Question

Which of the following does not give flame colouration? (a) \(\mathrm{Mg}\) (b) \(\mathrm{Ca}\) (c) \(\mathrm{Sr}\) (d) \(\mathrm{Ba}\)

Short answer

(a) \(\mathrm{Mg}\) does not give flame colouration.

Step-by-step solution

Understanding Flame Colouration

Flame colouration occurs when certain elements emit specific colours in a flame test due to the excitation of electrons. When these electrons return to their ground state, they release energy in the form of light, which is perceived as different colours.

Identify Elements Known for Flame Colouration

Calcium (Ca), strontium (Sr), and barium (Ba) are alkaline earth metals known to produce distinct flame colours. Calcium emits an orange-red colour, strontium gives a crimson red, and barium displays a pale green colour in flame tests.

Examine Magnesium (Mg)

Magnesium (Mg) is another alkaline earth metal but does not typically give a flame colour. Unlike the others mentioned, magnesium's electrons do not emit visible light in a flame test, making it the exception in flame colouration.

Conclusion: Identifying the Exception

Among the options given, magnesium (Mg) does not give flame colouration, setting it apart from options Ca, Sr, and Ba, which do emit characteristic flame colours.

Key concepts

Alkaline Earth Metals

Alkaline earth metals are a group of elements located in Group 2 of the periodic table. They include beryllium (\(\text{Be}\)), magnesium (\(\text{Mg}\)), calcium (\(\text{Ca}\)), strontium (\(\text{Sr}\)), barium (\(\text{Ba}\)), and radium (\(\text{Ra}\)). These metals share some common characteristics, such as having two electrons in their outer shell. This makes them highly reactive, though not as reactive as the alkali metals in Group 1.

The alkaline earth metals tend to form compounds by donating those two outer electrons, achieving a stable electronic configuration similar to noble gases. These reactions generally result in the formation of ionic compounds with metals having a +2 oxidation state. Alkaline earth metals are generally shiny and silvery-white in appearance. Most are found naturally in the form of various minerals in the Earth's crust.

• Calcium and strontium compounds are common in the Earth's crust in the form of carbonate minerals.
• Barium is often found in mineral barite.

Electron Excitation

Electron excitation is a fundamental concept in understanding how flame colouration occurs in elements. When an atom is heated, particularly in a flame, the energy supplied excites the electrons. This means the electrons absorb energy and jump to higher energy levels or orbits.

However, these excited electrons are not stable in their higher-energy state and soon return to their original orbit, also known as the ground state. As the electrons fall back to the ground state, they release the absorbed energy in the form of light, producing characteristic colours that are unique to each element.

• The specific wavelengths of light emitted correspond to the differences in energy levels between the excited state and the ground state.
• These light wavelengths are visible to us in distinct colours, which can be used to identify certain elements.

Understanding electron excitation helps explain why elements like sodium produce a strong yellow flame, while others may emit red, green, or even no visible light at all.

Flame Colouration

Flame colouration is a fascinating result of electron excitation in metallic elements. It occurs due to the release of energy when electrons transition between energy levels. Certain metals, such as those from the alkaline earth metal group, exhibit distinct flame colours, which can be used to identify their presence in compounds.

For example:

• Calcium emits an orange-red flame.
• Strontium gives a crimson red colour.
• Barium shows a pale green flame.

However, some metals like magnesium do not produce any bright visible flame colour. This is because the energy levels in magnesium do not allow the electrons to emit visible light when they move from an excited state back to a ground state.

This principle is a key practical application in laboratory settings known as flame tests. These tests are a quick qualitative analysis method used to detect the presence of particular metal ions based on the low-energy electromagnetic radiation they emit. Flame tests are an essential tool in chemistry education and practice, helping students and professionals visually identify elements based on their unique flame colouration.