Chapter 15: Visualizing the Atom Losing Valence Electrons The shell model described in Section 15.4 can be used to explain a wide variety of properties of atoms. Using the shell model, for example, we can explain how atoms within the same group tend to lose (or gain) the same number of electrons. Let's consider the case of three group 1 elements: lithium, sodium, and potassium. Look to a periodic table and find the nuclear charge of each of these atoms: Nuclear charge: Number of i nner shell electrons: +3 +11 10 2 +19 18 How strongly the valence electron is held to the nucleus depends on the strength of the nuclear charge-the stronger the charge, the stronger the valence electron is held. There's more to it, however, because inner-shell electrons weaken the attraction outer-shell electrons have for the nucleus. The valence shell in lithium, for example, doesn't experience the full effect of three protons. Instead, it experiences a diminished nuclear charge of about +1. We get this by subtracting the number of inner-shell electrons from the actual nuclear charge. What do the valence electrons for sodium and potassium experience? Diminished nuclear charge: ~+1 ~+1 ~+1 Question: Potassium has a nuclear charge many times greater than that of lithium. Why is it actually easier for a potassium atom to lose its valence electron than it is for a lithium atom to lose its valence electron? Potassium's valence electron is much farther from the nucleus. Because the electric force decreases with distance, the +1 charge for potassium's valence electron is not so effective at holding to the atom. Hence, it is easily lost. 79