Question

Why do alkali metals form unipositive ions and impart the characteristic colours to flame?

Answer 1

at first we will find out what are the alkali metals. Then we will see what happens inside of the atom when an alkali metal is heated. Now we will find out when our eyes see the colours. Then we will easily know the reason behind alkali metals imparting the colours.

Complete answer:
Step1. The metals found in the first column of the periodic table are called alkali metals. They lie in S-block . First element is Lithium and the last element is francium. All the metals have their outermost electron in S-orbital. They all have almost the same physical and chemical properties.
Step2. Now our eyes see colours when our eyes hit with the light with specific wavelengths. That wavelength must lie between the spectral wavelength. According to the wavelength of every light ray we can see the colours.
Step3. The ionization energy of alkali metals is very low because of their one electron in the S shell. The energy flame excites the electron and sends it to a higher energy level. Now to find the stability the electron will emit back the energy and go back in its original energy level. Now the wavelength of those lies in the electromagnetic spectrum. That is why alkali metals impart colour to us.

Note: The alkali metals are highly reactive. They often do not occur in free states in nature. Sodium and potassium are found in abundance. Sodium is so reactive that it is kept in the Kerosene so that it doesn't catch fire while reacting to oxygen or water.