In basic atomic structure, how many electron energy levels (shells) are occupied in a neutral magnesium atom?

Introduction / Context:
This question checks your understanding of electron configurations and energy levels (shells) in atoms. Magnesium is a common element in school chemistry, and its atomic number and electron configuration are used frequently as examples. Knowing how many principal energy levels, or shells, an atom occupies helps you place it correctly in the periodic table and predict its chemical behaviour, including valence electrons and reactivity. The question asks specifically how many electron levels are filled or partially filled in a neutral magnesium atom.

Given Data / Assumptions:
- The element is magnesium, with atomic number 12.
- A neutral magnesium atom has 12 electrons.
- The question is about the number of electron levels (shells), not subshells or orbitals.
- Standard electron configuration rules for main group elements apply (1st shell holds up to 2 electrons, 2nd and 3rd shells up to 8, and so on in the simple model).

Concept / Approach:
The atomic number of magnesium is 12, meaning a neutral atom has 12 protons and 12 electrons. In the Bohr model and basic electron configuration schemes, electrons fill shells in order of increasing energy: the first shell (n = 1) holds up to 2 electrons, the second (n = 2) holds up to 8, and the third (n = 3) begins to fill after the first two shells are filled. For magnesium, the electrons are arranged as 2 in the first shell, 8 in the second, and the remaining 2 in the third. Therefore, three distinct electron shells contain electrons in magnesium, so it has 3 occupied energy levels.

Step-by-Step Solution:
Step 1: Note that the atomic number of magnesium (Mg) is 12, so a neutral Mg atom has 12 electrons. Step 2: Fill the first shell (n = 1) with its maximum of 2 electrons, leaving 10 electrons. Step 3: Fill the second shell (n = 2) with its maximum of 8 electrons, leaving 2 electrons remaining. Step 4: Place the remaining 2 electrons in the third shell (n = 3). Step 5: Conclude that electrons occupy three shells in total: K (1st), L (2nd), and M (3rd), so the number of electron levels is 3.

Verification / Alternative check:
You can verify this using standard electron configuration notation. Magnesium has the configuration 1s2 2s2 2p6 3s2. Grouping by principal quantum number n, you have: n = 1 with 2 electrons (1s2), n = 2 with 8 electrons (2s2 2p6), and n = 3 with 2 electrons (3s2). This pattern confirms that shells 1, 2, and 3 are occupied. Additionally, in the simple distribution often taught in schools, magnesium is written as having 2, 8, 2 electrons in its shells, again showing three energy levels. No electrons occupy higher shells for the ground state of neutral magnesium, so the count remains three.

Why Other Options Are Wrong:
Option B (2) would correspond to an element whose electrons occupy only the first two shells, such as neon (2, 8). Option C (1) would suggest only the first shell is filled, as in helium (2). Option D (0) is impossible for any atom, as every atom must have at least one shell containing electrons. Therefore, options B, C, and D do not match the known electron configuration of magnesium. Only three occupied shells, as given by option A, are consistent with magnesium's atomic number and electron distribution.

Common Pitfalls:
Some students may confuse the number of valence electrons with the number of shells and mistakenly answer 2 because magnesium has two valence electrons in the third shell. Others may not recall its exact atomic number and try to guess based on its position in the periodic table. To avoid these mistakes, remember the simple shell distribution for magnesium as 2, 8, 2 and use it to count shells: each nonzero number in that sequence represents one occupied shell, giving a total of three shells for magnesium.

Final Answer:
A neutral magnesium atom has electrons in three energy levels (shells).