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Inorganic Structural Chemistry > Valence rules > Flashcards

Valence rules Flashcards

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1
Q

What is VEC_A?

A

The valence electron content/concentration of the anion.

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2
Q

How is VEC_A defined?

A

For compound CmAn:

VEC_A = (m*e_c + n*e_a) / n
e_c = valence electrons of cation
e_a = valence electrons of anion
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3
Q

What happens if VEC_A < 8?

A

Missing electrons obtained in A-A-bonds.

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4
Q

What happens if VEC_A > 8?

A

Excess electrons remain on C as C-C bonds or electron pairs.

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5
Q

What happens if VEC_A = 8?

A

Stable configuration for both atoms.

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6
Q

Calculate VEC_A for TlBr, and comment on bonding.

A 
e_c = 3
e_a = 7

VEC_A = 10.

Two excess electrons. Remain on Tl as electron pair.

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7
Q

Calculate VEC_A for SnCl2, and comment on bonding.

A 
e_c = 4
e_a = 7

VEC_A = (41 + 72)/2 = 9

One excess electron per anion, in total two per formula unit. Remain on Sn as electron pair.

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8
Q

Calculate VEC_A for SbCl3, and comment on bonding.

A 
e_c = 5
e_a = 7

VEC_A = (51 + 73)/3 = 8.667

2/3 excess electron per anion, in total two per formula unit. Remain on Sb as electron pair.

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9
Q

Calculate VEC_A for SF2, and comment on bonding.

A 
e_c = 6
e_a = 7

VEC_A = (61 + 72)/2 = 10

2 excess electrons per anion, in total four per formula unit. Remain on S as two electron pairs.

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10
Q

Calculate VEC_A for Hg2Cl2, and comment on bonding.

A 
e_c = 2
e_a = 7

VEC_A = (22 + 72)/2 = 9

1 excess electron per anion, in total two per formula unit. Will form Hg-Hg bonds to accommodate this.

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11
Q

Calculate VEC_A for H2O2, and comment on bonding.

A 
e_c = 1
e_a = 6

VEC_A = (12 + 62)/2 = 7

1 electron missing per anion, in total two per formula unit.
Will form O-O bonds to accommodate this.

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12
Q

Calculate VEC_A for SrO2, and comment on bonding.

A 
e_c = 2
e_a = 6

VEC_A = (21 + 62)/2 = 7

1 electron missing per anion, in total two per formula unit. Will form O-O bonds to accommodate this.

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13
Q

Calculate VEC_A for BaS2, and comment on bonding.

A 
e_c = 2
e_a = 6

VEC_A = (21 + 62)/2 = 7

1 electron missing per anion, in total two per formula unit. Will form S-S bonds to accommodate this.

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14
Q

Calculate VEC_A for CaC2, and comment on bonding.

A 
e_c = 2
e_a = 4

VEC_A = (21 + 42)/2 = 5

3 electron missing per anion, in total 6 per formula unit. Will form C-C bonds to accommodate this.

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15
Q

Calculate VEC_A for NaCl, and comment on bonding.

A 
e_c = 1
e_a = 7

VEC_A = 1 + 7 = 8

Stable configuration.

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16
Q

Calculate VEC_A for ZnS, and comment on bonding.

A 
e_c = 2
e_a = 6

VEC_A = 2 + 6 = 8

Stable configuration.

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17
Q

Calculate VEC_A for Mg3N2, and comment on bonding.

A 
e_c = 2
e_a = 5

VEC_A = (32 + 52)/2 = 8

Stable configuration.

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18
Q

Calculate VEC_A for SF6, and comment on bonding.

A 
e_c = 6
e_a = 7

VEC_A = (61 + 76)/6 = 8

Stable configuration.

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19
Q

What is the likely accommodation mechanism(s) if there’s an excess of 1 electron at A?

A

C-C bonds will form in dumbbells.

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20
Q

What is the likely accommodation mechanism(s) if there’s an excess of between 1 and 2 electrons at A?

A

C-C bonds will form in finite chains.

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21
Q

What is the likely accommodation mechanism(s) if there’s an excess of 2 electrons at A?

A

C-C bonds will form in infinite chains, ring or the two electrons will stay on C as electron pairs.

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22
Q

What is the likely accommodation mechanism(s) if there’s an excess of 3 electrons at A?

A

Three C-C bonds will form, or one C-C bond and one electron pair.

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23
Q

What is the likely accommodation mechanism(s) if there’s a deficit of 1 electron at A?

A

A-A bonds will form in dumbbells.

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24
Q

What is the likely accommodation mechanism(s) if there’s a deficit of between 1 and 2 electrons at A?

A

A-A bonds will form in finite chains.

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25
Q

What is the likely accommodation mechanism(s) if there’s a deficit of 2 electrons at A?

A

A-A bonds will form in infinite chains or rings.

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26
Q

What is the likely accommodation mechanism(s) if there’s a deficit of 3 electrons at A?

A

A forms 3 2e-bonds with another A.

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27
Q

What is the likely accommodation mechanism(s) if there’s a deficit of 4 electrons at A?

A

A forms 4 2e-bonds with another A.

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28
Q

Calculate VEC_A for GaSe, and comment on bonding.

A 
e_c = 3
e_a = 6

VEC_A = (31 + 61) = 9

One excess electron on A. Will form dumbbell Ga-Ga-bonds.

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29
Q

Calculate VEC_A for CdSb, and comment on bonding.

A 
e_c = 2
e_a = 5

VEC_A = (21 + 51) = 7

One electron missing on A. Will form dumbbell Sb-Sb-bonds.

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30
Q

In some cases, e.g. CO, the 8-N rule differs from octet rule. Which one tends to prevail?

A

8-N rule.

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31
Q

What are Zintl phases?

A

Compounds consisting of cations from groups 1 or 2 and post-transition metals or metalloids (groups 13-16).

32
Q

What are typical physical characteristics of Zintl phases?

A

Narrow-gap semiconductors, medium difference in electronegativity. Anion is short of electrons.

33
Q

Calculate VEC_A for KGe, and comment on bonding.

A 
e_c = 1
e_a = 4

VEC_A = (11 + 41) = 5

Three electrons missing on A. Will form three 2 electron bonds to other Ge-bonds, forming cages.

34
Q

Calculate VEC_A for CaSi, and comment on bonding.

A 
e_c = 2
e_a = 4

VEC_A = (21 + 41) = 6

Two electrons missing on A. Will form infinite chain of Si-bonds.

35
Q

Calculate VEC_A for SrGa2, and comment on bonding.

A 
e_c = 2
e_a = 3

VEC_A = (21 + 32)/2 = 4

Four electrons missing on per anion. Will form four 2 electron bonds to other Ga-bonds, forming a two-dimensional sheet (like C-graphite). WHYYYYYYYY?

36
Q

Calculate VEC_A for NaTl, and comment on bonding.

A 
e_c = 1
e_a = 3

VEC_A = (11 + 31) = 4

Four electrons missing on per anion. Will form four 2 electron bonds to other Tl-bonds, forming a three-dimensional network (like C-diamond).

37
Q

When does the generalised 8-N rule not apply?

A

For metals, which occur if:

1) The difference in electronegativity is low, and metallic bonds form (delocalisation).
2) Metallic compounds form with electrons as “anions” = Electrides (e.g. Ca2N)

38
Q

What are some pitfalls when working with the generalised 8-N rule?

A
  1. Fractional VEC_A
  2. Fractional bond order
  3. Low difference in electronegativity (metallic compounds occur)
  4. Compounds that form electrides
39
Q

Calculate VEC_A for Ba5P4, and comment on bonding.

A 
e_c = 2
e_a = 6

VEC_A = (25 + 64)/4 = 8 1/2

Half an electron missing per A. Half of A will form dumbells (1-connected) and rest will not form any bonds (0-connected).

40
Q

Calculate VEC_A for NaSn2, and comment on bonding.

A 
e_c = 1
e_a = 4

VEC_A = (11 + 42)/2 = 4 1/2

Three and a half electrons missing from Sn. Half Sn will be 3-connected and half will be 4-connected. (form a kind of connected set of cages)

41
Q

Calculate VEC_A for BaTe3, and comment on bonding.

A 
e_c = 2
e_a = 6

VEC_A = (21 + 63)/3 = 6 2/3

One and 1/3 electron is missing per Te. 1/3 of Te will be 2-connected, 2/3 will be 1-connected.

42
Q

Calculate VEC_A for LaAs2, and comment on bonding.

A 
e_c = 3
e_a = 5

VEC_A = (31 + 52)/2 = 6 1/2

3/2 electrons is missing per As. Could either have 3/4 1-connected and 1/4 3-connected or 1/2 1-connected and 1/2 2-connected. Difference in low-temp and room-temp modifications of this as well. Should look this up in lecture notes.

43
Q

Calculate VEC_A for KI3, and comment on bonding.

A 
e_c = 1
e_a = 7

VEC_A = (11 + 73)/3 = 7 1/3

2/3 electrons are missing per I. 3 I must share 2 electrons.

44
Q

Name all alkali metals in descending order (Group 1).

A 
Lithium (Li)
Sodium (Na)
Potassium (K)
Rubidium (Rb)
Cesium (Cs)
Francium (Fr)
45
Q

Name all earth alkali metals in descending order (Group 2).

A 
Beryllium (Be)
Magnesium (Mg)
Calcium (Ca)
Strontium (Sr)
Barium (Ba)
Radium (Ra)
46
Q

Name all chalcogens in descending order (Group 16).

A 
Oxygen (O)
Sulphur (S)
Selenium (Se)
Tellurium (Te)
Polonium (Po)
Livermorium (Lv)
47
Q

Name all pnictogens in descending order (Group 15).

A 
Nitrogen (N)
Phosphorous (P)
Arsenic (As)
Antimony (Sb)
Bismuth (Bi)
Moscovium (Mc)
48
Q

Name all Carbon group elements in descending order (Group 14)

A 
Carbon (C)
Silicon (Si)
Germanium (Ge)
Tin (Sn)
Lead (Pb)
Flerovium (Fl)
49
Q

Name all Boron group elements in descending order (Group 13).

A 
Boron (B)
Aluminium (Al)
Gallium (Ga)
Indium (In)
Thallium (Tl)
Nihonium (Nh)
50
Q

Name all Group 12 elements in descending order.

A

Zink (Zn)
Cadmium (Cd)
Mercury (Mg)
Copernicum (Cn)

51
Q

Name all noble gasses in descending order (Group 18).

A 
Helium (He)
Neon (Ne)
Argon (Ar)
Krypton (Kr)
Xenon (Xe)
Radon (Rn)
Oganesson (Og)
52
Q

What is the definition of valence compounds?

A

Compounds in which in general all atoms either accept or provide and/or share valence electrons to obtain a stable octet configuration ns2np6 (fully filled s- and p-orbitals).

53
Q

What are ionocovalent bonds?

A

Bonds that are an intermediate between pure covalent and pure ionic bonds, i.e. bonds where electrons are not in the middle of two atoms, but closer to one of them.

54
Q

What are normal valence compounds?

A

A compounds CmAn where the number of valence electrons of the cations is just sufficient to complete the octets of the anions. No shared electrons.

55
Q

What does Pauling’s first rule say?

A

That for a coordination figure of a small cation surrounded by large anions, such as a tetrahedron, there exists a critical ratio for it to be stable, so that the anions does not touch.

56
Q

What does Pauling’s second rule say?

A

The spatial arrangement of the cations and anions should preferably be one where the anions receive the exact number of necessary valence elctrons from cations of the coordination polyhedron.

This could be interpreted as local electroneutrality.

The sum of all bond valences of all electrostatic bonds which originate from surrounding cations and reach a particular anion should be equal to the charge of the anion.

57
Q

What is the definition of a general valence compound?

A

These are valence compounds where either cations don’t transfer all their valence electrons (either through forming cation-cation bonds or as lone pairs) or anions don’t need as many electrons to complete their octet shells due to bonds between themselves.

58
Q

What is the number of electrons the m cations in CmAn transfer to the n anions?

A

m * (e_c - e_cc)

e_c being number of valence electrons of C
e_cc being average number of valence electrons per cation which remain with the cation

59
Q

What is the number of electrons the n anions need to complete their octet shells?

A

n * (8 - e_a - e_aa)

e_a being number of valence electrons of A
e_aa being average number of valence electrons per anion that the anions acquire by sharing covalent bonds between themselves.

60
Q

What is a polyanionic valence compound?

A

A valence compound with VEC_A < 8, with AA > 0

61
Q

What is a polycationic valence compound?

A

A valence compound with VEC_A > 8, with CC > 0.

62
Q

How does the difference in electronegativity influence whether a lone pair or a cation-cation bond is formed when VEC_A > 8?

A

With a large difference in electronegativity, a lone pair is less likely.

63
Q

What are tetrahedral structure compounds?

A

A subset of general valence compounds. Each atom has at most four niehgbours which are positioned at the corner of a surrounding tetrahedron

64
Q

What is the difference between normal tetrahedral structures and defect tetrahedral structures?

A

Normal tetrahedral structures have four neighbours, while defect tetrahedral structures have less.

65
Q

What is the equation for valence electron offer and need for tetrahedral compounds?

A

me_c + ne_a = 4(m+n) + (m + n) * N_NBO

N_NO = average non-bonding orbitals per atom

This can be reformulated to:

VEC = 4 + N_NBO

Where VEC is the total valence electron concentration = (me_c + ne_a) / (m+n)

66
Q

What does it say about a tetreahedral structure if VEC < 4?

A

That no tetrahedral structures are possible.

67
Q

What does it say about a tetrahedral structure if VEC = 4?

A

That we have a normal tetrahedral structure with no non-bonding orbitals.

68
Q

What does it say about a tetrahedral structure if VEC > 4?

A

That we have a defect tetrahedral structure with more than 0 non-bonding orbitals.

69
Q

What are the possible coordination symbols for tetrahedral structures?

A

4t, 3n, 2n and 1.

70
Q

For VEC ≥ 6, what can be said about the tetrahedral structure?

A

That the number of bonding orbitals is ≤ 2. This means that one has a non-cyclic molecule with a finite number of atoms.

71
Q

For a finite sp3-chain with Nm atoms in it, how many bonds are there in such a chain?

A

N_BO = 2(N_m-1)/N_m

=> N_m = 2/(VEC-6)

72
Q

Calculate VEC and VEC_A for PI2 and comment on structure.

A 
VEC = 6 1/3 →  N_NBO = 7/3
VEC_A = 9.5 → CC = 3

Crystal chemical formula:
[P_2 [(2;1)n] I4[1;] ]

Lone pair on I.

73
Q

Calculate VEC and VEC_A for ZnP2 and comment on structure.

A 
VEC = (2+2*5)/3 = 4 →  N_NBO = 0
VEC_A = 6 →  AA = 2

Zn[4t;]P2[(2;2)t]

74
Q

Calculate VEC and VEC_A for GaSe and comment on structure.

A

VEC = (6 + 3)/2 = 4 1/2 → N_NBO = 1/2
VEC_A (6+3)/1 = 9 → CC = 1

Ga2[(3;1)t]Se2[3n;]

75
Q

Calculate VEC and VEC_A for Si2Te3 and comment on structure.

A 
VEC = (2*4 + 3*6)/5 = 5 1/5 →  N_NBO = 6/5
VEC_A = (2*4 + 3*6)/3 = 8 2/3 →  CC = (2/3)*3/2 = 1

Si2[(3;1)t]Te3[2n;] with lone pair on Te.

76
Q

Calculate VEC and VEC_A for InTeCl and comment on structure.

A 
VEC = (1*3 + 1*6 + 1*7)/3 = 5 1/3 →  N_NBO = 1 1/3
VEC_A = (1*3 + 1*6 + 1*7)/2 = 8 →  AA = CC = 0

In[(2,2)t;]Te[2,;]Cl[2,;] with lone pair on Te and Cl

or

In[(3,1)t;]Te[3,;]Cl[1,;] with lone pair on Te and black square on Cl (??)

77
Q

Calculate VEC and VEC_A for Al7Te10 and comment on structure.

A 
VEC = (7*3 + 10*6) / 17 = 81/17 = 4 13/17 → N_NBO = 13/17
VEC_A = (7*3 + 10*6)/10 = 8 1/10 →  CC = 1/7

Al6[4t;]Al[(3;1)t]Te7[3n;]Te3[2n;]

Inorganic Structural Chemistry (13 decks)

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