Structure and Bonding Flashcards
Describe polar covalent bonds
Polar covalent bonds are formed when the attractions of the atoms for the pair of bonding electrons are different (they elements involved have different electronegativity values).
When this occurs, one atom will become delta positive (δ+ ) and the other delta negative (δ- ). The atom with the HIGHER electronegativity will be delta negative (δ- ) – this is because it is ‘pulling the electrons closer’
Not all substances with polar covalent bonds will be polar molecules. This is because the spatial arrangement of the atoms has an effect - if the molecule has a symmetrical arrangement of polar and non-polar bonds, the overall polarity is cancelled out. See diagrams below:
State how covalent bonds are formed
A covalent bonds occurs between non-metal elements.
It is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons.
Remember covalent bonds share electrons.
Describe how physical properties help identify the type of bonding in a compound
State at room temperature
Covalent networks - solid
Covalent molecular - gas or liquids
Ionic substances - solid
Melting point
Covalent molecular - low melting points
Covalent network - high melting points
Ionic bonds - high melting points
Solubility – solubility decreases as molecules increase in size. Polar covalent compounds and ionic substances tend to be soluble in water. (Remember ‘like dissolves like’).
Electrical conductivity – covalent bonds do not conduct (with the exception of carbon in the form of graphite), ionic bonds can conduct when molten or in solution (aq).
State the definition of the term intermolecular force and state the 3 types of intermolecular (van der Waals) force
Intermolecular forces act between molecules. These are known as van der Waals (vdW) forces.
London dispersion forces (LDFs)
Permanent dipole-permanent dipole (Pd-Pd) forces
Hydrogen bonding
Describe how London Dispersion Forces are formed and explain what effects their strength
LDFs can operate between all atoms and molecules. They are formed by the electrostatic attraction between temporary dipoles and induced dipoles which are caused by the movement of electrons in atoms and molecules (electrons are continuously moving, why we get LDFs between all atoms and molecules, even monatomic atoms).
LDFs can operate between all atoms and molecules. They are formed by the electrostatic attraction between temporary dipoles and induced dipoles which are caused by the movement of electrons in atoms and molecules (electrons are continuously moving, why we get LDFs between all atoms and molecules, even monatomic atoms).
Explain when a molecule is described as polar
Molecules are polar when they have a permanent dipole (one end of the molecule is permanently negative, other positive).
Describe a permanent dipole-permanent dipole interaction
Dipole-Dipole interaction is the attraction between oppositely charged ends of polar molecules:
Pd-Pd interactions are stronger than LDFs.
Pd-Pd interactions are in addition to LDFs (this will cause these to have higher melting and boiling points).
Describe a hydrogen bond
Hydrogen bonds are Pd-Pd interactions found between molecules which contain highly polar bonds.
They can form between:
Hydrogen – Nitrogen
Hydrogen – oxygen
Hydrogen – Fluorine
(NOF elements)
Exam questions may ask you to indicate where a hydrogen bond would be - remember to draw they occur between molecules.
Hydrogen bonds are the strongest intermolecular force.
When hydrogen bonds are present, melting and boiling points will be much higher than those other compounds of similar molecular size.
Explain how intermolecular forces of attraction affect a molecule’s physical properties
The stronger the intermolecular force of attraction, the higher the melting and boiling point and increasing viscosity.
Explain why ammonia and water have high boiling points
Ammonia and water have high boiling due to hydrogen bonding.
Explain how to predict the solubility of a compound
‘Like dissolves like’. This means that polar substances will be soluble in other polar solvents (so any polar substance tends to be soluble in water) and non-polar substances tend to be soluble in non-polar solvents.
Non-polar substances will not be miscible in water (the reason oil and water do not mix).
When thinking about solubility, consider whether if there is hydrogen bonding. Also, what is the spatial arrangement of polar covalent bonds?
