Regents Chemistry Exam Explanations January 2009

A fluorescent light tube contains a noble gas and a drop of mercury. When the fluorescent light operates, the Hg is a vapor and there are free-flowing Hg ions and electrons in the tube. The electrons collide with Hg atoms that then emit ultraviolet (UV) radiation.

The inside of the tube is coated with a mixture of several compounds that absorbs UV radiation. Ions in the coating emit a blend of red, green, and blue light that together appears as white light. The compound that produces red light is Y2O3. The compound that produces green light is CeMgAl11O19. The compound that produces blue light is BaMgAl10O17.

75 Write the chemical name of the compound that produces red light. [1]

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yttrium oxide

76 Calculate the percent composition by mass of aluminum in the compound that produces green light. Your response must include both a correct numerical setup and the calculated result. [2]

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77 Explain, in terms of both electrons and energy, how ions in the coating emit light. [1]

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Excited electrons fall to lower energy levels losing energy

Electrons move from the ground state to an excited state as the compounds gain energy. Light energy is released when the electrons return to lower states.

Electrons lose energy as they move to lower shells.

Light is emitted as electrons return from higher to lower energy states.