37.6.3 Using the Nuclear Model

The nuclear model of the atom makes it easy to understand and picture such processes as ionization. Because electrons orbit a positive nucleus, an x-ray photon or a rapidly moving particle, such as another electron, can knock one of the orbiting electrons away, creating a positive ion. Removing one electron makes a singly charged ion, with $q = + e$ . Removing two electrons creates a doubly charged ion, with $q = + 2 e$ . This is shown for lithium (atomic number 3) in FIGURE 37.20.

The figure consists of three parts that illustrates different ionization stages of the lithium atom using the nuclear model.

FIGURE 37.20 Full Alternative Text

The nuclear model also allows us to understand why, during chemical reactions and when an object is charged by rubbing, electrons are easily transferred but protons are not. The protons are tightly bound in the nucleus, shielded by all the electrons, but outer electrons are easily stripped away.

EXAMPLE 37.7 The ionization energy of hydrogen

What is the minimum energy required to ionize a hydrogen atom?

SOLVE In Example 37.6 we found that the atom’s energy is $E_{i} = - 13.6 eV$ . Ionizing the atom means removing the electron and taking it very far away. As $r \to \infty$ , the potential energy becomes zero. Further, using the least possible energy to ionize the atom will leave the electron, when it is very far away, very nearly at rest. Thus the atom’s energy after ionization is $E_{f} = K_{f} + U_{f} = 0 + 0 = 0 eV$ . This is larger than $E_{i}$ by 13.6 eV, so the minimum energy that is required to ionize a hydrogen atom is 13.6 eV. This is called the atom’s ionization energy. If the electron receives $\geq 13.6 eV (2.17 \times 10^{- 18} J)$ of energy from a photon, or in a collision with another electron, or by any other means, it will be knocked out of the atom and leave a $H^{+}$ ion behind.