Molecular Models Trivia Quiz

Answer: two

Because each oxygen atom has 6 electrons in its bonding shell, it needs to gain two more to reach a stable configuration. If it combines with metallic atoms, it can do this by grabbing electrons that the metal atoms are happy to give away, forming an ionic compound. When it reacts with magnesium, for example, each magnesium atom gives its two outer electrons to an oxygen atom, forming magnesium ions with a charge of +2 and oxide ions with a charge of -2; their opposite charges mean there is a strong attractive force between them, and the compound magnesium oxide is formed.

When oxygen forms molecules with non-metallic substances (including the formation of molecules with other oxygen atoms), they form covalent bonds. As the name suggests, this involves sharing of electrons, so that each atom has a part-time ownership of the shared electrons. When two oxygen atoms combine to form a molecule of oxygen, there are two shared electrons; this is called a double bond. Diagrams using letters to represent atoms and lines to represent covalent bonds would resemble O=O.

Answer: water

The incorrect options are names that could be used according to the IUPAC guidelines for systematic naming of compounds, but they are rarely used, even by chemists, who just call it water unless there is some special reason for another name being used. For example, calling it hydric acid or hydrogen hydroxide could be appropriate in a discussion of alternative definitions of acids and bases.

The model in the diagram uses balls to represent the atoms of different elements, with standard (although not universal) colour coding. This allows the covalent bonds formed when the atoms share electrons to be represented by sticks, which is useful for more complex molecules when there may be more than one pair of electrons being shared between a pair of atoms. It also allows the shape of the molecule to be clearly shown, with the exact angle between the two hydrogen molecules (104.45 degrees) more obvious that would be the case for a space-filling model.

This model goes one step further than a simple ball-and-stick model, and adds a van der Waals surface. This cloud surrounding the molecule shows how it will interact with other molecules to form a liquid or solid. It is coloured red near the part of the molecule which is slightly negative due to unequal sharing of the bonding electrons, and blue near the slightly positive regions. This polarity of the water molecule is responsible for many of the unique properties of the substance, which are far too numerous to explain here.

Answer: carbon

The study of carbon compounds is called organic chemistry, because so many of the molecules are found in living organisms (or their remnants, in the case of fossil fuels). The simplest of these are the hydrocarbons, which contain only carbon and hydrogen. Sounds simple, but because carbon can form covalent bonds in a number of different ways, they get quite complex!

The name of a hydrocarbon is based on the number of carbon atoms in each molecule (or, if relevant, in the longest chain of carbon atoms in the molecule). A single carbon atom is indicated by the stem meth-, two atoms by eth-, three atoms by prop-, four by but-; after that, they get easier to remember because they are based on the Latin words for each number, at least through 12. If the only other atoms are hydrogen atoms, and all inter-carbon bonds involve a single shared pair of electrons, the ending is -ane: methane, ethane, propane, butane, etc.

When there are four or more carbon atoms in a molecule, they can be connected in arrangements other than a single chain. These are called branched alkanes, which are given names to indicate their structure. Butane has four carbon atoms in a row; if there is one central carbon atom that has three others attached to it, the longest chain to be found has three atoms, so the molecule is described as a propane molecule that has had a carbon atom (attached to three hydrogen atoms) joined in place of one of the usual hydrogen atoms. The systematic name, therefore, is methylpropane.

If there are five carbon atoms in the molecule, then there are three possible isomers (arrangements of the same atoms into different molecules): pentane has five carbon atoms in a row; methylbutane has a chain of four carbon atoms, with a methyl group attached to one of the carbon atoms that is in the middle of the chain; dimethylpropane has a chain of three carbon atoms, with two methyl groups attached to the central carbon atom.

Answer: ethanol

While the name hydroxyethane could possibly be used, to indicate that one of the hydrogen atoms in ethane has been replaced by an OH group, that is not the systematic name. Rather, the systematic name shows the substitution by changing the -ane ending to -anol. This is no longer a hydrocarbon, since it contains oxygen as well as carbon and hydrogen, but is one of many examples of organic chemicals whose properties are determined both by the molecular structure of an underlying hydrocarbon and by the atoms that have been substituted for one or more hydrogen atoms.

What chemists call alkanols most people call alcohols. And if someone just refers to alcohol, they are probably thinking of ethanol, the alcohol found in a wide range of intoxicating drinks. The molecule of ethanol can be indicated in symbols as CH3CH2OH (trying to show in letters and numbers the arrangement shown in the molecular model) or more simply as C2H5OH. In either case the OH bit is kept together, because it is what is called the functional group, the bit that gives the molecule properties which it shares with other members of its family.

Answer: silicon

This is a molecule of silanol, H3SiOH. The -SiOH group of molecules (rather than this compete molecule) is also sometimes given the name silanol when the field of silicon chemistry is involved, as that functional group is involved in a number of significant processes. One of the more common ones it trimethylsilanol, in which each of the three hydrogen atoms attached to the silicon atom is replaced by a methyl (CH3) group. This chemical, which forms when silicone-based materials deteriorate, is a common contaminant in the atmosphere of spacecraft.

Silicon, which is located directly under carbon in the periodic table, can form four covalent bonds, just like carbon. This has led speculative fiction writers to consider it as a possible base for the development of life forms analogous to the carbon-based forms we know. However, silicon does not share carbon's capacity for forming molecules involving more complex bonds, so most scientists consider this unlikely (but not impossible - it's a big universe).

Answer: 4

They are represented by the four white balls attached to the two linked carbon atoms. This sharing of two pairs of electrons, called a double bond, is an important reason why carbon atoms can form the wide variety of compounds they do. Double bonds (and triple bonds which share three pairs of electrons) are relatively unstable, so these molecules react with other substances to rearrange things so that there are more single C-C bonds. For example, a bunch of ethene molecules can join onto each other in a polymerization reaction to form polythene (the prefix poly- indicating many, a large but unspecified number), one of the world's most widely used plastics. Just as ethene used to be called ethylene, these macromolecules are also sometimes called polyethylene.

The diagram shows several useful bits of information about the ethene molecule. The particular stick used to connect the carbon atoms is shaped to indicate the double bond. The hydrogen atoms all lie in the same plane, and each pair form almost exactly an equilateral triangle with the carbon atom to which they are joined. The green region indicates the pi bond, the region where the second pair of bonding electrons locate themselves. Since this bond is between parallel, rather than overlapping, orbitals, it is weaker than the primary (sigma) bond. This is what makes alkenes so reactive, a property indicated by referring to them as unsaturated hydrocarbons.

Answer: graphite

Graphite (powdered and mixed with clay) forms the core of the standard "lead" pencil. It is one of the allotropes of the element carbon - chemicals formed when the atoms combine in different ways. A single layer is called graphene. As the diagram shows, each carbon atom is attached to three other carbon atoms, which means each of them is still looking for a fourth bond. This is done by way of weak pi bonds between the layers, which allows the layers to slide past each other readily. These delocalized electrons also allow graphite to conduct electricity, unlike most covalently bonded elements, although not as well as most metals.

The most familiar other allotrope of carbon is diamond, in which each atom is bonded to four others in a tetrahedral structure which produces a very hard material. Coal is often considered to be made of carbon, but it is produced from decomposition of organic matter, and generally contains some amount of impurity, usually in the form of sulfur and nitrogen compounds. Both coal and graphite can be transformed into diamond under appropriate conditions of temperature and pressure.

Answer: benzene

All of these molecules have six carbon atoms, but only benzene has the formula C6H6. Hexane is C6H14, cyclohexane (which has the six atoms in a ring) is C6H12, and hexyne (which contains a double bond between two of its carbon atoms) is C6H10.

Benzene is the simplest example of an aromatic hydrocarbon. This name is based on the fact that the earliest known examples of substances in this group of chemicals had distinctive smells. The structure of benzene was famously mysterious for a long time, as a proposed structure involving three single bonds and three double bonds in the ring accounted for the molecular formula, but not for the amazing stability of the ring. According to that structure, benzene should have engaged readily in addition reactions to remove the double bonds, but actually is known for being involved in substitution reactions, in which one or more of the hydrogen atoms in benzene is replaced with another atom or group of atoms. Increased understanding of atomic structure led to the current model, with each carbon atom having one single bond with hydrogen or another atom, and two bonds with adjacent carbon atoms which involve delocalized electrons in such a way as to make each bond effectively a one-and-a-half bond, also sometimes thought of as alternating between single and double. This is shown in diagrams by a circle drawn inside the ring of six carbon atoms.

Aromatic compounds are those with one or more phenyl (C6H5-) groups. This includes four of the amino acids that are the building blocks of proteins, as well as the five nucleotides that combine to form RNA and DNA. Hemoglobin, which carries oxygen around our bloodstream, and chlorophyll, which plants use to produce glucose in the process of photosynthesis, also contain aromatic sections in their molecules.

Answer: Fullerenes

Harold Kroto, Robert Curl and Richard Smiley were awarded the Nobel Prize in recognition of the work (in which James Heath and Sean O'Brien also participated) that led to the discovery of this molecular form of carbon. The first ones discovered, found by vapourising carbon in a helium atmosphere, had molecular formulae of C60, C70 and higher. C60 (shown in the image) has a shape much like that of the geodesic domes popularised by Buckminster Fuller. The structure of the molecule was nicknamed 'buckyballs', and later given the full name of buckminsterfullerene. The -ene suffix indicates that the molecule is unsaturated because each carbon atom is only joined to two other carbon atoms, not three. Close inspection of the image will show that it is made up of a combination of pentagons (five-sided shapes) and hexagons (with six sides), similar to the shape of the ball used in association football (soccer). Once discovered in the lab, buckminsterfullerene has been found in everyday soot - it just hadn't been noticed!

Further investigation of fullerenes has involved both closed (or ball-shaped) molecules, and open (tubelike) molecules. The latter, called carbon nanotubes, are expected to have many significant applications in technological developments in electronics, optics and other areas of material science. This is an area where the 21st century is exploring possibilities that were not even suspected back when I first studied high school chemistry!

While it is relatively straightforward to explain how organic molecules are given systematic names, it is not so simple for fullerenes. According to IUPAC guidelines, the name must include the number of carbon atoms in each of the rings that form the structure of the molecule, information about the molecule's structural symmetry using the standard Schoenflies notation to describe three-dimensional figures, and the total number of carbon atoms. For example, buckminsterfullerene is systematically named (C60-Ih)[5,6]fullerene. That's the start, before any substitutions (whose position needs to be precisely identified) are made. 'Nuff said.

Answer: Yes

DNA, or deoxyribonucleic acid, is called a macromolecule, meaning that it is (in molecular terms) very large. The largest human chromosome has approximately 220 million linked pairs of its constituent bases; it would be about 85 mm (over 3 inches) long if uncoiled and straightened out.

The double helix structure describes the way a DNA molecule can be seen to be constructed. There are two long chains, called polynucleotides, which are crosslinked to form a stable shape which naturally forms a coil. Each polynucleotide is made up of smaller units called nucleotides. Each nucleotide is made up of a molecule of the sugar 2-deoxyribose which has had an hydroxyl group replaced by a phosphate group, and one of four nitrogen-containing units usually represented by a single letter ([C] for cytosine, [G] for guanine, [A] for adenine, [T] for thymine) attached to the 1'ribose carbon atom. The phosphate groups link successive molecules to form the chain, while the nitrogenous units are responsible for forming the crosslinking between chains to form the double helix.

Why is DNA important? DNA molecules contain the biological information known as genes, which determine the details of the living organism of which it is a part. That's for an entirely different quiz.