"Kind of atom" in this context refers to the number of protons in its nucleus. An element, on the other hand, is -as stated in its definition -made up of only one kind of atom. Every item of matter that exists, except for the subatomic particles discussed in this essay, is made up of atoms. In fact, these two definitions do not form a closed loop, as they would if it were stated that an element is something made up of atoms. The definitions of atoms and elements seems, at first glance, almost circular: an element is a substance made up of only one kind of atom, and an atom is the smallest particle of an element that retains all the chemical and physical properties of the element. So important are electrons to the interactions studied in chemistry that a separate essay has been devoted to them. Furthermore, chemical reactions are the result of changes in the arrangement of electrons, not of any activity involving protons or neutrons. In fact, what defines an atom's ability to bond with another atom, and therefore to form a molecule, is the specific configuration of its electrons. Ions are critical to the formation of some kinds of chemical bonds, but the chemical role of the electron is not limited to ionic bonds.
Conversely, sometimes an atom takes on one or more electrons, thus acquiring a negative charge. Moving rapidly through the space between the nucleus and the edge of the atom, electrons sometimes become dislodged, causing the atom to become a positively charged ion. All the chemical "work" of an atom is done by particles vastly smaller in mass thanĮither the protons or neutrons -fast-moving little bundles of energy called electrons. However, as important as elements and even isotopes are to the work of a chemist, the components of the atom's nucleus have little direct bearing on the atomic activity that brings about chemical reactions and chemical bonding. The number of protons in an atom is the critical factor in differentiating between elements, while the number of neutrons alongside the protons in the nucleus serves to distinguish one isotope from another. Rather, the defining characteristic of an element is the atom that forms its basic structure. And what differentiates the elements, ultimately, from one another is not their color or texture, or even the phase of matter -solid, gas, or liquid -in which they are normally found. Just as a writer in English works with the 26 letters of the alphabet, a chemist works with the 100-plus known elements, the fundamental and indivisible substances of all matter. At heart, chemistry is about the interaction of different atomic and molecular structures: their properties, their reactions, and the ways in which they bond. While it is true that physicists study atomic structure, and that much of what scientists know today about atoms comes from the work of physicists, atomic studies are even more integral to chemistry than to physics.
People who are not scientifically trained tend to associate studies of the atom with physics, not chemistry. In addition, we will explore the many insights added along the way as, piece by piece, the evidence concerning atomic behavior began to accumulate. Only then will we examine the many challenges scientists faced in developing the current atomic model: false starts, wrong theories, right roads not taken, incomplete models. Many accounts of the atom begin with a history of the growth in scientists' understanding of its structure, but here we will take the opposite approach, first discussing the atom in terms of what physicists and chemists today understand. Nonetheless, much still remains to be explained about the atom -particularly with regard to the smallest items it contains. The particulars of this theory, including the means by which it evolved over the centuries, are as dramatic as any detective story. Atomic theory explains a great deal about the universe, including the relationship between chemical elements, and therefore (as with Darwin's theory concerning biological evolution), it is generally accepted as fact. After all, the atom cannot be seen, even with electron microscopes -yet its behavior can be studied in terms of its effects. By definition, an idea that is dubbed a theory has yet to be fully proven, and such is the case with the atomic theory of matter. A theory, on the other hand, attempts to explain why things happen.
Laws address the fact that certain things happen, as well as how they happen. Our world is made up of atoms, yet the atomic model of the universe is nonetheless considered a "theory." When scientists know beyond all reasonable doubt that a particular principle is the case, then it is dubbed a law.
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