Chemical Elements in the Solar System

With most of the mass in the Solar System in the Sun, and the Sun composed almost entirely of hydrogen and helium, the chemical composition of the Solar System is dominated by these two elements. Hydrogen is the lightest element. Its most common isotope (by far) has a nucleus consisting of a single proton. You saw in Section 1.1.3 that this isotope is represented as :H.

Helium is the next lightest element, with the nucleus of its most common isotope (again by far) consisting of two protons and two neutrons. Recall that an element is defined by the number of protons in its nucleus - this is the atomic number - and that the isotopes are distinguished by different numbers of neutrons. To denote a particular isotope the number of neutrons plus protons is included with the chemical symbol, as you have seen for helium's common isotope, 4He (Section 1.1.3).

The Solar System contains all 92 naturally occurring chemical elements with atomic numbers from 1 (hydrogen) to 92 (uranium). The relative abundances of these elements have been determined through observations of the Sun and through analyses of primitive meteorites (Section 3.3.2).

Most of the mass outside the Sun is in Jupiter and Saturn, and these are also composed largely of hydrogen and helium, though they contain larger proportions of the other elements -the so-called heavy elements. For the Solar System as a whole, Table 1.5 gives the relative abundances of the 15 most abundant of the chemical elements. Note that the value for helium is for the Sun outside its fusion core. This region has not been depleted in helium by its conversion into hydrogen by nuclear fusion, such as occurs in the core of the Sun.

Except in very high-temperature regions, most of the atoms of most elements are combined with one or more other atoms, either of the same element, or of other elements. The important exceptions are helium, neon, argon, krypton, and xenon, which are so chemically unreactive that they remain monatomic and have been given the name inert gases or noble gases. If an element is combined with itself, as in H2, then we have the element in molecular form, whereas if it is combined with other elements, then we have it as a chemical compound.

Water (H2O) is the most abundant chemical compound of hydrogen in the Solar System. Table 1.5 suggests the reason. □ What is the reason?

It is because oxygen has a high abundance. But hydrogen is so overwhelmingly abundant that there is plenty left over after the formation of hydrogen compounds. Most of the uncompounded hydrogen outside of the Sun is in the giant planets, as H2, or as a fluid of hydrogen with metallic properties. Water is the main repository of hydrogen in most of the other bodies.

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