3.1.3 Bonding Flashcards
What happens in ionic bonding?
Metal atoms lose electrons to form +ve ions. Non-metal atoms gain electrons to form -ve ions.
What is the structure and bonding of ionic substances?
Structure: Giant ionic lattice.
Bonding: Strong electrostatic forces between oppositely charged ions.
What are the properties of ionic substances? (bp/mp, conductivity, solubility)
Boiling and melting points: High - giant ionic lattice with strong electrostatic forces between oppositely charged ions.
Conductivity when solid: Poor - ions can’t move/fixed in lattice.
Conductivity when molten: Good - ions free to move and carry a charge through the structure.
Solubility in water: Generally good.
What factors affect the strength of ionic bonding?
- Ionic radius - the smaller the ion, the stronger the bond.
- Charge on ions - the higher the charge, the stronger the bond.
Why are positive ions smaller compared to their atoms?
What are negative ions larger compared to their atoms?
Positive ions are smaller compared to their atoms because they have one less shell of electrons and the ratio of protons to electrons has increased so there is greater net force on remaining electrons holding them more closely.
The negative ions formed from groups five to seven are larger than the corresponding atoms. The negative ion has more electrons than the corresponding atom but the same number of protons. So the pull of the nucleus is shared over more electrons and the attraction per electron is less, making the ion bigger.
What happens in covalent bonding?
Electron pairs are shared.
What are the 2 structures of covalent bonding?
- Simple molecular
- Macromolecular (e.g. diamond, graphite)
What are the properties of simple molecular substances?
Boiling and melting points: Low - weak intermolecular forces between molecules (specify type e.g van der waals/hydrogen bond).
Conductivity: Poor - no ions & no delocalised electrons to carry a charge through the structure.
Solubility in water: Generally poor.
What are the properties of macromolecular substances?
Boiling and melting points: High - many strong covalent bonds take a lot of energy to overcome.
Conductivity in diamond: Poor - no delocalised electrons.
Conductivity in graphite: Good - delocalised electrons between layers free to carry charge through structure.
Solubility in water: Insoluble.
Diamond is hard - each carbon atom bonded to 4 others by strong covalent bonds.
Graphite is soft - structured in layers with fairly weak forces between layers, can easily slide over one another.
What is a dative covalent bond?
Forms when the shared pair of electrons in the covalent bond come from only one of the bonding atoms. One atom donates both of the electrons in the bonding pair.
Also called co-ordinate bonding.
What happens in metallic bonding?
Positive metal ions and their delocalised electrons are attracted to each other.
What is the structure and bonding of metallic substances?
Structure: Giant metallic lattice.
Bonding: Strong electrostatic forces between positive metal ions and delocalised electrons.
What are the properties of metallic substances?
Boiling and melting points: High- strong electrostatic forces between positive ions & sea of delocalised electrons.
Conductivity: Good: delocalised electrons free to move and carry charge through structure
Solubility in water: Insoluble.
Malleable - the positive ions in the lattice are identical, the planes of ions can slide easily over one another.
What factors affect the strength of metallic bonding?
- Nuclear charge (number of protons) - The more protons the stronger the bond.
- Number of delocalised electrons per atom (the outer shell electrons are delocalised) - The more delocalised electrons the stronger the bond.
- Size of ion - The smaller the ion, the stronger the bond.
What is the shape & bond angle of a molecule with 2 bonding pairs & 0 lone pairs?
Linear
180 degrees
What is the shape & bond angle of a molecule with 3 bonding pairs & 0 lone pairs?
Trigonal planar
120 degrees
What is the shape & bond angle of a molecule with 4 bonding pairs & 0 lone pairs?
Tetrahedral
109.5 degrees
What is the shape & bond angle of a molecule with 3 bonding pairs & 1 lone pair?
Trigonal pyramidal
107 degrees
What is the shape & bond angle of a molecule with 2 bonding pairs & 2 lone pairs?
Bent
104.5 degrees
What is the shape & bond angle of a molecule with 5 bonding pairs & 0 lone pairs?
Trigonal bipyramidal
120 & 90 degrees
What is the shape & bond angle of a molecule with 6 bonding pairs & 0 lone pairs?
Octahedral
90 degrees
How do you explain a shape of a molecule in an exam question?
- State number of bonding pairs and lone pairs of electrons.
- State that electron pairs repel and try to get as far apart as possible (or to a position of minimum repulsion.)
- If there are no lone pairs state that the electron pairs repel equally
- If there are lone pairs of electrons, then state that lone pairs repel more than bonding pairs.
- State actual shape and bond angle.
(Remember lone pairs repel more than bonding pairs and so reduce bond angles by about 2.5° per lone pair in previous examples)
What is electronegativity?
The relative tendency of an atom in a molecule to attract a pair of electrons in a covalent bond to itself.
Measured on the Pauling scale (0 to 4)
What are the most electronegative atoms?
F, O, N & Cl are the most electronegative atoms.
The most electronegative element is fluorine & its given a value of 4.0 on the Pauling scale.
What factors affect electronegativity?
- Atomic radius
- Nuclear charge
- Shielding
What are the general trends in electronegativity across a period & down a group?
Across a period:
Electronegativity increases
- Number of protons increases = increased nuclear charge.
- Atomic radius decreases - electrons in the same shell are pulled in more.
- No difference in shielding.
Down a group:
Electronegativity decreases
- Nuclear radius increases.
- Shielding increases.
- Despite an increase in nuclear charge.
How does electronegativity relate to the bonding type in molecules?
lonic and covalent bonding are 2 extremes of a scale (not 2 separate distinct bonding types) - some molecules may have both ionic and covalent character.
Differences in electronegativity between elements can determine where a compound lies on this scale:
- A compound containing elements of similar electronegativity & hence a small electronegativity difference (< 0.5) will be purely covalent
- A compound containing elements of very different electronegativity & hence a very large electronegativity difference (> 1.7) will be ionic
- Compounds with an electronegativity difference between these will either be polar covalent or polar ionic.
What is a polar covalent bond?
What is a dipole?
A polar covalent bond forms when the elements in the bond have a difference in electronegativities.
When a bond is a polar covalent bond it has an unequal distribution of electrons in the bond which causes a slight charge difference, inducing a dipole in the molecule.
(The element with the larger electronegativity in a polar compound will be the partially negative end.)
Dipole - a permanent or temporary difference in charge between 2 atoms in a covalent bond.
What else, other than polar covalent bonds, determines whether a molecule is net polar or not?
Symmetry of the molecule
A symmetric molecule (all bonds identical & no lone pairs) will not be polar even if individual bonds within the molecular are polar.
The individual dipoles on the bonds ‘cancel out’ due to the symmetrical shape of the molecule.
There is no net dipole moment: the molecule is non-polar.
What are the 3 types of intermolecular force?
- Van der Waals’ forces
- Permanent dipole-dipole forces
- Hydrogen bonding
What are Van der Waals’ forces?
These are also called induced dipole-dipole interactions.
They occur between all simple covalent molecules & the separate atoms in noble gases.
- In any molecule the electrons are moving constantly and randomly.
- As this happens the electron density can fluctuate & parts of the molecule become more or less negative i.e. small temporary dipoles form.
- These instantaneous dipoles can cause dipoles to form in neighbouring molecules (induced dipoles). The induced dipole is always the opposite sign to the original one.
- A temporary dipole forms between the partially +ive charge in one molecule and the partially -ive charge in a neighbouring molecule.
What factors affect the size of Van der Waals’ forces?
- The amount of electrons - the more electrons there are in the molecule the higher the chance that temporary dipoles will form. This makes the Van der Waals stronger between the molecules.
(The increasing boiling points of the halogens down the group 7 series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the Van der Waals between the molecules. This is why l2 is a solid whereas Cl2 is a gas.)
(The increasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the Van der Waals between molecules.)
- The shape of the molecule - long chain alkanes have a larger surface area of contact between molecules for Van der Waals to form than compared to spherical shaped branched alkanes & so have stronger Van der Waals.
What are permanent dipole-dipole forces?
- Occur between polar molecules.
- Stronger than Van der Waals & so the compounds have higher boiling points.
- Polar molecules have a permanent dipole.
- Polar molecules are asymmetrical & have a bond where there is a significant difference in electronegativity between the atoms.
Occurs in addition to vdw.
What is hydrogen bonding?
- Occurs in compounds that have a hydrogen atom attached to one of the 3 most electronegative atoms of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons (e.g. a -O-H -N-H F-H bond).
- There is a large electronegativity difference between the H & the O, N, F.
- Strongest IMF.
Occurs in addition to vdw.
