EL5 - Covalent Bonding and Shapes of Molecules Flashcards
The molecules of life
What is a dative covalent bond?
- What is the difference relative to a covalent bond?
A bond where there is one pair of electrons shared between two atoms.
- In a dative covalent bond, these electrons both come from one atom (rather than one electron from each) e.g. the ammonium ion, NH4^+.
What is a lone pair?
A pair of valence electrons that are not shared with another atom in a covalent bond.
Unshared pair.
Non-bonding pair.
How are individual ions held together to form ionic compounds?
By the electrostatic forces between the charge distribution
NOTE:
Many of the substances found in gas clouds (breakup of old stars) contain carbon atoms bonded to elements other than just oxygen – they are organic species. These elements are major constituents of the human body.
What experiment suggests that life on Earth could have originated from the molecules in the dense gas clouds in outer space?
Miller (1950)
put methane, ammonia, carbon dioxide and water (simple molecules like those present in the dense gas clouds) into a flask.
He heated and subjected the mixture to an electrical discharge to stimulate the effect of lightning.
Miller found that some of the reaction mixture had been converted into amino acids - the building blocks of proteins.
Why don’t non-metallic elements react to form ions?
It’s not energetically favourable to form ions.
How is a dative covalent bond presented?
With an arrow, which points away from the atom that donates the pair of electrons.
Why do elements and compounds with a simple molecular structure have relatively low melting and boiling points?
The covalent intramolecular bonds within simple molecules such as water and ammonia are strong, however the electrostatic attractions between different simple molecules are weak.
This means relatively small amounts of energy are needed to separate one molecule from another.
Therefore, elements and compounds with a simple molecular structure have relatively low melting and boiling points.
What does there being no charged particles in simple covalent molecules mean?
Covalent simple molecules do not conduct electricity.
They mostly don’t dissolve readily in water.
What is the bond angle in a tetrahedral shape?
Give an example of a molecule with this shape.
109.5 degrees
Methane, CH4
Ethane
Ammonium ion, NH4^+
What is the shape of ammonia?
Pyramidal
What is the shape of water?
Bent
What decides the shape of covalent molecules?
The pairs of electrons in the outer shell repelling one another and moving as far apart as possible.
(Doesn’t matter if bonding or lone pairs)
Which repel more more strongly:
bonding pairs or lone pairs?
Lone pairs.
NOTE:
This explains the different bond angles observed in methane, ammonia and water even though all three molecules have four groups of electrons around a central atom.
BeCl2 is a linear molecule.
Explain the shape, and why it differs to methane.
Give another example of a linear molecule.
In BeCl2 there are two groups of electrons around the central atom.
Because there are fewer groups of electrons than in methane, they can get further apart.
The furthest apart they can get is at an angle of 180 degrees, so BeCl2 is linear with a
Cl – Be – Cl angle of 180 degrees.
Another example of a linear molecule is carbon dioxide as there are only two groups of electrons around the central atom.
BF3 is a planar molecule.
Explain the shape.
Give another example of a planar molecule.
In BF3 there are three groups of electrons around the central atom.
The F – B – F bond angle is 120 degrees.
BF3 is flat and shaped like a triangle with the boron atom at the centre and the fluorine atoms at the corners. It is described as a trigonal planar.
Other examples of planar structures are methanal and ethane.
In methanal there are three groups of electrons around the carbon atom, and in ethene both carbon atoms have three groups of electrons.
There are four pairs of electrons around the carbon atoms in these molecules, but the bonds are not tetrahedrally directed because there are only three groups of electrons.
