Structure and Bonding Flashcards
What is a covalent bond?
In a covalent bond atoms share pairs of electrons. The covalent bond is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons.
What are polar covalent bonds?
- Polar covalent bonds are formed when electronegativity is different.
- δ+ and δ- notations can be used to indicate the partial charges on an atom. The atom with the higher electronegativity value is δ-.
What are non-polar (pure) covalent bonds?
If electrons are distributed evenly across the molecule the molecule is non-polar.
What are ionic bonds?
- Ionic bonds are the electrostatic attraction between positive and negative ions.
- Ionic compounds form lattice structures of oppositely charged ions.
- Compounds formed between metals and non-metals are often, but not always, ionic.
What is ionic character?
- The difference in electronegativity values between bonded atoms gives an indication of the ionic character.
- The larger the difference, the more polar the bond will be.
- If the difference in electronegativity is great, then the movement of electrons will be complete, resulting in the formation of ions.
What are the properties of ionic compounds?
- High melting and boiling points
- Mostly soluble in water
- Electrically conductive as a liquid but not as a solid.
What are London Dispersion Forces?
- LDFs are forces of attraction that can operate between all atoms and molecules.
- They are formed as a result of electrostatic attractions between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.
- The strength of LDFs is related to the number of electrons within an atom or molecule. The more electrons present the higher attractive force of the LDF.
What are permanent dipole permanent dipole interactions?
- Permanent dipole interactions are additional electrostatic forces of attraction between polar molecules.
- A molecule is polar if it has a permanent dipole.
What molecular shapes are polar (non symmetrical)?
- Angular (X2Y)
- Trigonal pyramidal (XY3)
What molecular shapes are non-polar (symmetrical)?
- Tetrahedral (XY4)
- Linear (XY)
What are hydrogen bonds?
- Hydrogen bonds are electrostatic forces of attraction between molecules that contain highly polar bonds.
- This a special type of permanent dipole interaction.
- The highly polar bonds are the result of a hydrogen atom bonded to an atom of a strongly electronegative element (N.O.F)
What is the order of strength for Van der Waal’s forces?
- LDFs (much weaker)
- Pd-pd interactions (stronger than LDFs for molecules with similar numbers of electrons.)
- Hydrogen bonds (stronger than other forms of pd-pd interaction but weaker than covalent bonds.)
Describe the effect of Van der Waal’s forces on viscosity
- Substances with only LDFs require very little energy to break so the substance will have a low melting point, a low boiling point and will not be very viscous. The more electrons a substance has, the more LDFs present; this will result in a higher melting or boiling point, and will be more viscus than a substance with fewer electrons.
- The attraction of pd-pd interactions in polar molecules will require more energy to break and so melting and boiling points
will be higher and the substance will be more viscous. - Substances with hydrogen bonding will require even more energy to break the intermolecular bond and so melting and boiling points are much higher and the substance is even more viscous.
Describe the effect of hydrogen bonding on water, ammonia and hydrogen fluoride.
- The high melting and boiling points of water (H2O), ammonia (NH3) and hydrogen fluoride (HF) is due to the presence of hydrogen bonding.
- Hydrogen bonding between molecules in ice results in an expanded structure that causes the density of ice to be less than that of water at low temperatures.
Describe the effect of polarity on solubility
- Ionic compounds and polar molecular compounds are soluble in polar solvents and insoluble in non-polar substances. Non-polar substances tend to be soluble in non-polar solvents.
- “like dissolves like”.
