Chapter 8: Periodic Relationships Among the Elements

Transcription

1 10/25 Chapter 8: Periodic Relationships Among the Elements Development of the Periodic Table Early chemists: chemical properties related to atomic mass Newlands: Law of octaves >Didn t work past Ca Mendeleev (and Meyer): grouped by properties >D.M. predicted element properties Mosely: atomic # (Z) >Z doesn t always go with atomic mass >Sorting by Z makes groups make sense Electron Configuration Core elements >Inner electrons >Filled Valence electrons: outermost >Likely to be involved in reactions >Higher energy Same valence configuration similar properties Periodic Law Same group=same valence electron configuration Example: Group 2: Be 2,2 Mg2,8,2 Ca 2,2,8,2 Are the following pairs of elements similar in chemical properties? Examples:

2 Cl and Br? Yes 1s 2 2s 2 2p 5 and [Ne] 3s 2 3p 4 No 1s 1 2s 2 2p 2 and [Ne] 3s 2 3p 2 Yes P and S No S and O Yes Representative (1A 7A) >Incomplete s or p Transition metals (1B, 3B 8B) >Incomplete d >Number=outer e, except 8B 2B (no name) >Full s and d, empty p f block (Lanthanides, Actinides) >Incomplete f Noble gases (8A) >Full s and p (except He) Elements in Equations Metal: empirical formula >Sn, Cu, Ag, Nb Non metals: empirical or molecular >Empirical: C, noble gases >Molecular: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2, P 4 >Either: S or S8 Metalloids: empirical >Sb, Te, Po Representative Elements: Cations and Anions Na [Ne]3s 1 makes Na + [Ne] Ca [Ar]4s 2 makes Ca 2+ [Ar] Al [Ne]3 2 3p 1 makes Al 3+ [Ne]

3 ^Lose electrons to get a noble gas configuration H 1s 1 makes H 1s 2 or [He] F 1s 2 2s 2 2p 5 makes F 1s 2 2s 2 2p 6 or [Ne] O 1s 2 2s 2 2p 4 makes O 2 1s 2 2s 2 2p 6 or [Ne] N 1s 2 2s 2 2p 3 makes N 3 1s 2 2s 2 2p 6 or [Ne] ^Gain electrons to get a noble gas configuration Na +, Al 3+, F, O 2, and N 3 are all isoelectronic with Ne What atom is isoelectronic with H? He Transition Metals Cations Electrons are always removed first from the ns orbital and then from the (n 1)d orbitals Examples: Fe: [Ar]4s 2 3d 6 makes Fe 2+ : [Ar]3d 6 or Fe 3+ : [Ar] 3d 5 Mn: [Ar]4s 2 3d 5 makes Mn 2+ : [Ar]3d 5 Effective nuclear charge (Zeff): positive charge felt by an outer electron Zeff=Z σ 0<σ<Z (σ=shielding constant, Z=atomic #) Z Core Zeff Radius Na Mg Al Si Increasing Zeff a period Atomic Radius=1/2 x d d=distance between nuclei

4 Atomic Radius: Increases as you go down, decreases across a period 10/28 Ge has an atomic radius of 123pm. What is the distance between two Ge atoms in solid germanium? 123 X 2=246pm The distance between W atoms in tungsten metal is 274pm. What is the atomic radius of W? 274/2=137pm Estimate the length of an As I bond given the atomic radii: As: 120pm I:133pm =253pm Estimate the atomic radius of Bi if Bi I is 281pm =148pm Arrange the following in order of increasing atomic radius: a. F, B, Be b. S, Te, Po c. Be, Mg, Na d. F, S, P, As e. S, P, Se, As (Down is a bigger effect than going left) Cation is always smaller than atom from which is formed. Anion is always larger than atom from which it is formed. Arrange the following in order of increasing radius: a. Mg +2, Ca +2, Ca b. Se, Se 2, Te c. Co +3, Fe +3, Fe +2 *Br and Hg are liquids, and possibly Fr

5 Ionization energy is the minimum energy (kj/mol) required to remove an electron from a gaseous atom in its ground state. I1 + X(g) X + (g) +e I2 + X(g) X 2+ (g) +2e I3 + X(g) X 3+ (g) +3e I1 first ionization energy I2 second ionization energy I3 third ionization energy Across a period, ionization increases Down a group, ionization decreases Arrange the following in order of increasing 1 st ionization energy: K, Na, P, Ar, Ne Use: B, Al, C, Si Which has the lowest 1 st ionization energy? Al Which has the highest? C Use: S, Mg, Li Which has the largest second ionization energy? Li Use: Ca, S Which has the largest third ionization energy? Ca 10/30 Electron affinity (EA): negative of the ΔH when an electron is accepted by an atom (g) to for an anion. X (g) + e X (g) F (g) + e F (g) ΔH= 328 kj/mol EA=238 kj/mol

6 O (g) + e O (g) ΔH= 141 kj/mol EA=141 kj/mol Top to bottom: ~same Left to right: increases (metals<nonmetals) Exception: Noble gases: ~0 5A<4A 2A<1A 2A lowest EA in a period Similar charge to radius ratio=similar behavior Li and Mg Be and Al B and Si Group 1A Elements (ns 1, n 2) Note: H doesn t really fit M M e 2M (s) + 2H 2 O (l) 2MOH (aq) + H 2(g) 4M (s) + O 2(g) 2M 2 O (s) Group 2A Elements (ns 2, n 2) M M e, Be forms covalent Be (s) + 2H 2 O (l) No reaction Mg (s) + 2H 2 O (g) Mg(OH) 2(aq) + H 2(g) Slow M (s) + 2H 2 O (l) M(OH) 2(aq) + H 2 M=Ca, Sr, or Ba M (s) + 2H + (aq) M +2 (aq) + H 2(g) ALL Group 3A Elements (ns 2 np 1, n 2) Boron: Metalloid forms covalent bonds, no reaction with O 2 Aluminum: Metal forms some covalent bonds (AlH 3 ) 4Al (s) + 3O 2(g) 2Al 2 O 3(s) Ions: Al, Ga: +3 2Al (s) + 6H + (aq) 2Al 3+( aq) + 3H 2(g) In: +3 (+1) Ti: +1 (+3) Group 4A Elements (ns 2 np 2, n 2) Carbon: nonmetal Si and Ge: metalloids

7 Sn (s) + 2H + (aq) Sn 2+ (aq) + H 2(g) Ions: +2 or +4 Pb (s) + 2H + (aq) Pb 2+ (aq) + H 2(g) Group 5A Elements (ns 2 np 3, n 2) 2 nonmetals 2 metalloids 1 metal N 2 O 5(s) + H 2 O (l) 2HNO 3(aq) Bi=very unreactive Group 6A Elements (ns 2 np 4, n 2) Nonmetals: O 2, S 8, Se 8 No metals Po=radioactive SO 3(g) + H 2 O (l) H 2 SO 4(aq) Group 7A Elements (ns 2 np 5, n 2) All nonmetals X + 1e X 1 X 2(g) + H 2(g) 2HX (g) Group 8A Elements (ns 2 np 6, n 2) Completely filled ns and np subshells. Highest ionization energy of all elements. No tendency to accept extra electrons. 1A vs. 1B All valence s: half filled Low IE (under 530 kj/mol) Very reactive High IE (over 730 kj/mol Very stable Have d electrons that shield nucleus 2A (reactive) vs. 2B (stable) All valence s: filled Similar argument explains reactivity difference