Atomic number is the number of protons in an atom’s nucleus. For a neutral atom, the number of electrons equals that proton number. Electron-shell diagrams are simplified models; distinguish a shell’s maximum capacity from the order in which orbitals fill.
Start with proton number
Identify the element by its atomic number, not by a guessed electron-shell drawing. An ion has gained or lost electrons, while its proton number still identifies the element. Do not confuse atomic number with mass number, which counts protons and neutrons for a specified isotope. State the kind of atom or ion being described before counting electrons.
The third shell can hold eighteen electrons
The maximum capacity of the n = 3 shell is 18 electrons, from its s, p and d subshells. Simplified arrangements for early elements often show eight electrons in the third shell before the fourth begins to fill. These statements concern different ideas: capacity and the energy-based filling order. A basic eight-electron diagram does not establish the shell’s full capacity.
Use a shell diagram at the expected level
A Bohr-style classroom diagram places electrons in schematic shells around a nucleus. It is a model for organisation, not a literal picture of fixed planetary paths. Use the representation prescribed by the course. If the question asks for electron configuration, a shell sketch may not provide the orbital detail required.
Connect elements to the periodic table
The alkaline earth metals are the Group 2 elements, including magnesium and calcium. Use the group relationship to support discussion at the level taught, but do not infer every chemical property from one simple rule. Check whether the assignment concerns an isolated atom, an ion or a substance, since these are different objects of explanation.
Check your final explanation
Verify proton and electron counts, charge, notation and the distinction between capacity and filling. Explain why a diagram answers the question rather than supplying it without commentary. Where a simplified model is used, state the relevant limitation. That makes the answer clearer than treating apparently different textbook diagrams as a contradiction.
Sources and further reading
Related study guides
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For general study guidance, visit the knowledgebase. Follow your own course requirements when applying this article to assessed work.