The atoms of the elements germanium, arsenic, selenium and bromine each contain how many electron shells according to their positions in the modern periodic table?

Introduction / Context:
This question checks understanding of how the periodic table is organised into periods and how the period number relates directly to the number of electron shells in an atom. Germanium, arsenic, selenium and bromine are a sequence of elements located in the same period. Recognising their common period allows us to deduce how many electron shells each atom possesses in its ground state electronic configuration.

Given Data / Assumptions:

• Elements considered: germanium (Ge), arsenic (As), selenium (Se) and bromine (Br).
• We use the modern long form periodic table.
• Each period in the periodic table corresponds to a principal quantum number n and to the number of occupied electron shells.
• All four elements are placed in the same horizontal row (period).

Concept / Approach:
In the modern periodic table, the period number tells us how many electron shells are occupied in the ground state of an element. Elements in period 1 have 1 shell, elements in period 2 have 2 shells, and so on. Germanium, arsenic, selenium, and bromine are in period 4. Therefore, each of these atoms has electrons occupying four principal energy levels or shells (K, L, M and N shells). Once we identify the period, we can answer the question without detailed electronic configurations.

Step-by-Step Solution:

Verification / Alternative check:
A more detailed verification can be done by writing approximate electronic configurations. For example, germanium has configuration [Ar] 3d10 4s2 4p2, which clearly uses four shells (levels 1, 2, 3, and 4). Similarly, arsenic has [Ar] 3d10 4s2 4p3, selenium has [Ar] 3d10 4s2 4p4, and bromine has [Ar] 3d10 4s2 4p5. In every case electrons occupy up to the fourth shell. This confirms that each atom has four electron shells.

Why Other Options Are Wrong:

• 3 shells: This would correspond to elements in period 3, such as sodium or chlorine, not to the period 4 elements listed in the question.
• 5 shells: That would apply to period 5 elements like rubidium or iodine, not to germanium, arsenic, selenium and bromine.
• 7 shells: This is characteristic of very heavy elements in period 7 and is far above the given elements.
• 6 shells: This would correspond to period 6 elements, again not matching the positions of the elements in the question.

Common Pitfalls:
A frequent mistake is to confuse group number with period number or to assume that the number of valence electrons equals the number of shells. Another error is to memorise configurations without linking them back to the periodic table arrangement. To avoid this, always remember that periods run horizontally and indicate the number of shells, while groups run vertically and indicate the pattern of valence electrons. Focusing on the period makes questions like this straightforward.

Final Answer:
Each of the atoms of germanium, arsenic, selenium, and bromine contains 4 electron shells.