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Basic Structure of an Atom

As was mentioned in Chapter 14, an atom consists of a tiny positive charge center (10–15 m) called the nucleus and a surrounding cloud of electrons (10–10 m). Atoms connect together by interactions of their electrons to form molecules and ionic solids, which comprise almost all of the matter around us.
The nucleus of the atom contains protons and neutrons. Each proton has a charge +1.6 x 10–19 C (chemists call this +1), each neutron has zero net charge, and each electron has charge –1.6 x 10–19 C (chemists call this –1).
The mass of the proton and the mass of the neutron are about the same (1.7 x 10–27 kg), and electrons are about 2000 times lighter. As you can see, kilograms are really too large a unit to use to talk of these masses, so we often use another mass unit, called the atomic mass unit, or [amu] (or sometimes just u), so that
1 amu = 1.6606 x 10–27 kg
It is very nearly the mass of a proton or neutron, so we have
mproton = 1.0073 amu
mneutron = 1.0087 amu
melectron = 0.00055 amu
The atomic mass (sometimes erroneously called the atomic weight) is the mass of an atom in amu. The mass number is the total number of protons and neutrons in the nucleus. The mass number is approximately the atomic mass of an atom in amu, since most of the mass comes from the protons and neutrons.
Thus we can describe a nucleus by specifying the number of protons in it, or atomic number, and the total number of protons and neutrons, or mass number. The atomic number also determines which element the atom makes up. The notation we use for a single atom contains all this information. For instance, we denote common helium by one of the two symbols:


(pronounced "helium four"), where the mass number is 4 and the atomic number is 2. We really do not need to specify the atomic number, since we can obtain that from the periodic table. The mass of such a nucleus is about 4 amu (actually 4.00260 amu).
Several atoms which differ in the number of neutrons but have the same number of protons are called isotopes of that element, for example, 35Cl and 37Cl. The chemical properties of 35Cl and 37Cl are nearly identical, since the neutrons of the nucleus have hardly any effect on the surrounding electron cloud.
The symbol 42He may refer either to a nucleus having two protons and two neutrons or to an atom having such a nucleus. We write nuclear reactions using similar notation as that for chemical reactions, except now we are concerned with changes in the protons and neutrons in the nuclei.


When a slow neutron collides with a 17O nucleus, the products include a 4He nucleus and what other nucleus?



The information given in the problem can be written

where we have used the symbol 10n for the neutron (do you see why?). The number of protons mentioned among the reactants is 0 + 8 = 8, of which 2 went into the 42He. The number of protons and neutrons all together among the reactants is 1 + 17 = 18, of which 4 are in 42He. Thus the final nucleus has a 6 in the lower left position and a 14 in the upper left, corresponding in the periodic table to 14C or carbon-14, so we write

When writing reactions, the sum of left superscripts for the reactants must equal the sum of left superscripts for the products. This assures that the number of heavy particles stays constant. Also, the sum of left subscripts for the reactants must equal the sum of left subscripts for the products. This assures that charge is conserved in the reaction.
There are always at least two product particles in any nuclear reaction. (Exceptions are vanishingly rare.) One of these particles is often a particle of light, called a photon and symbolized by γ.



When a proton collides with a nucleus of carbon-12, generally the result is one new nucleus (if there is a reaction at all). How do we write the reaction?



We write the reaction as follows:


where we have used  for the proton (again, do you see why?).

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