0.4 Electron shell model of an atom  (Page 2/6)

$V\mathrm{=}\frac{(q_1)(q_2)}{r}$

This is Coulomb’s Law. We will very rarely do any calculations with this equation. Instead, we will apply it to understand when V is expected to be a large number or a small number, positive or negative. When V is a large negative number, the potential energy is very low and the two charges are strongly attracted to one another. To see this, think about what must happen to pull two charges apart which have a very negative potential energy. If we want r to become very large, then in Coulomb’s Law, we want V to get close to zero. If V is a large negative number, then we have to add a lot of energy to bring V up to zero. Therefore, a large negative value of V means that the two particles are strongly attracted to each other since it requires a lot of work to pull them apart.

In the equation above, V will be a large negative number when several things are true: the charges must have opposite signs, so that multiplying them together gives a negative number. All this means is that opposite charges attract. The larger the charges, the stronger the attraction. In addition, r must not be large and preferably will be fairly small. These simple conclusions must be kept in mind. Two particles with large opposite charges close to one another must be strongly attracted to one another. The smaller the charges or the larger the distance, the weaker the attraction.

In many ways, it is fair to say that Coulomb’s Law forms the foundation of everything we know about the chemistry of atoms and molecules. Therefore, it is very important to understand the conclusions of the previous paragraph. Without them, we can make no further progress in our understanding of atoms.

Observation 1: periodic properties of the elements

We now have much more information about the differences between the atoms of different elements. We know how many electrons and protons each atom contains, and we know where these charged particles are in the atom, with the protons in a very small nucleus and the electrons occupying the vast empty space around the nucleus. It seems that we should be able to account for the chemical properties of these atoms by using this information. However, we rapidly run into a surprising result.

Remember that the atomic number tells us how many protons and electrons an atom contains. We observe that atoms with very similar atomic numbers often have very different chemical properties. For example, carbon’s atomic number is 6 and nitrogen’s is 7, so they have very similar numbers of protons and electrons. But as we have seen, elemental carbon is a solid and elemental nitrogen is a gas. Oxygen’s atomic number is 8, just one greater than nitrogen, but oxygen reacts with most other elements, sometimes violently, whereas nitrogen is so unreactive that it is often used to provide an “inert” atmosphere to store chemicals.

Also surprisingly, elements with very different atomic numbers can have quite similar chemical properties. The elements fluorine and chlorine are both gases and both exist as diatomic molecules in nature, F ₂ and Cl ₂ . Both are highly reactive and will combine with hydrogen to form acids, HCl and HF. They both combine with metals like sodium and magnesium to form solid salts with similar molecular formulas, like NaF and NaCl. But their atomic numbers are quite different: F’s atomic number is 9, and Cl’s is 17.