Na and K have low first and high second ionization potentials, whereas Mg and Ca have

low values for both first and second potentials.
These observations suggest that

To me, Mg and Ca have a normal valanece of +2.

these observations suggest that Na and K have a relatively low tendency to lose their first electron and a high tendency to lose their second electron. On the other hand, Mg and Ca have a relatively low tendency to lose both their first and second electrons.

To understand why this is the case, we need to look at the electronic configurations of these elements.

Na has an atomic number of 11, which means it has 11 electrons. Its electronic configuration is 1s² 2s² 2p⁶ 3s¹. The first ionization potential refers to the energy required to remove the outermost (valence) electron, which in this case is the 3s¹ electron. Since this is a relatively weakly held electron, it requires a relatively low amount of energy to remove it.

After losing the 3s¹ electron, the electronic configuration of Na+ becomes 1s² 2s² 2p⁶. To remove the second electron, we need to overcome the increased repulsion from the remaining 10 electrons and the increased attraction from the positively charged Na+ ion. This requires a much higher amount of energy, resulting in a high second ionization potential for Na.

K, on the other hand, has an atomic number of 19 and an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Similarly to Na, the first ionization potential for K refers to the energy required to remove the outermost electron, which in this case is the 4s¹ electron. Again, this electron is weakly held, so it requires a relatively low amount of energy to remove.

However, after losing the 4s¹ electron, the electronic configuration of K+ becomes 1s² 2s² 2p⁶ 3s² 3p⁶. To remove the second electron, we again need to overcome the increased repulsion and increased attraction, resulting in a high second ionization potential for K.

Moving on to Mg and Ca, Mg has an atomic number of 12, and its electronic configuration is 1s² 2s² 2p⁶ 3s². The first ionization potential for Mg refers to the energy required to remove the outermost 3s² electrons. These electrons are relatively weakly held, so it requires a relatively low amount of energy to remove them.

After losing the 3s² electrons, the electronic configuration of Mg²+ becomes 1s² 2s² 2p⁶. To remove the second electron, we need to overcome the increased repulsion and increased attraction, but since the 3s electrons have already been removed, it requires a lower amount of energy compared to Na and K. Hence, Mg has a comparatively low second ionization potential.

Similarly, Ca has an atomic number of 20, and its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². The first ionization potential for Ca refers to the energy required to remove the outermost 4s² electrons. These electrons are relatively weakly held, so it requires a relatively low amount of energy to remove them.

After losing the 4s² electrons, the electronic configuration of Ca²+ becomes 1s² 2s² 2p⁶ 3s² 3p⁶. To remove the second electron, we again need to overcome the increased repulsion and increased attraction, but since the 4s electrons have already been removed, it requires a lower amount of energy compared to Na, K, and even Mg. Hence, Ca also has a comparatively low second ionization potential.

In summary, the low first ionization potentials of Na and K indicate that their valence electrons are relatively weakly held, while the high second ionization potentials indicate that their second outermost electrons are more tightly held due to increased repulsion and increased attraction. On the other hand, the low first and second ionization potentials of Mg and Ca suggest that their valence electrons are weakly held and that the subsequent outermost electrons are not as strongly bound due to the absence of the previously removed electrons.