Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 51

Each element is described by the number of protons, neutrons, and electrons it possesses. These are represented by two quantities: the mass number (A) and the atomic number (Z) The mass number indicates the total number of protons and neutrons in the nucleus of the atom (p + n). The atomic number gives the number of protons in a neutral atom of the element (p). A neutral atom must have the same number of protons as electrons, so Z also indicates the number of electrons (e).

The mass number gives the total number of protons and neutrons: A = p + n. The atomic number gives the total number of protons or electrons present in a neutral atom: Z = p or e.

For any element X, we can represent the information about the mass number, A, and atomic number, Z, as shown in Figure 1.2a. As you can see, the mass number is written as a superscript and the atomic number as a subscript in front of the symbol for the element.

Figure 1.2 (a) General representation of mass number and atomic number for the element X. (b) The mass number and atomic number for beryllium.

Information about the element beryllium is shown in Figure 1.2b using this convention.

Beryllium (Be) has an atomic number, Z, of 4; therefore, in a neutral molecule, there are 4 protons and 4 electrons. Beryllium has a mass number, A, of 9; therefore it has 5 neutrons (because the mass number is 9 and we already know that it has 4 protons from the atomic number, so 9 − 4 = 5 neutrons). The mass number is written as a superscript and the atomic number as a subscript before the symbol for the element.