Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 67

The hydrogen atom has just one electron, which occupies the first principal quantum shell. There is just one atomic orbital in this shell, called a 1s orbital. The number 1 tells us the orbital is in the first principal quantum shell (n = 1).

An s orbital is spherical in shape, and in any shell, there is only one s orbital. A 1s orbital is shown in Figure 1.6a. Also shown in this figure are a 2s orbital and a 3s orbital (Figures 1.6b and c). They are all spherical in shape and get larger as the value of n increases. Within an atom, all the s orbitals are concentric – this means they all have the nucleus as their centre and lie on top of each other. The shading in the figure represents the electron density or the probability of finding an electron in each orbital. You can see that in the 1s orbital, there is a high probability of finding the electron close to the nucleus. However, in the higher s orbitals, there are areas that are unshaded and therefore are unlikely to contain electrons. These areas are called nodes.

Figure 1.6 (a) The 1s orbital; (b) the 2s orbital; (c) the 3s orbital.

Source: Redrawn from https://www.quora.com/What‐is‐a‐simple‐explanation‐of‐the‐Quantum‐mechanical‐model‐of‐the‐atom.

Elements that either have fully or partially filled s orbitals in their outer shells are called s‐block elements. These elements occupy Groups 1 and 2 of the periodic table.

Each s orbital can contain a maximum of two electrons. The second element in the periodic table, helium, has a total of two electrons. Both of the electrons in helium are therefore in the 1s orbital because the s orbital can hold up to two electrons and this is the lowest‐energy orbital. To allow both negatively charged electrons to occupy the same space and not fly apart, the electrons have opposite spins. This creates a very tiny magnetic field about each electron, causing them to behave as magnets with opposite polarity, and so holds them together in the orbital.

The concept of spin is beyond the scope of this textbook. But when representing electrons in orbitals, if there are two electrons in one orbital, make sure they are represented by arrows (or half arrows) pointing in opposite directions: i.e. one pointing up and one pointing down, as in Fig. 1.10.