Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 65

The position of oxygen in the periodic table and its atomic number (Z = 8) tell us that a neutral atom of oxygen has 8 protons and therefore must have 8 electrons. However, oxygen is in the second row of the periodic table; therefore the electrons are arranged into two shells. The first shell contains two electrons because there is only space for two electrons, and the second shell contains six electrons. We can draw this as shown:

We have seen that each successive shell in an atom can hold an increasing number of electrons. The reason for the precise maximum number of electrons that each shell can hold is due to the arrangement of the electrons in each shell. The next section will explain how electrons are arranged in shells or energy levels.