Particle, Grain, Atomic Particle
noun
A grain's child is even smaller than a grain. It's a particle, sometimes even an atomic particle.
りゅうし
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
光が粒子なのか波なのかを考えていて、昨日は眠れませんでした。
I was thinking about whether light was a particle or a wave and I couldn’t sleep last night.