Question:
Which pair of the following elements do not give flame test?
Which pair of the following elements do not give flame test?
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Flame test is not shown by Be and Mg because of very high ionization energy and absence of easy electronic excitation.
Updated On: Jun 20, 2026
- Be, Ca
- Be, Mg
- Mg, Ca
- Ca, Ba
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The Correct Option is B
Solution and Explanation
Step 1: Understand flame test principle.
Flame test is based on excitation of valence electrons of metal atoms. When electrons return to ground state, they emit visible light of characteristic color. Only those metals whose electrons get easily excited show flame color.
Step 2: Identify elements that show flame test.
Alkali and alkaline earth metals generally show flame colors due to low ionization energy. However, very small or strongly hydrated ions may not show flame test.
Step 3: Analyze Be (Beryllium).
Be has very high ionization energy and small size. Its electrons are strongly held, so they cannot be easily excited in flame. Hence Be does not give flame test.
Step 4: Analyze Mg (Magnesium).
Mg also has relatively high ionization energy and its electrons are not easily excited in flame conditions. Therefore Mg also does not show a distinct flame color.
Step 5: Compare with Ca and Ba.
Ca and Ba have larger atomic size and lower ionization energy. Their electrons get easily excited and they show characteristic flame colors (brick red for Ca, green for Ba).
Step 6: Final conclusion.
Since both Be and Mg fail to give flame test due to high ionization energy and strong nuclear attraction, they form the correct pair.
Final Answer: \[ \boxed{\text{Be, Mg}} \]
Flame test is based on excitation of valence electrons of metal atoms. When electrons return to ground state, they emit visible light of characteristic color. Only those metals whose electrons get easily excited show flame color.
Step 2: Identify elements that show flame test.
Alkali and alkaline earth metals generally show flame colors due to low ionization energy. However, very small or strongly hydrated ions may not show flame test.
Step 3: Analyze Be (Beryllium).
Be has very high ionization energy and small size. Its electrons are strongly held, so they cannot be easily excited in flame. Hence Be does not give flame test.
Step 4: Analyze Mg (Magnesium).
Mg also has relatively high ionization energy and its electrons are not easily excited in flame conditions. Therefore Mg also does not show a distinct flame color.
Step 5: Compare with Ca and Ba.
Ca and Ba have larger atomic size and lower ionization energy. Their electrons get easily excited and they show characteristic flame colors (brick red for Ca, green for Ba).
Step 6: Final conclusion.
Since both Be and Mg fail to give flame test due to high ionization energy and strong nuclear attraction, they form the correct pair.
Final Answer: \[ \boxed{\text{Be, Mg}} \]
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