‐ 12 e-­‐ lex O C O
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0 e-­‐ lex Octet rule violaJon #1: sub-­‐octet systems •  B and Be are prone to this: they can form stable molecules that don’t obey the octet rule •  Examples: BF3 (boron trifluoride), BeCl2 (beryllium chloride), BCl3 (boron trichloride) F
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Octet rule violaJon #2: valence shell expansion •  tends to happen for third period elements (Na, Mg, Al, Si, P, S, Cl)
•  More than eight electrons can surround one of these elements in a molecule and it can sJll form a stable bond
•  e.g. ClF3 (chlorine trifluoride), PCl5 F
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Cl
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Octet rule violaJon #3: unpaired valence electrons •  these are free radicals: –  they can be very reacJve because of their unpaired electron •  e.g. ClO2 chlorine dioxide O Cl O
Unpaired electron Recap: learning goals for today Hopefully, you now feel able to: •  Describe and explain some basic properJes of an element based on periodic trends •  Figure out the number of valence electrons for an atom •  Understand electronegaJvity; use this to predict formaJon of compounds in general terms •  State the octet rule –  name and define the types of bonds that atoms can engage in to saJsfy it –  recognize the three major types of violaJons of this rule •  Draw Lewis dot structures of atoms, ionic compounds, and covalent compounds If not, please ask your ques=ons during sec=on, on Piazza, or come to office hours! (Thursdays at noon by appointment)