Final answer:
After examining the valence electrons for each pair of elements listed, none of them have the same number of valence electrons in their stable states. Elements from the same group in the periodic table typically share the same number of valence electrons, which defines their chemical properties and reactivity.
Step-by-step explanation:
The question relates to the concept of valence electrons in chemistry, asking which pairs of elements have the same number of valence electrons. Looking at the periodic table and knowing the typical electron configurations of elements, we can determine the number of valence electrons for each element presented. Valence electrons are important because they determine the chemical properties of an element, particularly its reactivity and the types of bonds it can form with other elements.
- Oxygen (O) is in group 16 and has 6 valence electrons, while bromine (Br) is in group 17 with 7 valence electrons, so they do not match.
- Selenium (Se) is also in group 16 like oxygen, having 6 valence electrons, and bromine (Br) also being in group 17 with 7 valence electrons, means they do not match.
- Carbon (C) is in group 14 with 4 valence electrons, while lead (Pb) is also in group 14, but due to the inert pair effect lead frequently exhibits only 2 valence electrons in its stable state.
- Sodium (Na) is in group 1 with 1 valence electron, and magnesium (Mg) is in group 2 with 2 valence electrons, so they do not have the same number of valence electrons.
Therefore, none of the pairs of elements listed have exactly the same number of valence electrons when considering their usual stable states.