Строение атомов и принцип Паули

Принцип Паули помогает объяснить разнообразные физические явления. Следствием принципа является наличие электронных оболочек в структуре атома, из чего, в свою очередь, следует разнообразие химических элементов и их соединений. Количество электронов в отдельном атоме равно количеству протонов, так как электроны являются фермионами; принцип Паули запрещает им принимать одинаковые квантовые состояния. В итоге, все электроны не могут быть в одном квантовом состоянии с наименьшей энергией (для невозбуждённого атома), а заполняют последовательно квантовые состояния с наименьшей суммарной энергией (при этом не стоит забывать, что электроны неразличимы, и нельзя сказать, в каком именно квантовом состоянии находится данный электрон). Примером может служить невозбуждённый атом лития (Li), у которого два электрона находятся на 1S орбитали (самой низкой по энергии), при этом у них отличаются собственные моменты импульса и третий электрон не может занимать 1S орбиталь, так как будет нарушен запрет Паули. Поэтому, третий электрон занимает 2S орбиталь (следующая, низшая по энергии, орбиталь после 1S).

UNIT 26 Electronegativity and Strong Bonds

Although the propensity that a given atomic species displays for losing or gaining electrons is determined by the dual factors of ionization potential and electron affinity, an adequate qualitative measure of the same thing is provided by a single parameter known as the electronegativity, e_N. Atoms with large electronegativities tend to capture electrons, whereas the opposite is the case for atoms with small electronegativities (these being said to be electropositive). A reliable scale of electronegativity has been derived by Linus Pauling and it is, as indicated above, a dual measure of ionization energy and electron affinity.

If the difference in the electronegativities of two atoms is quite small, there will be no clear tendency for one to lose an electron while the other gains this subatomic particle. There is thus no basis for ionic binding in such a situation. Instead, one has either covalent bonding or metallic bonding, the first of these occurring if the two atoms are both electronegative, and the latter arising when they are both electropositive. An example of covalent bonding is seen if the two atoms involved are both fluorine. The nine electrons in an atom of this element are arranged in such a way that there are two in the 1s orbital, two in the 2s orbital, two in each of the 2p_x and 2p_y orbitals, and finally a single electron in the 2p_z orbital. It is only the latter orbital, therefore, which lacks an electron and, as we saw in the case of ionic bonding, it can fulfil this need by acquiring an electron from another atom. However, in the case of the covalent bond, it does this not by completely removing an electron from that other atom, but rather by entering into a mutual sharing of atoms, in which the unfilled orbitals of both atoms are filled by the other's lone 2p_z electron.

On the example of a hydrogen fluoride molecule one can see, that the 2p_z orbitals alone are involved in the binding. Although the interatomic bond is highly directional, as mentioned earlier, it possesses rotational symmetry and there is very little resistance to rotation of one of the atoms with respect to the other, about the z-axis. This form of covalent bonding is known as a sigma bond and numerous examples are encountered in molecules having biological relevance. In the case of the HF molecule the incomplete orbitals, the 1s of hydrogen and the 2p of fluorine, overlap and the two electrons are shared by a sigma-bond.

Define the following words using a dictionary

Propensity, affinity, to acquire, to derive, to gain, the latter, reliable

Complete the sentences

1. Atoms with large electronegativities tend to capture electrons, whereas…
2. If the difference in the electronegativities of two atoms is quite small,…
3. A reliable scale of electronegativity…
4. An example of covalent bonding is seen if…
5. The nine electrons in an atom of this element are arranged in such a way that…
6. Although the interatomic bond is highly directional,…
7. In the case of the HF molecule the incomplete orbitals…