Crossword Dictionary
atom
atom - n
(physics and chemistry) the smallest component of an element having the chemical properties of the element
Atoms are the basic units of matter. Everything in the universe apart from energy is made of matter therefore atoms make up everything in the universe, according to Northwestern University. The term "atom" comes from the Greek word for indivisible, because it was once thought that atoms were the smallest things in the universe and could not be divided. We now know that atoms are made up of three particles known as subatomic particles: protons, neutrons and electrons — which are composed of even smaller particles, such as quarks.
Atoms were created after the Big Bang 13.7 billion years ago. As the hot, dense new universe cooled, conditions became suitable for quarks and electrons to form. Quarks came together to form protons and neutrons, and these particles combined into nuclei. This all took place within the first few minutes of the universe's existence, according to CERN.
It took 380,000 years for the universe to cool enough to slow down the electrons so that the nuclei could capture them to form the first atoms. The earliest atoms were primarily hydrogen and helium, which are still the most abundant elements in the universe, according to Jefferson Lab. Gravity eventually caused clouds of gas to coalesce and form stars, and heavier atoms were (and still are) created within the stars and sent throughout the universe when the star exploded (supernova).