Chemistry
Two elements which can form a large number of compounds are

Carbon and Oxygen
Carbon and Sulphur
Carbon and Nitrogen
Carbon and Hydrogen

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Chemistry
Sodium vapour lamps glow with yellow colour. This is due to

Its ability to absorb all other colours except yellow
sublimation of sodium to emit yellow colour
The low ionisation energy of sodium
The emission of excess energy absorbed by sodium atoms, in the yellow region of the spectrum

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