Atoms | Middle school chemistry | Khan Academy

The video starts with a thought experiment about breaking down a piece of gold repeatedly. It poses the question: can you keep breaking it down forever, or will you reach a point where it can no longer be divided?

Ancient philosophers pondered whether elements, the roughly 100 building blocks of all matter, could be infinitely broken down. Many concluded that there must be an ultimate, indivisible smallest piece. The Greeks named this smallest piece the 'atom,' meaning 'uncuttable'.

Atoms are defined as the smallest pieces of an element that retain all the properties of that element. They are the fundamental building blocks. Examples include gold atoms having all the properties of gold, and carbon atoms having all the properties of carbon.

Atoms are incredibly tiny, impossible to see with microscopes. To illustrate their scale, a gold ring contains sextillions (1 followed by 21 zeros) of gold atoms, a number comparable to the total number of stars in the observable universe.

The video concludes by reiterating that almost all matter in the universe, from microbes to stars, is made of elements. These elements, in turn, are fundamentally made of atoms, which are the smallest and most basic building blocks containing all the element's properties.