Chemistry From Different Angles Quiz

Answer: Atoms of low atomic number react to achieve 8 electrons in their outermost shell

For elements of low atomic number (less than 20), their outermost electron shell is full when it contains eight electrons, and this is its most stable form. One way of achieving a full outer shell of electrons is to simply "borrow" electrons from other elements.

For example, fluorine reacts and gains one electron to add to the seven it already has to make eight, giving it a full outer shell and an overall negative charge. Forming chemical bonds is another common way of achieving a full outer shell of electrons, as the electrons in these bonds are shared between the two entities involved.

Answer: Lone pairs

Electrons "like" to exist as pairs and are more stable this way. Unpaired electrons are a defining characteristic of free radicals, which are highly reactive.

Answer: The Valence Shell Electron Pair Repulsion Theory

The Valence Shell Electron Pair Repulsion Theory (VSEPR theory) describes how electron pairs repel one another and so push each other as far away as possible. Lone pairs repel each other more than they repel bonding pairs, and lone pairs repel bonding pairs more than bonding pairs repel one another.

This rationale can be used to predict bond angles of molecules based on the number of bonding pairs and lone pairs about the central atom.

Answer: The wedge means the atom is coming towards you. The dashed line means the atom is going away from you

Methane (CH4, as pictured) has four bonding electron pairs, which repel one another equally. However, the molecule is not 2-dimensional, and so the bond angle is not 90 degrees, as one may expect if the molecule were 2-dimensional (360/4). Instead, because methane is 3-dimensional, the bonding pairs can make use of all of the space around the central carbon atom in order to get as far away as possible from the other bonding pairs.

Answer: 180 degrees

In VSEPR theory, double (and triple) bonds are usually treated as single bonds. Therefore, there are essentially two bonding pairs about the central carbon atom, and no lone pairs. The maximum angle between two mutually repulsive electron pairs in 3D space is 180 degrees, and so that is what we see in carbon dioxide.

Answer: Trigonal planar

The logic here is relatively simple and is similar to the carbon dioxide example. By knowing that the bonds are all in plane with the screen, and that there are three mutually repulsive bonding pairs of electrons, we can deduce that the bond angle will be 120 degrees (360/3), since this gives maximum space between the pairs and is therefore the most stable form.

Answer: Tetrahedral

Though not stated explicitly in the question, carbon has four valence electrons and so tends to form four covalent bonds in order to achieve a full outer shell of electrons. The central carbon here therefore has four bonding pairs of electrons and no lone pairs.

As seen by the wedges and dashed lines, this should also tell you that the bonds are not all in plane with the screen and so the same simplistic logic used for the previous two questions cannot be used. Instead, the bonds are in different planes, and the maximal bond angle that is possible is 109.5 degrees.

Answer: The bond angle will be less than 109.5 degrees

Nitrogen has five valence shell electrons and so only needs to form three covalent bonds in order to achieve a full outer shell of eight electrons, with the remaining two electrons existing as a lone pair. The lone pair (as opposed to a bonding pair) is the main difference between ammonia and methane, and the lone pair basically squashes the bonding pairs closer together than they would be in methane.

This gives a bond angle of 107 degrees and a molecular shape described as pyramidal.

Answer: Octahedron

The bonds about the central sulphur atom define the vertices of an octahedron, hence the term octahedral for this molecular shape. I feel this warranted an explanation as I was always a little confused why sulphur hexafluoride had the prefix "octa-" despite having six bonds.

The more eagle-eyed may have noticed that sulphur has six bonding pairs of electrons and so has twelve electrons in its valence shell, which may seem at first to disobey the octet rule. The explanation here is that sulphur is relatively large (its atomic number is greater than 20) and so it can accommodate more than eight electrons in its outermost shell.

Answer: Bent

Water may also be described as "V-shaped". In accordance with the VSEPR theory, the two lone pairs about the oxygen atom repel each other more than they repel the bonding electron pairs (which link the oxygen to the hydrogen atoms), which in turn is greater than the repulsion experienced between the two bonding pairs. The resultant bond angle is 104.5 degrees.