she is (a ) newer comer to (不填)chemistry是这样吧 but she has already made some important discoveries.
newcomer是个可数名词,而且是第一次出现,前面应该是不定冠词a,类似于I am a teacher.而chemistry是表示学科的名词,前面不需要加任何冠词,例如:He is good at physics.
其他参考：
new comer是可数名词的单数形式,前面必须有限定词(冠词、物主代词、指示代词或不定代词等)；chemistry是不可数名词,前面不用限定词.