Electrons in Different Orbits (Shells) – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain the formula 2n² for the maximum number of electrons in a shell. Use the first four shells as examples and state their capacities.
Answer:
Electrons in an atom occupy shells labeled by the principal quantum number n. The formula 2n² gives the maximum number of electrons that a shell can hold. This comes from quantum ideas where each energy level has sublevels and orbitals that together allow up to 2 electrons per orbital. For example:
- For n = 1 (K-shell): maximum = 2 × 1² = 2 electrons.
- For n = 2 (L-shell): maximum = 2 × 2² = 8 electrons.
- For n = 3 (M-shell): maximum = 2 × 3² = 18 electrons.
- For n = 4 (N-shell): maximum = 2 × 4² = 32 electrons.
In simple words, as n increases, the shell can hold more electrons because it has more orbitals. This rule helps us arrange electrons correctly in atoms and predict their chemical behaviour. - Key points: formula = 2n², shells get larger with n, and each orbital holds 2 electrons.
Q2. Why is the outermost shell limited to a maximum of 8 electrons for most elements? Explain using the idea of stability and give examples.
Answer:
The outermost shell is important for an atom’s chemical behaviour. For many elements (especially the first three rows of the periodic table), having 8 electrons in the outer shell gives a stable arrangement known as the octet. This stability is similar to the arrangement of noble gases like Neon (Ne). Because of this, atoms tend to gain, lose, or share electrons to reach an octet. For example, sodium (Na) with configuration 2, 8, 1 tends to lose 1 electron to become Na⁺, and chlorine (Cl) with 2, 8, 7 tends to gain 1 electron to become Cl⁻. When they combine, they form NaCl, where both achieve an octet. Thus, while shells can hold more electrons by 2n², the outer shell behaves as if capped at 8 for chemical stability in many common elements.
Q3. Describe the step-wise filling of shells (stepwise filling rule). Give examples with Carbon, Oxygen and Fluorine to show how electrons fill shells.
Answer:
Step-wise filling means electrons occupy lower energy shells first, then fill higher shells. This ensures the atom is in the lowest energy and most stable state. For example:
- Carbon (C) has 6 electrons. They fill as K-shell = 2, then L-shell = 4, giving configuration 2, 4.
- Oxygen (O) has 8 electrons. They fill K = 2, then L = 6, making 2, 6.
- Fluorine (F) has 9 electrons. They fill K = 2, L = 7, so 2, 7.
This rule is often stated as: fill the lowest energy level available before moving to the next. It explains why the first shell (K) is filled with 2 electrons before any electron goes to L-shell, and why outer shells determine how atoms interact in chemical reactions. Step-wise filling helps predict valence electrons and reactivity.
Q4. Compare Sodium (Na) and Magnesium (Mg) in terms of their electronic distribution and explain how that affects their chemical properties.
Answer:
Sodium (atomic number 11) and Magnesium (atomic number 12) differ by one electron in total, which changes their valence and reactivity. Their distributions are:
- Sodium (Na): 2, 8, 1 — outermost shell has 1 electron.
- Magnesium (Mg): 2, 8, 2 — outermost shell has 2 electrons.
Because Na has a single valence electron, it easily loses one electron to form Na⁺, making it very reactive and forming ionic bonds (for example NaCl). Mg tends to lose two electrons to form Mg²⁺, and so it forms compounds where it gives away two electrons (for example MgO). Both aim to achieve a stable octet (like Neon). In short, the number of outer electrons determines whether an element loses one or two electrons, and thus its usual ionic charge and chemical behaviour.
Q5. How can electronic configuration help predict whether an element is reactive or inert? Use Neon, Fluorine and Sodium as examples and explain in simple terms.
Answer:
Electronic configuration shows how many electrons are in the outermost shell (valence electrons). Atoms with a complete outer shell (usually 8 electrons) are inert or less reactive. For example, Neon (Ne) has configuration 2, 8 and is inert because its outer shell is full. Atoms with incomplete outer shells are usually reactive because they can gain, lose, or share electrons to become stable. Fluorine (F) with 2, 7 needs 1 electron to complete its octet, so it is highly reactive and tends to gain 1 electron forming F⁻. Sodium (Na) with 2, 8, 1 tends to lose 1 electron to reach a full shell and becomes Na⁺. So, by looking at the outer shell electrons, we can predict whether an element is likely to react and what kind of bond (gain or lose) it might form.
High Complexity (Analytical & Scenario-Based)
Q6. An atom has more than 8 electrons available to place using the formula 2n². Explain how such atoms behave chemically and why many elements still follow the octet rule. Give examples of bonding that show this behaviour.
Answer:
Although the formula 2n² allows shells to hold many electrons (for example the 3rd shell can hold up to 18), many common elements aim for an octet in their outermost shell to reach stability. This leads to three main behaviours: losing, gaining, or sharing electrons. For instance, sodium (2,8,1) loses one electron to form Na⁺, while chlorine (2,8,7) gains one to form Cl⁻; together they form ionic NaCl, each with an octet. In covalent bonding, atoms share electrons: two oxygen atoms (each 2,6) share two electrons each to form O₂, so each effectively attains an octet. For elements with larger shells (beyond the first 20), expanded valence can occur in special cases, but for most simple chemical reactions taught in Class 9, the pursuit of an octet governs reactivity and bond formation.
Q7. Element X has atomic number 17. Write its electron distribution, state its valency, and explain how it will react with sodium. Provide the final formula and reason.
Answer:
Element X with atomic number 17 is Chlorine (Cl). Its electron distribution is 2, 8, 7. The valency is 1, because it needs one electron to complete its outer shell and reach an octet. Sodium (Na) has distribution 2, 8, 1 and valency 1 (it easily loses one electron). When chlorine reacts with sodium, sodium donates its one valence electron to chlorine. Sodium becomes Na⁺ and chlorine becomes Cl⁻. The opposite charges attract to form an ionic bond, giving the compound NaCl. This reaction occurs because both atoms achieve stable octet configurations: Na resembles Neon and Cl resembles Argon, making the ionic compound energetically favourable and stable.
Q8. Explain why the third shell (M-shell) can hold up to 18 electrons according to 2n², yet for many elements in the first three periods it appears limited to 8 electrons. Use examples like Aluminium and Calcium to clarify.
Answer:
The formula 2n² predicts that the third shell (n = 3) can hold 18 electrons, because it includes more sublevels. However, for elements in the first three periods (first 20 elements), electrons fill lower energy sublevels in such a way that the outermost usable valence behaves like it has up to 8 electrons. For example, Aluminium (Al) with atomic number 13 has distribution 2, 8, 3 (not using full 18 in M-shell as valence), and it commonly shows valency 3. Calcium (Ca) with atomic number 20 is 2, 8, 8, 2 — here the 3rd shell has 8 electrons and the 4th shell starts with 2 electrons. The reason is that the 4s orbital (part of the 4th shell) has lower energy than some 3d orbitals, so electrons begin filling higher shells or different sublevels. In simple terms, energy ordering and orbital filling cause the outermost shell to act like it holds up to 8 valence electrons for many common elements.
Q9. You are explaining to your class why noble gases are inert. Use Neon and Argon to describe their electron distribution and relate that to chemical inactivity.
Answer:
Noble gases are inert because they have complete outer shells, making them stable and unlikely to react. For example, Neon (Ne) has electron distribution 2, 8, meaning its outer shell is full with 8 electrons. Argon (Ar) has distribution 2, 8, 8, again with a completely filled outer shell. A filled outer shell means there is no tendency to gain or lose electrons, so these atoms are not driven to form bonds to achieve stability. Because of this stable configuration, noble gases rarely combine with other elements under normal conditions. Their chemical inactivity is directly due to their complete valence shells, which satisfies the octet (or duet for the first shell) requirement and leads to minimal chemical reactivity.
Q10. Element Y has electron distribution 2, 8, 5. Identify the element, state its valency, and predict the formulas it would form with sodium and with oxygen. Explain your reasoning.
Answer:
An atom with distribution 2, 8, 5 has 15 electrons, so the element is Phosphorus (P). Its outermost shell has 5 valence electrons, so its usual valency to attain an octet is 3 (it can gain 3 electrons) or it may share electrons covalently. With sodium (Na), which tends to lose 1 electron (Na⁺), three sodium atoms will each donate one electron to phosphorus, forming Na₃P where phosphorus becomes P³⁻ and each sodium is Na⁺. With oxygen (O), phosphorus commonly forms covalent oxides such as P₂O₅ (which can be thought of as P in +5 oxidation state) or simpler oxide formulas depending on bonding. The predictions fol...