Tag Archives: Questions of science

Science

Questions of Science: Plant poser

Q: Did all the oxygen in Earth’s atmosphere come from photosynthesising plants? If not, where did it come from?

WITHIN the Earth’s crust, oxygen combines with all the most common atoms to form water, rock, organic compounds and almost everything else around us. Spontaneous free oxygen is about as likely as finding round rocks perched on steep slopes. Such rocks would imply that something had pushed them uphill more strongly than they could roll downhill.

Similarly, any free oxygen about us has been torn from its compounds with more than its bonding force. And that is a lot of force that only a few things are able to produce. Ionising radiation, such as X-rays, can do it, but there is little of that about. Visible light does it laboriously, step-by-step through photosynthesis, the only process that could release the level of oxygen that we see about us. That amount is calculated to be in region of 10¹⁵ tonnes.

How much oxygen plants actually produce is another matter entirely. The chloroplasts used by plants to photosynthesise are thought to have originated as symbiotic cyanobacteria. So, in effect, all our oxygen came from photosynthesising bacteria. Hence, practically all the atmospheric oxygen is of biological origin – and is not from plants but cyanobacteria.

These single-cell organisms, which were present on Earth more than 3.5 billion years ago and pre-date plants, were initially responsible for all oxygen production and are still responsible for more than 60 per cent of current oxygen production.

Cyanobacteria come in many varieties and are sometimes called blue-green algae, although they are not really algae. A species of cyanobacteria present in the ocean, Prochlorococcus marinus, is both the smallest photosynthetic organism known and the most abundant of any photosynthetic species on the planet. It was only discovered in 1988.

Previously Questions of Science: Free the atoms

Science-in-motion: a series of short articles following topics in science.

. Biodiversity

This measures how much variety there is between all the different species of life on Earth, from single-celled bacteria through insects to blue whales, the largest known animal ever to have existed. Biodiversity also describes the genetic diversity within a single species, or even the diversity of ecosystems like wetlands and forests.

Around 1.75 million species of living organisms have been identified on Earth so far, mostly small ones like bacteria and insects, and estimates suggest the true number could be as high as 100 million. But in recent centuries, there has been a rapid increase in the rate of species extinctions due to human activities such as habitat destruction for farming.

Between 1500 and 2009, international organisations documented more than 800 species becoming extinct, including the Javan tiger that died out completely in the 1980s, but the vast majority of disappearances probably go unnoticed. Conservationists grade the vulnerability of species according to a continuum scale that runs from ‘extinct’ to ‘least concern’.

Biodiversity is of huge significance. The development of new medicines, for example, is inhibited during rainforest destruction, of which many tiny organisms are either destroyed or not even previously discovered.

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Science

Questions of Science: Free the atoms

Oxygen has a slightly greater density than nitrogen. Why, then, don’t these main constituents of air separate out?

MD overlapGAS MOLECULES move rapidly at room temperature, with oxygen and nitrogen travelling at around 500 metres per second, so they obviously collide frequently. This allows the oxygen and nitrogen molecules to mingle and mix, rather like large numbers of people on a nightclub dance floor, in a process known as diffusion. Convection, the transfer of heat within the atmosphere, also plays an important role in this gas mixing process.

Gas mixing is a spontaneous process. This means that if you had a container with two compartments separated by a barrier, with one compartment containing pure nitrogen and the other pure oxygen, the two gases would automatically mix or diffuse as soon as the barrier was removed.

A change in the ratio of oxygen to nitrogen would be expected in a hypothetical quiescent atmosphere. However, constant mixing occurs in the real atmosphere, driven by the Earth’s rotation and by differences in density between hot air at the Earth’s surface and colder air higher up.

Up to altitudes of between 80 and 120 kilometres this mixing results in a uniform concentration of oxygen and nitrogen – which respectively make up approximately 21 per cent and 78 per cent of the atmosphere.

This region is known as the homosphere. Partial stratification of the two gases does occur above 120 kilometres, in the heterosphere, where the density of air is much lower than at the surface and the efficiency of bulk mixing processes is reduced.

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If there were no circulation in the atmosphere, the oxygen would tend to concentrate in the lower strata. This process would take millions of years once circulation ceased because molecules of oxygen (and, indeed, nitrogen) are constantly colliding with other molecules. This means it would take a long time for a particular molecule to fall from its starting point to the ground. Once it hit the ground, it would bounce and eventually rise again to a great height, only to fall again. This would be repeated frequently if no other variable, such as temperature, changed.

Although the individual molecules continue to travel up and down, each ‘species’ of oxygen and nitrogen would eventually reach an equilibrium distribution of molecules per unit volume as a function of height. This species density will decrease with height by an amount that depends on the molecular weight of the species. So, the oxygen would fall off with height slightly faster than the nitrogen. At high altitudes, the air would become richer in nitrogen, but then other gases such as water vapour, neon, methane, helium and hydrogen would dominate.

In fact, atmospheric circulation and turbulence prevents this from happening in the lower atmosphere. But in the very high atmosphere there is not much circulation and the composition does become dominated by atomic oxygen. Above 600 kilometres this is superseded by helium, and eventually by atomic hydrogen.

science in motion

Science-in-motion: a series of short articles following topics in science.

. Molecular geometry

This 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 (SF6).

‘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 (which are the building blocks of proteins).

Molecular Geometry

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Education, Science

Questions of Science: Push and pull

Questions of Science

Intro: Why, in an atom, does the negatively charged electron not collapse into the positively charged nucleus? Is this in any way similar to the reason why large systems like stars and planets do not collapse into each other under the pull of gravity?

When Ernest Rutherford, the New Zealand-born founder of nuclear physics, first discovered the atomic nucleus he did indeed propose that electrons did not fall toward the nucleus of the atom because the attractive forces of the nucleus were being balanced by the orbital velocity of the electron in much the same way as a planet orbiting a star.

However, the Danish physicist Niels Bohr modified this theory after Albert Einstein and Max Planck found that energy could only exist in certain discrete amounts, or quanta. This meant that electrons could be seen to have both wave and particle properties, and required that the circumference of the orbit of an electron could not be zero. This means, of course, it could never reach the nucleus.

We have since adopted the model proposed by the Austrian theoretical physicist Erwin Schrödinger. Instead of orbiting the nucleus like planets, his model has electrons occupying ‘clouds’ where it is statistically probable that they will exist, although we may never determine an electron’s position and velocity at the same time.

Niels Bohr’s questioning in 1913 deserves further explanation. The atom was known to have a small heavy nucleus, and the much lighter electrons were thought to orbit it like planets around the sun. As long as a planet does not lose energy, it can continue its orbit indefinitely.

According to the laws of electromagnetism, charged particles moving in a circle ought to radiate energy as waves. Bohr calculated that a hydrogen atom should collapse with a flash of light in a matter of femtoseconds. Because this does not happen, he proposed what has become known as the ‘old’ quantum mechanics. It asserted that the electron’s angular momentum had to be a multiple of Planck’s constant.

The rule meant that electrons could only occupy particular orbits, and there was a minimum size of orbit. Using this, Bohr was able to predict the entire spectrum of excited states of hydrogen, which was a quite astounding achievement.

But Bohr’s theory was hard to apply to more complex atoms and was superseded by Erwin Schrödinger’s wave mechanics in 1927, which is the start of modern quantum theory.

Schrödinger’s formulation shows that an electron has a wave character, and a stable atom can be thought of as a box confining the wave. An electron has a wavelength equal to Planck’s constant divided by its momentum, so the faster an electron moves, the shorter its wavelength. To confine the electron near the nucleus the electron must move very quickly.

Conversely, a fast-moving electron can escape the pull of the nucleus. So you can think of the size of an atom as resulting from a compromise between the electrons having enough kinetic energy for their waves to fit in the box, but not so much that they can escape.

Large solar systems don’t collapse for quantum-mechanical reasons. They don’t collapse because the planets’ velocities keep them in freefall.

Science in motion

Science-in-motion: a series of short articles following topics in science.

. Atomic structure

Atoms consist of a tiny dense nucleus containing positively charged protons and uncharged neutrons, surrounded by clouds of electrons. Because protons and neutrons are much heavier than electrons, most of an atom’s mass resides in the central nucleus.

Each chemical element has a unique number of protons in its nucleus, its ‘atomic number’. For instance, the element carbon with six protons has the atomic number six. However, single elements can have different numbers of neutrons in the nucleus. For example, carbon has three naturally occurring ‘isotopes’ with six, seven or eight neutrons. The sum of protons and neutrons in an atom’s nucleus is called the atomic mass number.

Normally, the net electric charge of an atom is zero because the number of electrons is the same as the number of protons, and their equal and opposite electric charges cancel out. However, it’s possible to knock electrons out of atoms or add extra ones to create positively or negatively charged particles known as ‘ions’.

Atomic Structure

. Negatively charged electrons in orbit around the nucleus . Nucleus containing positively charged protons and neutrons

 

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