– ‘Science Book: Chemistry’ seeks to address all of the fundamental concepts within chemistry. This is Book 2 in the science series. ‘Science Book: Physics’ can be found on a separate page on this site by clicking on the link in the main header section

Chemical elements. Chemical elements are the simplest substances found in nature, consisting of individual atoms that all have the same number of protons [the “atomic number”] in the atomic nucleus. Each nucleus is surrounded by shells of negatively charged electrons that usually cancel out the positive nuclear charge, making the atom neutral overall.

Each element has a standard chemical symbol, such as H for hydrogen and Fe for iron. Hydrogen is the lightest element, consisting of just one proton and one electron, while elements heavier than uranium, which has atomic number 92, are all unstable and rapidly undergo radioactive decay. A detailed description of radioactive decay can be found in ‘Science Book: Physics‘.

The periodic table displays chemical elements in rows that highlight repeating trends. Atomic number increases from left to right, while the chemical properties of elements in each column are similar. For instance, the far-right column contains neon and argon, inert gases that don’t easily form compounds. That’s because they have the same configurations of outer electrons, the key factor in chemical properties.

Isotopes. Chemical elements can exist as two or more isotopes that have different numbers of neutrons in the nucleus. For example, while carbon always has six nuclear protons, it exists as three different naturally occurring isotopes with six, seven or eight neutrons. These isotopes are often written carbon-12, carbon-13 and carbon-14.

Chemically, different isotopes of an element are usually identical because their chemical properties are determined by their outer electrons. But different isotopes undergo nuclear decay at different rates. For instance, while most carbon on Earth is the stable isotope carbon-12, the isotope carbon-14 is radioactive and decays with a half-life of 5,700 years.

This underpins the technique of carbon dating. Constant interchange with the environment makes the ratio of carbon-14 to carbon-12 constant in a living tree, for instance, but the ratio drops with time in a predictable way after the tree dies. If ancient wood has just half the expected “living” value of carbon-14, it must be about 5,700 years old.  

Allotropes. The atoms of some elements sometimes bind together into different structures called allotropes. For example, oxygen in the Earth’s atmosphere exists as two allotropes – stable diatomic oxygen [O₂] and ozone [O₃]. Ozone is an unstable molecule that forms when diatomic oxygen absorbs ultraviolet light from the Sun.

Solid carbon has three common allotropes. Diamond consists of carbon atoms bonded in a tetrahedral lattice, while graphite consists of flat sheets of carbon atoms bound in hexagons. Fullerenes are spheres [“buckyballs”] or tubes of carbon atoms, including the soccer-ball shaped molecule C60.

Allotropes of an element can have very different physical and chemical properties. Diamond is the hardest known mineral in nature because each carbon atom is bonded rigidly to four other carbons in a tetrahedron; graphite is relatively soft because the flat sheets are weakly bonded and can slide over each other. While diatomic oxygen forms a colourless, odourless gas, ozone is a pale blue gas with a pungent smell.

Solutions and compounds. Atoms of different elements can join together in chemical reactions to form compounds. For example, the elements hydrogen and oxygen react to form water [H₂O]. The properties of compounds are usually very different from the properties of the elements they contain; hydrogen and oxygen are gases at room temperature, for instance, but water is liquid.

Compounds always have a fixed ratio of atoms that are held together in set arrangements by chemical bonds, and they can only be separated into elements by chemical reactions. Unlike compounds, mixtures consist of two or more substances that do not combine chemically and can usually be separated by simple mechanical means such as filtering or evaporation.

Mixtures include alloys, such as steel [iron with carbon], as well as solutions, such as salt dissolved in water. “Colloids” are substances with particles evenly dispersed throughout them, such as emulsion paint, while suspensions are fluids containing solid particles that are large enough to gradually settle out of the fluid.

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(1) A dilute copper sulphate solution contains relatively few dissolved copper sulphate molecules [the solute] compared to the amount of water [the solvent]. (2) When more copper sulphate is added, the solution becomes increasingly concentrated. (3) Eventually the solvent can hold no more solute, and is said to be “saturated”.  

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Chemical bonds. Chemical bonds bind elements together to form compounds. Chemical bonding occurs because atoms are most stable when their outer electron shell – also called the valence shell – is either completely full or empty.

Covalent bonds form when atoms team up to fill their valence shells by sharing their outer electrons. For instance, atoms of hydrogen have only one valence electron in a shell that can hold a maximum of two electrons. Hydrogen molecules form because two hydrogen atoms join up to share their outer electrons and attain full valence shells. Oxygen has two electron vacancies in its valence shell, so it covalently bonds to two hydrogen atoms to form water.

Ionic bonding occurs when a substance, usually a metal, donates an electron to another atom. For instance, sodium chloride [common salt] forms when sodium donates an electron to chlorine. The sodium and chlorine ions then have opposite electric charge and the electrostatic force between them holds the molecule together.

Chemical reactions. In chemistry, a reaction happens when two or more atoms or molecules interact and transform into a different compound. For example, rusting occurs when an “oxidation” reaction makes iron combine with oxygen to form rust-coloured iron oxide.

The opposite process, such as reactions that remove oxygen from iron ores like haematite (Fe 2 O 3) is called reduction. More generally, oxidation means an atom loses electrons as it bonds, while it gains electrons during reduction. Combustion or burning involves reactions between a fuel and an oxidant, with the release of heat. For example, methane or natural gas burns in oxygen to form water vapour and carbon dioxide.

“Catalysts” are substances that can increase the rate of a chemical reaction, without chemically changing themselves. Some reactions are reversible, such as the “Haber process” in which nitrogen and hydrogen combine to form ammonia (NH3). The reaction is said to be in equilibrium when the forward reaction occurs at the same rate as the reverse reaction, which breaks ammonia back down into nitrogen and hydrogen.

Acids and bases. Generally speaking, acids are solutions containing an excess of positive hydrogen ions, while bases or alkaline solutions contain an excess of negatively charged hydroxide [OH^-1] ions. Acids and bases also have more general definitions as electron acceptors and electron donors.

An example of an acid is hydrochloric acid, which forms when hydrogen chloride [H^+CI^-] dissolves in water and the bonds between the hydrogen and chloride ions break, liberating free positive hydrogen ions. Likewise, dissolved sodium hydroxide [Na^+OH^-] creates an alkaline solution. The pH scale measures acidity and ranges from 0 [highly acidic] to 14 [highly alkaline]. Car battery acid has a pH of around 0 to 1, while milk of magnesia has a pH around 10. Perfectly pure water has a neutral pH of 7.

Acids and bases neutralise each other because excess hydrogen ions combine with excess hydroxide ions to form water. These neutralisation reactions also form various salts. For example, hydrochloric acid reacts with sodium hydroxide to produce water and sodium chloride [common table salt].

Electrolysis. Electrolysis is a process that uses electricity to drive a chemical reaction. When positive (anode) and negative (cathode) electrodes are placed in a fluid, positively charged ions in the fluid drift towards the negative electrode, where they accept electrons, while negatively charged ions move to the positive electrode, where they are oxidised. For example, molten aluminium oxide can be electrolysed to produce pure aluminium at the negative electrode, while oxygen bubbles off at the positive electrode.

Batteries effectively reverse this process, with chemical reactions generating electrical energy. When plates of copper and zinc are placed in a sulphuric acid solution, a current flows between them. The zinc electrode gives up electrons that flow along a wire to a copper plate. Once there, they combine with hydrogen ions to liberate hydrogen gas. Many modern batteries use a paste of potassium hydroxide as their electrolyte.

Fuel cells are like batteries, but consume fuel from an outside source. For example, they can generate electricity by oxidising a constant feed of hydrogen gas to form water.

Molecular geometry. Molecular geometry describes the overall shape of a molecule in terms of how the atoms inside it are arranged. Examples of simple structures are linear molecules like carbon dioxide [O=C=O] and tetrahedral molecules like methane, which consists of a carbon atom with four hydrogen atoms surrounding it at the corners of a tetrahedron.

Trigonal-bipyramidal molecules are shaped like two pyramids back-to-back, while octahedral molecules have a shape like an eight-sided solid. Octahedral molecules include the compound sulphur hexafluoride [SF 6].

“Isomers” are compounds that have the same chemical formula but different molecular structures. For instance, the sugar fructose is an isomer of glucose – they have the same formula C₆H₁₂O₆, but their atoms are arranged in different ways. Sometimes, two isomers are mirror images of each other, in which case the molecule is said to be “chiral” and the two mirror-image forms are called enantiomers. Chiral molecules include most amino acids (the building blocks of protein).

Structural formulas. The structural formula of a molecule indicates how the atoms inside it are bound together. For example, ethanol has the chemical formula C2H6O, but its structural formula is CH3-CH2-OH, indicating that a methyl group [CH3] is attached to the carbon of a methylene group [CH2], which in turn is attached to the oxygen of a hydroxyl group [OH].

There are various graphical ways of representing a structural formula, including the simple flat “Lewis structure” that shows how atoms bond together. A representation called the Natta projection shows molecules in three dimensions, with solid and dotted triangular bonds indicating bonding directions towards and away from the viewer respectively.

Often, “skeletal formulas” are used to describe complex organic molecules, with a hexagon representing the benzene ring C6H6, for instance. To keep things simple, skeletal formulas don’t label carbon and hydrogen atoms specifically – carbon is assumed to sit at the vertices with as much hydrogen as it needs to use up four bonds.

Chemical polarity. Polar molecules are ones in which the electric charge is unevenly distributed, so that one side of the molecule is positively charged while another region is negatively charged.

Water is an example of a polar molecule. There is an excess of positive charge on the side of the molecule where two hydrogen atoms sit, covalently bonded to oxygen via a shared pair of electrons. The oxygen also has two unshared electron pairs on the opposite side of the hydrogen atoms, making that side negatively charged.

Water molecules tend to align themselves so that the negatively charged side of one molecule sits next to the positively charged end of an adjacent one. This creates a weak type of secondary bonding called hydrogen bonding. These bonds give water a crystal structure when it freezes, and this explains why water ice is less dense than liquid water. As a result, when ice forms on a lake during a cold winter, it floats to the surface, forming an insulating blanket that protects the entire lake from freezing.

Molecular engineering. Molecular engineering, or nanotechnology, is the manipulation of matter on tiny scales down to a billionth of a metre (about one hundred-thousandth of the width of a human hair). It creates materials with useful nanoscale properties, including invisible coatings just 3 micrometres (millionths of a metre) thick that protect shiny stainless steel exhaust pipes on cars from corrosion.

Opticians apply nanocoatings to spectacles to make them more scratch resistant and easier to keep clean, while other nanomaterials help strengthen composite materials used in lightweight tennis rackets and bicycles, for instance. However, scientists have expressed concern that nanoparticles in commercial products could cause serious diseases if inhaled.

Nanomachines or “nanbots” are in an early research and development phase. In future, tiny nanosensors in packaging could detect the pathogens that cause food poisoning, while nanobots could swim through your bloodstream to repair DNA damage in cells or identify and kill tumours.  

Crystal structures. A crystal or crystalline solid is a material in which the atoms or molecules are arranged in a rigid and orderly repeating pattern. Table salt, snowflakes and diamonds are common examples of crystals. Crystalline rocks can form in solutions or when molten magma cools. For instance, completely crystallised granite forms when magma cools and solidifies very slowly under high pressure.

Crystals can have a simple cubic lattice, with one lattice point on each corner of a cube, while the body-centred cubic system also has a lattice point in the centre of the cube. The face-centred cubic system has lattice points in the middle of the cube faces. Common salt forms a face-centred cubic lattice with alternating atoms of sodium and chlorine.

Some crystals can also form more complicated shapes, including double pyramids and eight-faced octahedra. Scientists often study the structures of crystals by passing X-rays through them and examining the resulting diffraction patterns.  

Metals. In chemistry, a metal is an element or alloy that has high electrical and thermal conductivity. A metal’s ability to conduct electricity and heat stems from the fact that its outer electrons are extremely loosely bound to the atoms and readily flow through a metal wire. Iron and aluminium are the two most common metals on Earth.

Metals are typically denser than non-metallic elements and they readily form positively charged ions by losing electrons, although their levels of reactivity vary – iron for example, rusts over years as it converts to iron oxide in the atmosphere, while pure potassium burns up in seconds as it oxidises. Some metals, such as the precious metals platinum and gold, do not react with the atmosphere at all. Others, including aluminium and titanium, form a thin oxide layer on their surfaces that protects them from further oxidation.

Confusingly, however, astronomers often use the term “metal” to refer to any element in the universe that is heavier than hydrogen or helium.

Semiconductors. A semiconductor is a material that conducts electricity better than an insulator (including most ceramics) but less well than an electrical conductor like copper. Semiconductors can be pure elements, such as silicon or germanium, or compounds, including gallium arsenide or cadmium selenide.

When electrons move in a semiconductor, they leave behind “holes” with relative positive charge, and this makes semiconductors useful for electronic devices such as transistors, often used as switches. An example is the NPN transistor, which sandwiches a P-type semiconductor (with an excess of positive holes) between two N-type semiconductors (with an excess of negatively charged electrons).

When an electric current is applied to the “base” input of the transistor, it increases the conductivity of the P-type region, which in turn increases current flow across the transistor from the “collector” to the “emitter”. Today, transistors are miniaturised on microchips. Semiconductors play vital roles in just about all modern electronic devices.

Polymers. Polymers are materials made from large molecules composed of many repeating units. Naturally occurring polymers include starch, which is made up of many repeating units of the sugar glucose, and proteins, which are long chains of amino acids. Most polymers are organic, using carbon bonds as their backbone.

Plastics are synthetic polymers. Polyethylene, commonly called polythene, is one of the simplest, composed of chains of repeating CH2 [ethylene] units. Depending on the temperatures and pressures that are applied during production, the ethylene molecules can link up to create high-density polyethylene, used in containers like milk bottles, or low-density polyethylene, used to produce plastic films and sandwich bags, for instance.

Polyvinyl chloride is a polymer similar to polyethylene, but also includes chlorine atoms. This rigid polymer is used for pipes, window frames and vinyl siding for houses. It can be blended with other compounds to make it soft for products like raincoats and shower curtains.  

Composites. Composite materials are ones that combine two or more materials without completely blending them. An example is reinforced concrete, which uses cement to bind rough gravel and sand, often with steel bars running through it for extra strength. Wood is a natural composite, composed of cellulose fibres in a matrix of the complex polymer lignin.

Usually, composite materials are designed to be lightweight yet strong and stiff. Typically, one material (the matrix or binder) surrounds and binds together clusters of fibres of a much stronger material (the reinforcement). For instance, fibreglass is a plastic matrix reinforced by threads of glass.

When building an aircraft, engineers need lightweight, strong materials that can withstand the stresses from turbulent air. One material they use is carbon-fibre-reinforced plastic, which is similar to fibreglass, but even stronger. Aerospace engineers often build spacecraft with more exotic composite materials designed to withstand extremely low temperatures in Earth’s orbit or interplanetary space.

Nanomaterials. Nanomaterials are substances that have at least one dimension smaller than about 100 nanometres [billionths of a metre], which is about one-thousandth of the width of a human hair. Nanomaterials can be nanoscale in just one dimension [such as surface films], two dimensions [fibres or strands] or three dimensional [tiny particles].

Many nanomaterials have unusual properties because they reach the ultra-small realms where the quantum behaviours of atoms start to show their hand. Products containing nanomaterials are already in commercial use, including sunscreens with nanoparticles that absorb ultraviolet sunlight without generating skin-damaging free radicals in the process. Other products include stain-resistant textiles.

A nanoscale coating of titanium dioxide (a catalyst) makes windows “self-cleaning”. When the coating absorbs UV sunlight, it breaks down organic dirt. The coating is also “hydrophilic”, or water loving, so that rain forms a sheet on the glass rather than individual droplets, keeping the glass evenly clean. Once the nanolayer breaks down the pollutant under ultraviolet light, carbon dioxide and water are released.

Metamaterials. Metamaterials are materials that have been artificially engineered to have exotic properties unlike anything we’ve seen in nature. One example is material with the potential to act like the “cloaking” devices that make spaceships or people invisible in sci-fi films.

In this case, materials are carefully engineered to manipulate light. Sometimes they alternate tiny layers to produce a substance with a negative refractive index that bends light in unexpected directions. In theory, such a material could cloak an object by making light waves pass around it and then continue along their original straight paths, so the object would be invisible to someone standing “downstream”.

Research is at a very early stage and is unlikely to lead to viable invisibility cloaks. However, similar materials could be extremely useful in future microscopes for imaging tiny viruses and molecules, because they don’t suffer from the diffraction limit that prevents normal materials focusing light to a tiny super-sharp spot. Optics is another area where a range of possibilities exist for incorporating metamaterials.

You will find more on diffraction in Science Book: Physics

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_{This completes Science Book (Chemistry). Amendments to the above entries may be made in future.}