2020年高中化学竞赛路易斯共价键理论

1. Odd Number of Electrons
NO
Number of valence electrons = 11
NO
NO
Resonace Arrows
Resonance occurs when more than one valid Lewis structure can be written for a particular molecule (i.e. rearrange electrons)
O2
OO
OO
Oxygen is a ground state "diradical"
Radicals and Biradicals
Species having electrons with unpaired spins are called radicals.
One example is the methyl radical, CH3, which is so reactive that it cannot be stored. Radicals are of crucial importance for the chemical reactions that take place in the upper atmosphere, where they contribute to the formation and decomposition of ozone. A biradical is a molecule with two unpaired electrons.
Cl
H
Cl Al + N H
Cl
H
Lewis acid Lewis base
Cl
Cl_ Al
H N+ H
Cl H
salt
AlX3
Aluminium chloride is an ionic solid in which Al3+ is surrounded by six Cl-. However, it sublimes at 192 °C to vapour Al2Cl6 molecules
Elements from the third Period and beyond, have ns, np and unfilled nd orbitals which can be used in bonding
PCl5
P : (Ne) 3s2 3p3 3d0
Number of valence electrons = 5 + (5 x 7) = 40
and a non-metal anion
Covalent Bonding results from the sharing of two electrons between two atoms
(usually non-metals) resulting in molecules
H +H
Octet Rule applies
HH
number of electrons around each atom = He
Cl + Cl N+ N
Cl Cl
number of electrons around each atom = Ar
NN
Triple bond
number of electrons around each atom = Ne
10 electrons around the phosphorus
Cl
Cl Cl
P
SF4
Cl Cl
S : (Ne) 3s2 3p4 3d0 Number of valence electrons = 6 + (4 x 7) = 34
The Larger the central atom, the more atoms you can bond to it – usually small atoms such as F, Cl and O allow central atoms such as P and S to expand their valency.
bonds
PBiblioteka Baidul3 Total Number of valence electrons = 5 + (3 x 7) = 26
Cl P Cl Cl
Cl P Cl Cl
Cl P Cl Cl
CHBr3
Total Number of valence electrons = 4 + 1 + (3 x 7) = 26
B2H6
A Lewis structure cannot be written for diborane. This is explained by a three-centre bond – single electron is delocalized over a B-H-B
HH
H
BB
HHH
3. More than an Octet
One of the most important examples is the oxygen atom itself. Its electron configuration is [He]2s22px 22py 12pz 1 and its Lewis symbol is . The O atom has two unpaired electrons, and so it can be regarded as a special type of biradical.
Rules for Drawing Lewis Structures
• First sum the number of valence electrons from each atom • The central atom is usually written first in the formula • Complete the octets of atoms bonded to the central atom (remember that H can
NO2
Number of valence electrons = 17
ONO
ONO
ONO
Molecules and atoms which are neutral (contain no formal charge) and with an
unpaired electron are called Radicals
Carbon has 4 valence electrons
H HCH
H methane
H HCH
H
HC
Ne Neon
Stable Octet required
Covalent Bonding – Atoms Share Electrons
Hydrogen molecule, H2
第 2 节 共价键理论
一 路易斯理论 1916 年，美国科学家 Lewis 提出共价键理论。认为分子中
的原子都有形成稀有气体电子结构的趋势，求得本身的稳定。 而 达到这种结构，可以不通过电子转移形成离子和离子键来完成， 而是通过共用电子对来实现。
例如 H • + • H = H •• H 通过共用一对电子，每个 H 均 成
Concentration of negative charge between two nuclei occurs in a covalent bond
7A elements (e.g. F) have one valence electron for covalent bonding, so to achieve octet 6A elements (e.g. O) use two valence electrons for covalent bonding, so to achieve octet 5A elements (e.g. N) use three valence electrons for covalent bonding, so to achieve octet 4A elements (e.g. C) use four valence electrons for covalent bonding, so to achieve octet
Each Covalent Bond contains two electrons
Carbon has 4 valence electrons
H HCH
H methane
H HCH
H
HC
Ne Neon
Stable Octet required
Covalent Bonding – Atoms Share Electrons
可以通过共用电子对形成分子，共价键，共价分子。 八隅体规则
Lewis Symbols
Represent the number of valence electrons as dots Valence number is the same as the Periodic Table Group Number
2. Less than an Octet
Includes Lewis acids such as halides of B, Al and compounds of Be
BCl3
Cl
Group 3A atom only has six electrons around it
B
Cl
Cl
However, Lewis acids “accept” a pair of electrons readily from Lewis bases to establish a stable octet
Ne
Atoms tend to gain, lose or share electrons until they are surrounded by 8 electrons
Octet Rule
Nobel Gas Has a Stable Electron Configuration
Ne; 1s2, 2s2, 2p6 Ar; [Ne] 3s2, 3p6
Br Br C H
Br
共价键八隅体规则的特例
1. Molecules with an odd number of electrons 2. Other Natural Radicals, which do not obey Lewis Structures (e.g.
O2) 2. Molecules in which an atom has less than an octet 3. Molecules in which an atom has more than an octet
H
He
Li Be B C N O F Ne
Groups 1
2
3
4
5
6
7
8
For example,
Na; Is2, 2s2, 2p6, 3s1 = [Ne] 3s1
Lewis Structure = Na
Elements want to achieve the stable electron configuration of the nearest noble gas
为 H又e 如的电子H构型＋，形C成l 一个共价键。H Cl
Lewis 的贡献，在于提出了一种不同于离子键的新的键型， 解释了 X 比较小的元素之间原子的成键事实。
:: ::
:: :: ::
1、Lewis 符号 H He: K
2、Lewis 符号和离子键 :Cl+ Ca: + :Cl
:Cl [:Cl:]-Ca2+ [:Cl:]-
only have two electrons) • Place any left over electrons on the central atom, even if doing so it results in
more than an octet • If there are not enough electrons to give the central atom an octet , try multiple
Ar
Example of Ionic Bonding
Na + F
11
9
Na+ +
[
F
_ ]
10
Electronic configuration of Neon achieved in both cases
There are two types of bonding;
Ionic Bonding refers to electrostatic forces between ions, usually a metal cation