Beryllium and magnesium do not give specific colour to flame whereas other alkaline earth metals do so. Why?

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#1

(a) Beryllium and magnesium do not give specific colour to flame whereas other alkaline earth metals do so. Why ?
(b) Why are potassium and cesium used in photoelectric cells ?
© Why ${{Li}{2}}$C${{O}{3}}$ decomposes at lower temperature whereas ${{Na}{2}}$C${{O}{3}}$ at higher temperature ?


#2

(a) Beryllium and magnesium atoms are smaller in size and their electrons are strongly bound to the nucleus. They need large amounts of energy for excitation of electrons to higher energy levels which is not available in the bunsen flame. So they do not impart colour to the flame.
(b) Due to low ionization energies, alkali metals, specially potassium and caseium, eject electrons when exposed to light, therefore, potassuim and caseium are used in photoelectric cells.
© Due to very small size and high polarising j power of lithium ion, {{Li}{2}}$C${{O}{3}}$ is unstable and easily decompose on heating.