We know that Kinetic Energy (\(KE\)) is equal to the negative of Total Energy (\(KE = -E_n\)). Therefore:
\( KE = 13.6 \times \frac{Z^2}{n^2} \text{ eV} \)
Thus, kinetic energy is directly proportional to \( \frac{Z^2}{n^2} \). We just need to calculate this ratio for each given option.
Step 2: Evaluate Each Option
(A) First orbit of H atom:
Here \( Z = 1 \) and \( n = 1 \).
\( \text{Ratio } \left(\frac{Z^2}{n^2}\right) = \frac{1^2}{1^2} = \mathbf{1} \)
(B) First orbit of He\(^+\):
Here \( Z = 2 \) and \( n = 1 \).
\( \text{Ratio } \left(\frac{Z^2}{n^2}\right) = \frac{2^2}{1^2} = \mathbf{4} \)
(C) Second orbit of He\(^+\):
Here \( Z = 2 \) and \( n = 2 \).
\( \text{Ratio } \left(\frac{Z^2}{n^2}\right) = \frac{2^2}{2^2} = \frac{4}{4} = \mathbf{1} \)
(D) Second orbit of Li\(^{2+}\):
Here \( Z = 3 \) and \( n = 2 \).
\( \text{Ratio } \left(\frac{Z^2}{n^2}\right) = \frac{3^2}{2^2} = \frac{9}{4} = \mathbf{2.25} \)
Conclusion: Comparing the values (1, 4, 1, and 2.25), the highest ratio is 4. Therefore, the highest kinetic energy is associated with the first orbit of He\(^+\). The correct option is (B).
Mastering Bohr Model Proportionalities
In JEE Advanced, many questions can be solved without calculating the exact numerical value by simply comparing proportionalities. Remembering how Energy, Velocity, and Radius scale with \(Z\) (Atomic Number) and \(n\) (Orbit Number) is a massive time-saver.
For example: \( v \propto \frac{Z}{n} \), \( r \propto \frac{n^2}{Z} \), and \( E \propto \frac{Z^2}{n^2} \).
To build absolute confidence in these relationships and tackle complex multiple-choice questions, we highly recommend reading our detailed conceptual guide on the
Structure of Atom Class 11 Chemistry.
Continue Your Chemistry Prep
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Class XI Chemistry.
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Class XII Chemistry.
7 Common Mistakes in IUPAC Nomenclature of Organic Compounds (With Examples)
Organic Chemistry
7 Common Mistakes to Avoid While Doing IUPAC Nomenclature
Whether you are preparing for JEE (Main & Advanced), NEET, CBSE Boards, or AP Chemistry, mastering IUPAC rules is crucial. Stop losing easy marks with this comprehensive guide.
Updated: June 2026 10 Min Read
Naming organic compounds is like learning a new language. The International Union of Pure and Applied Chemistry (IUPAC) nomenclature system provides a logical, rule-based method to ensure every chemical structure has a unique, unambiguous name worldwide.
However, students often fall into familiar traps. A misplaced comma, a miscounted carbon chain, or forgetting the hierarchy of functional groups can turn a correct answer into a minus marking on highly competitive exams like JEE and NEET. Let's break down the 7 most common IUPAC nomenclature mistakes and exactly how to fix them so you score full marks on your chemistry paper.
1. The "Straight Chain is Always the Principal Chain" Trap
One of the most frequent errors is assuming that the longest carbon chain drawn horizontally is automatically the parent chain. Rule Check: The principal (parent) chain is the longest continuous carbon chain that must contain the principal functional group and the maximum number of multiple bonds, even if it is not the longest overall chain or isn't drawn in a straight horizontal line.
Example: Choosing the Parent Chain
Structure: A 7-carbon horizontal chain with a double bond branching downwards off carbon 3.
Incorrect Name
3-vinylheptane
Why? The student chose the 7-carbon straight chain (heptane) but left the double bond as a substituent.
Correct Name
3-butylpent-1-ene
Why? The chain must include the double bond. The longest chain containing the double bond has 5 carbons (pentene).
2. Misapplying the Lowest Locant Rule
Students often number from left to right out of habit. However, numbering must proceed from the end that gives the lowest numbers (locants) to the principal functional group first, then multiple bonds, and finally substituents (alkyl groups or halogens).
If there is a tie, apply the first point of difference rule. Compare the locant sets term by term; the set with the lowest number at the first point of difference wins.
Example: Numbering the Chain
Incorrect Name
2,5,5-trimethylhexane
Locant set: 2, 5, 5
Correct Name
2,2,5-trimethylhexane
Locant set: 2, 2, 5. Comparing (2,5,5) and (2,2,5), the second number is lower in the correct set.
3. Forgetting Functional Group Priority
When a molecule contains multiple functional groups, one acts as the principal functional group (suffix), and the rest are treated as substituents (prefixes). Messing up this hierarchy is guaranteed to lose you points.
A molecule has an -OH group and a C=O (ketone) group.
Incorrect Name
4-oxopentan-2-ol
Why? Ketones have higher priority than alcohols. The suffix must be '-one'.
Correct Name
4-hydroxypentan-2-one
Why? The ketone gets the lowest number and suffix, while -OH becomes the 'hydroxy' prefix.
4. Alphabetical Order Mishaps with Prefixes
Substituents must be listed in alphabetical order. The common trap is counting numerical prefixes (di, tri, tetra) or hyphenated structural prefixes (sec, tert) when alphabetizing. You ignore them. However, you do count non-hyphenated prefixes like iso, neo, and cyclo.
Example: Alphabetizing Substituents
Incorrect Name
2,2-dimethyl-4-ethylhexane
Why? The student alphabetized 'dimethyl' under 'D' before 'ethyl' (E).
Correct Name
4-ethyl-2,2-dimethylhexane
Why? 'di' is ignored. 'Ethyl' (E) comes before 'methyl' (M).
5. Dropping the 'e' Incorrectly
When adding a suffix to the parent alkane/alkene/alkyne name, you drop the terminal 'e' only if the suffix begins with a vowel (a, e, i, o, u) or 'y'. If the suffix begins with a consonant (like 'di' or 'tri' or 'nitrile'), you keep the 'e'.
Hexane + -ol = Hexanol (drop 'e' because of 'o')
Hexane + -diol = Hexanediol (keep 'e' because of 'd')
Butane + -nitrile = Butanenitrile (keep 'e' because of 'n')
6. Punctuation Errors: Commas vs. Hyphens
In the digital age where graders are often automated systems (especially in online NTA mock tests and homework platforms), formatting is everything. IUPAC rules are strict about punctuation:
Use commas to separate numbers from numbers (e.g., 2,2).
Use hyphens to separate numbers from letters (e.g., 2-methyl).
There are no spaces in an IUPAC name, except for carboxylic acids (e.g., ethanoic acid) and esters (e.g., ethyl ethanoate).
2, 2 dimethyl propane2,2-dimethylpropane
7. Multi-Bond Numbering Directions
When a carbon chain contains both a double bond (ene) and a triple bond (yne), how do you number them? The rule is: give the lowest possible locants to the double and triple bonds as a set. However, if there is a choice (symmetry), the double bond gets the lower number.
Example: Ene vs. Yne Suffix
Structure: A 5-carbon chain with a double bond at one end and a triple bond at the other.
Incorrect Name
pent-4-en-1-yne
Correct Name
pent-1-en-4-yne
Why? The numbering set is 1,4 either way. In a tie, the '-ene' wins the lower number over '-yne'. Also note the dropped 'e' from 'ene' before 'yne'.
Frequently Asked Questions
The lowest locant rule dictates that when numbering a carbon chain, the substituents or functional groups should receive the lowest possible numbers (locants) at the first point of difference. You compare numbering from left-to-right vs right-to-left.
No, multiplying prefixes like di-, tri-, tetra-, sec-, and tert- are ignored when alphabetizing substituents. However, prefixes that are part of the fundamental structural name like iso-, neo-, and cyclo- are considered.
Carboxylic acids (-COOH) have the highest priority in standard IUPAC nomenclature. When a molecule has a carboxylic acid, it dictates the parent chain numbering and the suffix (-oic acid), forcing other groups like alcohols or ketones to be named as substituents (hydroxy- or oxo-).
