( ( Quantum mechanics ) ( ( is ( ( the branch ) ( of ( mechanics ( that ( deals ( with ( ( ( the motion ) ( of ( ( ( ( ( ( ( electrons , ) protons ) , ) neutrons ) , ) and ) ( other ( subatomic particles ) ) ) ) ) ( that ( ( make up ) ( ( atoms and ) molecules ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Quantum) (NNS mechanics)) (VP (VBZ is) (NP (NP (DT the) (NN branch)) (PP (IN of) (NP (NP (NNS mechanics)) (SBAR (WHNP (WDT that)) (S (VP (VBZ deals) (PP (IN with) (NP (NP (DT the) (NN motion)) (PP (IN of) (NP (NP (NNS electrons)) (, ,) (NP (NNS protons)) (, ,) (NP (NNS neutrons)) (, ,) (CC and) (NP (JJ other) (JJ subatomic) (NNS particles)))) (SBAR (WHNP (WDT that)) (S (VP (VBP make) (PRT (RP up)) (NP (NNS atoms) (CC and) (NNS molecules)))))))))))))) (. .)))
Quantum mechanics is the branch of mechanics that deals with the motion of electrons, protons, neutrons, and other subatomic particles that make  up atoms and molecules.
(ROOT (S (NP (NP (DT The) (JJ specialized) (NN study)) (PP (IN of) (NP (NP (DT the) (NN motion)) (PP (IN of) (NP (NP (NNS objects)) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (ADJP (JJ atomic/subatomic) (PP (IN in) (NP (NN size)))))))))))) (VP (VBZ is) (VP (VBN called) (NP (NN quantum) (NNS mechanics)))) (. .)))
The specialized study of the motion of objects that are atomic/subatomic in size is called quantum mechanics.