02 Kémiai kötések
2014.04.09
Chemical Bondings
„First class” bondings: strong
• Metallic
• Covalent
• Ionic
Intermolecular forces: weaker
Metallic bonding
Electronegativity: a measure of how strongly an atom attracts electrons in a bond
Metal atoms can lose their outer electrons – electronegativity is low
Metallic lattice: metal ions and delocalised electrons
Metallic bond: electric attraction between ions and delocalised electrons
Good conductors of electricity and heat - Delocalised electrons can move easily
High density, high melting point -ions (and free electrons) are packed closely together, a lot of bonds in a space
Malleable and ductile - Ion layers can slide over each other easily
Covalent bonding
Atoms need full outer shells
H atom: 1 electron is needed
Solution: sharing
Covalent bonding: atoms are bonded by shared pair(s) of electrons
Molecular orbital: the space where an electron can be found in 90% of probability in a molecule
Simple bond: is located between the atoms - Sigma (σ) bond
Double bonds: second pair of electrons (π –bond) is located over and below the σ-bond – it is weaker
Triple bonds: 1 σ-bond and 2 π –bonds
Properties
Bond energy: the measure of bond strength in a chemical bond (kJ/mole)
Bond length: the average distance between the centres of the nuclei of two bonded atoms in a molecule
Bond angle: the angle formed between two bonds surrounding an atom in a molecule
Dative bonding
Bonding pair came from the same atom
Shape of molecules
• Depends on the number of electron
• pairs surrounding the central atom
• Electron pairs repel each other
• Lone pair needs more place than bonding pair
Shapes: Linear, Trigonal planar, Tetrahedral, Pyramidal, V-shaped
•
Bond polarity
Bond between similar atoms: electronegativity is the same – nonpolar bond
Bond between different atoms: electronegativity is different – polar bond
Polarity of molecules
It depends on the polarity of bonds and the shape of the molecule
• H – H:
nonpolar bond and molecule
• H – Cl:
polar bond, dipole molecule
• H2O:
polar bonds, dipole molecule
• CH4:
polar bonds and nonpolar molecule
Ionic bonding
Two ways to get full outer shell:
• To lose electron(s): make cation (+ charge), group 1-3
• To gain electron(s): make anion (- charge), group 5-7
Ionization energy: the energy required to remove electron(s) from 1 mole of gaseous atoms or ions
• in a group decreases (distance of electrons increases - attraction decreases)
• in a period increases (charge of the nucleus increases - attraction increases)
• Electron affinity
• Energy is required or released to gain electron(s) in 1 mole of gaseous atoms or ions
Changing of atom radius
• Losing electrons: atom radius decreases
• Gaining electrons: atom radius increases
• Ionic compounds
Are formed by metals and non-metals
Ionic lattice: anions and cations, neighbours have opposite charge
Ionic bonds: the electrostatic forces of attraction between oppositely charged ions
Lattice energy: is required to break down the lattice to ions
Intermolecular forces
Van der Waals’ forces
• The weakest intermolecular force
• Electron cloud is not symmetric
• Temporary dipoles induce a dipole in their neighbours
• Larger the molecule – stronger the VdW forces
• E. g. iodine, hydrogen, sulphur
Dipole-dipole forces
• Dipole molecules: electrostatic attraction between partially + and – parts
• Larger the attraction – higher the melting and boiling point
Hydrogen bonding
• H-O-H
• A partially + H atom
• A strongly negative O atom with lone electron pairs and high electronegativity
H-bonding: attraction between the partially positive H atom and the lone pair of the O (or F, N) atom
• Importance in Nature
Water melting point and boiling point is quite high: evaporation needs a lot of energy - thermoregulation
Water density: the highest at 4 oC
Density of ice is lower than density of water:
• Living creatures can survive, they are protected in winter
• Rivers can flow in winter too
• Ocean currents can work all over the world
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Lattice types |
Atomic lattice |
Ionic lattice |
Metallic lattice |
Molecular lattice |
|
Particles |
Atoms |
Ions |
Metal ions and delocalised electrons |
Molecules (or noble gas atoms) |
|
Type of attraction |
Covalent bonding |
Ionic bonding |
Metallic bonding |
Intermolecular forces (Van der Waals, dipol-dipol, Hydrogen bonding) |
|
Strength of attraction |
Strong |
Strong |
Strong (usually) |
Weak |
|
Hardness |
Hard |
Hard |
Hard (usually) |
Soft |
|
Melting point |
High |
High |
High (usually) |
Low |
|
Can conduct electricity? |
No (insulator) |
No (insulator) |
Yes (conductor) |
No (insulator) |
|
Can conduct heat? |
No (insulator) |
No (insulator) |
Yes (conductor) |
No (insulator) |
|
Solubility |
Insoluble |
In polar solvents |
Physically no (some react with water, e. g. Na) |
In polar or nonpolar solvents |
|
Types, examples, specialities |
Diamond, quartz |
Salts (NaCl, KF) |
Cubic body centered (Fe, Cr), Cubic face centered (Al, Ag, Cu, Ag), hexagonal (Zn, Mg) |
H2O, HCl, I2 |