Definition of "atom"

Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the water-oxidizing complex, made up of manganese atoms and a calcium atom.

2013 September–October, Katie L. Burke, “In the news: Photosynthesis precursor”, in American Scientist‎, archived from the original on 13 April 2016

Towards the following morning, the thermometer fell to 5°; and at daylight, there was not an atom of water to be seen in any direction.

1835, John Ross, James Clark Ross, “Chapter XXXIV. Labour in Cutting through the Ice—Become Fixed for the Winter—Summary of the Month.”, in Narrative of a Second Voyage in Search of a North-west Passage, and of a Residence in the Arctic Regions, during the Years 1829, 1830, 1831, 1832, 1833; by Sir John Ross, C.B., K.S.A., K.C.S., &c. &c. Captain in the Royal Navy. Including the Reports of Commander (now Captain) J. C. Ross, R.N., F.R.S., F.L.S., &c. and the Discovery of the Northern Magnetic Pole, Philadelphia, Pa.: E. A. Carey & A. Hart; Baltimore, Md.: Carey, Hart & Co., pages 283–284

But at this critical moment the pirate astern sent a mischievous shot and knocked one of the men to atoms at the helm.

1863, Charles Reade, Hard Cash‎

"Doctor, tell me one word more," said Theodore, quivering with suppressed emotion. "How do you think it will end?" / "I have hardly the faintest atom of hope," answered this honest, earnest man.

1873, “Pansy” [pseudonym; Isabella Macdonald Alden], “A Double Crisis”, in Three People, Cincinnati, Oh.: Western Tract and Book Society, 176 Elm Street, page 325

Now, comrades, what is the nature of this life of ours? Let us face it, our lives are miserable, laborious, and short. We are born, we are given just so much food as will keep the breath in our bodies, and those of us who are capable of it are forced to work to the last atom of our strength; and the very instant that our usefulness has come to an end we are slaughtered with hideous cruelty. No animal in England knows the meaning of happiness or leisure after he is a year old. No animal in England is free. The life of an animal is misery and slavery: that is the plain truth.

1945 August 17, George Orwell [pseudonym; Eric Arthur Blair], chapter 1, in Animal Farm […], London: Secker & Warburg