Final answer:
Silicon is the element with 4 electrons in energy level 3; its electronic configuration ends in 3s2 3p2, while the other options do not match the criterion of having exactly 4 electrons in that energy level.
Step-by-step explanation:
The question is regarding which element has 4 electrons in energy level 3. The answer to the question is silicon, as silicon has an atomic number of 14 and its electronic configuration ends with 3s23p2, which means it has 4 electrons in the third energy level. Options carbon and nitrogen do not have electrons in the third energy level because they are lighter elements with fewer electrons overall, and phosphorus has five electrons in its third energy level.
To further break down the concept, the third energy level, also known as the third shell, is denoted by the principal quantum number n=3. Now, looking at the periodic table and applying knowledge of electron configurations, option b (Carbon), with 6 electrons, fills up its second energy level with 2 electrons in the 2s orbital and 4 electrons in the 2p orbitals and has no electrons in the third energy level. On the other hand, option c (Nitrogen) with 7 electrons, fills the second energy level with 5 electrons and also does not reach the third energy level. Option d (Phosphorus), however, has 15 electrons with the last 5 being in the third energy level, which is more than 4 electrons. Hence, based on these configurations, we can confidently say that option a (Silicon) is the correct answer.