chemical bonds example: electronic interaction between two atoms

Transcription

1 chemical bonds example: electronic interaction between two atoms 3 =? 3 problem: how can a chemical bond connect two atoms and overcome the electrostatic repulsion?

2 3 solutions nature has found to bind atoms: covalent bond ionic bond metallic bond

3 1 st solution: the ionic bond a single electron (or several electrons) is (are) completely transferred from one atom to the other, forming ions: anion cation 3 electrostatic attraction 3

4 the ionic bond typical representatives: typical structures: typical properties: most compounds of elements of the groups I a and II a with elements of the groups VI a and VII a of the periodic table ion lattices (solid state) or ion pairs (gas state), in solution: hydrated ions often dissolvable in water under dissociation, high melting and evaporation temperatures, solutions and melts show electrical conductivity, geometry of ion pairs and ion lattices determined by the radius of the ions, crystals are always very brittle (easy to brake by deformation);

5 periodic table of elements (groups I a to VIII a and groups I b to VIII b ) H Li Be Na Mg K Ca Rb Sr Cs Ba Sc Ti Y Zr La Hf V Cr Nb Mo Ta W Mn Fe Tc Ru Re Os Co Ni Cu Zn B C N O Al Si P S Ga Ge As Se He F Ne Cl Ar Br Kr Rh Pd Ag Cd In Sn Sb Te I Xe Ir Pt Au Hg Tl Pb Bi Po At Rn I a II a III a IV a V a VI a VII a VIII a III b IV b V b VI b VII b VIII b I b II b lanthanides and actinides

6 2 nd solution: the covalent bond one or several electrons are shared between two or more atoms 3 electrostatic attraction 3

7 the covalent bond typical representatives: typical structures: typical properties: compounds between two or more elements of the groups III a, IV a und V a covalent bonds lead to a large variety of geometrical structures formed by two or more atoms up to macromolecules high mechanical strength possible (diamond), low electrical conductivity, molecular geometry determined by geometry of the bonds, small molecules easily evaporated at low temperatures, in the solid phase they often form molecular crystals;

8 periodic table of elements (groups I a to VIII a and groups I b to VIII b ) H Li Be Na Mg K Ca Rb Sr Cs Ba Sc Ti Y Zr La Hf V Cr Nb Mo Ta W Mn Fe Tc Ru Re Os Co Ni Cu Zn B C N O Al Si P S Ga Ge As Se He F Ne Cl Ar Br Kr Rh Pd Ag Cd In Sn Sb Te I Xe Ir Pt Au Hg Tl Pb Bi Po At Rn I a II a III a IV a V a VI a VII a VIII a III b IV b V b VI b VII b VIII b I b II b lanthanides and actinides

9 3 rd solution: the metallic bond electrons move freely in a regular lattice formed by one or more types of metal cations (example: lithium):

10 the metallic bond typical representatives: typical structures: elements from groups I a and II a as well as all transition elements small number of structural varieties depending on the radius of the metal atoms (or ions) typical properties: shiny metallic appearance, high electrical and thermal conductivity, high variability of melting and boiling points, high ductility (easily being brought into a given shape by external force);

11 periodic table of elements (groups I a to VIII a and groups I b to VIII b ) H Li Be Na Mg K Ca Rb Sr Cs Ba Sc Ti Y Zr La Hf V Cr Nb Mo Ta W Mn Fe Tc Ru Re Os Co Ni Cu Zn B C N O Al Si P S Ga Ge As Se He F Ne Cl Ar Br Kr Rh Pd Ag Cd In Sn Sb Te I Xe Ir Pt Au Hg Tl Pb Bi Po At Rn I a II a III a IV a V a VI a VII a VIII a III b IV b V b VI b VII b VIII b I b II b lanthanides and actinides

12 In reality, chemical bonds in real compounds exhibit different contributions of the ideal bond characteristics. Therefore, they have to be attributed to various positions in a bond triangle : covalent bond HCl NaCl NaK ionic bond metallic bond

13 theoretical models for covalent bonds: 1) ValenceBond(VB)theory Bonds are formed by the overlap between atomic orbitals which are occupied by single electrons. Only a small number of bond electrons contribute to the bond. 2) Molecular Orbital(MO)theory All electrons of the two atoms are organized in common molecular orbitals around the nuclei and contribute to binding forces. The original atomic orbitals do not exist any more.

14 VBTheory example: water H 2 O oxygen: hydrogen: water: 4 pelectrons 1 selectron 2 e 2 e e e e e e e e problem: bond angle (90 instead of 105!)

15 atomic number name symbol n = 1 n = 2 l = 0 n = 2 l = 1 n = 3 l = 0 1 hydrogen H 1 2 helium He 2 3 lithium Li beryllium Be boron B carbon C nitrogen N oxygen O fluorine F neon Ne sodium Na magnesium Mg aluminum Al n = 3 l = 1

16 VBTheory example: water H 2 O oxygen: hydrogen: water: 4 pelectrons 1 selectron 2 e 2 e e e e e e e e problem: bond angle (90 instead of 105!)

17 VBTheory example: methane CH 4 How is methane being formed? We try CH 2 : carbon: hydrogen: CH 2 2 pelectrons 1 selectron e e e e e e e problems: CH 2 instead of CH 4, bond angle (90 instead of 109!)

18 atomic number name symbol n = 1 n = 2 l = 0 n = 2 l = 1 n = 3 l = 0 1 hydrogen H 1 2 helium He 2 3 lithium Li beryllium Be boron B carbon C nitrogen N oxygen O fluorine F neon Ne sodium Na magnesium Mg aluminum Al n = 3 l = 1

19 VBTheory example: methane CH 4 How is methane being formed? We try CH 2 : carbon: hydrogen: CH 2 2 pelectrons 1 selectron e e e e e e e problems: CH 2 instead of CH 4, bond angle (90 instead of 109!)

20 solution in the VB theory: hybridization hybridized carbon: carbon: methane CH 4 : 2s, 2p sp 3 hybrid e e e e 2 e e e e e tetrahedron, bond angle 109

21 theoretical models for covalent bonds: 1) ValenceBond(VB)theory Bonds are formed by the overlap between atomic orbitals which are occupied by single electrons. Only a small number of bond electrons contribute to the bond. 2) Molecular Orbital(MO)theory All electrons of the two atoms are organized in common molecular orbitals around the nuclei and contribute to binding forces. The original atomic orbitals do not exist any more.

22 MOtheory example: water 1) estimated positioning of nuclei (first approximation) 2) calculation of molecular orbitals based on the given position of the nuclei 8 8

23 MOtheory example: water 3) optimization of the position of the nuclei based on the given molecular orbitals 4) calculation of new molecular orbitals based on the new position of the nuclei repeat step 3, step 4, step 3,... etc.

24 electronegativity and polarity: Elements differ in their tendency to attract the electrons in a covalent bond. This tendency is called electronegativity. Generally, the electronegativity increases from left to right and from the bottom to the top in the periodic table. All elements of groups VI a and VII a show high electronegativity. The highest electronegativity is found for the element flourine (F). If a chemical bond connects two elements of different electronegativity, it is called a polar bond. In this case, the more electronegative element adopts a partial negative charge, the less electronegative element a partial positive charge. examples: H Cl H F H O H δ δ δ δ δ δ δ