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The Elements of Life

Atoms are the building blocks of matter. Every atom can be identified with a particular chemical element on the basis of the number of protons in its nucleus, which we discuss more below.

Although the variety of the chemical elements is very rich and diverse, many of the elements are not available in large quantities. Certain environments also have a greater or lesser concentration of different elements. The cosmic abundance of each chemical element in the universe (rare or common) is determined by the relative ease of their production (synthesis) and destruction, mostly inside stars.

The figure below shows the relative abundances of the ten most common elements. The vertical axis is measured in parts per million - the taller the bar, the more of the universe's mass that is composed of that element. The bars for hydrogen and helium are so tall that they go far off the top of the chart!

In addition, the cosmic abundances of the elements in the universe change over time due to nuclear fusion within stars (combining lighter elements to create heavier elements). Throughout their lifetimes, stars synthesize small amounts of heavier elements. The early universe, before the first stars formed, was completely devoid of any chemical element heavier than Lithium (the third lightest element in existence). It took many generations of stars to enrich our galaxy by creating the trace amounts of elements like O, C, Fe, S (oxygen, carbon, iron, sulfur) which are all essential components for any life to emerge.

Astronomers have developed very precise tools - spectrometers - to determine the abundances of different elements in all kind of cosmic environments. Hydrogen and helium are by far the most prevalent. Galaxies and stars are all composed of more than 98% H and He. However, planets - among the most numerous and significant of astronomical objects - break this general pattern.

This is illustrated below in the comparison of the universe as a whole (same image as above) and a rocky planet (the Earth) in terms of the abundances of the elements that make them. Watch the scale on the left-hand side carefully - it is still measured in parts per million, but the universal abundance graph has large gaps.

The Origins and Structure of the Elements

Where do the elements come from? There are two basic cosmic sources of the elements:

The elements, from hydrogen to the heaviest metals, are all made of atoms with increasingly larger (and "heavier," i.e. more massive) nuclei. Under normal conditions, the nucleus of an atom is surrounded by a specific number of electrons, which contribute very little to the mass. The configuration of the electrons in an atom determines the chemical properties of the atom - how the atoms of an element bond with each other or with atoms of different elements. The periodic table of elements is laid out to show both mass and chemical properties of each element.

A chemical element is defined as those atoms which have a specific number of protons in their nucleus. For example, an atom of helium has two protons in its nucleus (making it very light), while an atom of lead has 82 protons (making it very heavy). Normally, an atom will have the same number of electrons as it does protons. In this case, the negative charge on the electrons exactly balances the positive charge on the protons, making the atom electrically neutral.

An ion is the name for an atom of a certain element in which one or more electrons are missing from the atom, e.g. having been stripped off in very hot gas. This leaves the ion with a slight positive charge, which influences its chemical properties. On occasion, it is possible to attach an additional electron to an atom - that will be a "negative ion."

In addition to the protons, the nucleus of an atom contains a different massive particle, the neutrons. It is similar to the proton, except that it has no electric charge. Because of its charge neutrality, the number of neutrons does not matter in defining a chemical element. While neutrons do not contribute to the electrical charge of an atom, they do contribute to the mass.

An isotope is the name for an element that has a varying number of neutrons in the atomic nuclei. Some isotopes are stable; others will undergo nuclear decay. Every element has more than one isotope, but not all isotopes are stable. Some instead undergo radioactive decay. There are 90 stable elements, and 266 stable isotopes, but a very large number of unstable isotopes that can be found in nature or created in a laboratory.

As an example, the element carbon always has 6 protons. Carbon has many different isotopes but only two are stable forms: carbon with 6 neutrons is called Carbon-12 (6 protons + 6 neutrons = 12); carbon with 7 neutrons is called Carbon-13. Carbon with 8 neutrons is called Carbon-14 and is not stable, but decays at a predictable rate, so is used effectively as a tool by geologists and archaeologist to determine the age of organic remains through radiocarbon dating.

Because neutrons have mass, each isotope of an element has a different total atomic mass: Oxygen-16 is lighter than Oxygen-18. While each element has its unique set of chemical properties, the slight weight differences between the isotopes of the same element can lead to the accumulation of isotopic imbalances over time as the lighter isotope is prone to certain reactions, and the heavier form is prefered by other reactions. The accumulating imbalances are excellent tracers of climate change. This makes isotope chemistry a valuable tool in studying atmospheric conditions.

Солнечная система и ее тайны