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The air we breathe has about 10 19 atoms in each

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The air we breathe has about 1019atoms in each cubic centimeter—and we usually think of air as empty space. In theinterstellar gas of the Galaxy, there is about one atom in every cubic centimeter. Intergalactic space is filled so sparsely thatto find one atom, on average, we must search through a cubic meter of space. Most of the universe is fantastically empty;places that are dense, such as the human body, are tremendously rare.Even our most familiar solids are mostly space. If we could take apart such a solid, piece by piece, we would eventuallyreach the tiny molecules from which it is formed. Molecules are the smallest particles into which any matter can be dividedwhile still retaining its chemical properties. A molecule of water (H2O), for example, consists of two hydrogen atoms andone oxygen atom bonded together.Molecules, in turn, are built of atoms, which are the smallest particles of an element that can still be identified as thatelement. For example, an atom of gold is the smallest possible piece of gold. Nearly 100 different kinds of atoms (elements)exist in nature. Most of them are rare, and only a handful account for more than 99% of everything with which we come incontact. The most abundant elements in the cosmos today are listed inTable 2.1; think of this table as the “greatest hits”of the universe when it comes to elements.The Cosmically Abundant ElementsElement[1]SymbolNumber of Atoms perMillion Hydrogen AtomsHydrogenH1,000,000HeliumHe80,000CarbonC450NitrogenN92OxygenO740NeonNe130MagnesiumMg40SiliconSi37SulfurS19IronFe32Table 2.1All atoms consist of a central, positively charged nucleus surrounded by negatively charged electrons. The bulk of the matterin each atom is found in the nucleus, which consists of positive protons and electrically neutral neutrons all bound tightlytogether in a very small space. Each element is defined by the number of protons in its atoms. Thus, any atom with 6 protonsin its nucleus is calledcarbon, any with 50 protons is calledtin, and any with 70 protons is calledytterbium. (For a list ofthe elements, seeAppendix F.)1.This list of elements is arranged in order of the atomic number, which is the number of protons in eachnucleus.Chapter 2 | The Universe at Its Limits59
The distance from an atomic nucleus to its electrons is typically 100,000 times the size of the nucleus itself. This is whywe say that even solid matter is mostly space. The typical atom is far emptier than the solar system out to Neptune. (Thedistance from Earth to the Sun, for example, is only 100 times the size of the Sun.) This is one reason atoms are not likeminiature solar systems.Remarkably, physicists have discovered that everything that happens in the universe, from the smallest atomic nucleus tothe largest superclusters of galaxies, can be explained through the action of only four forces: gravity, electromagnetism(which combines the actions of electricity and magnetism), and two forces that act at the nuclear level. The fact that thereare four forces (and not a million, or just one) has puzzled physicists and astronomers for many years and has led to a questfor a unified picture of nature.

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Physics, National Aeronautics and Space Administration, OpenStax University Physics, Nuclear Physics and Comparative Geology

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