Harley-Davidson XA Into the war, the government asked Harley to build a bike that was similar to the BMWs used by the Germans. So they didâto exacting specification. The 740cc flat twin and shaft drive aped the R71 perfectly. Harley built 1,000 XAs and even used the platform to test its first telescoping fork.
Definition of atom for English Language Learners. : 1  the smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule. : 2  a very small amount of something.
1 Atom-smallest unit of all matter, that is composed of 3 sub-atomic particles called protons, electrons and neutrons. 2  Proton-the 'heavy' positively-charged particle in the nucleus of an atom. 3  Electron-the very 'light' negatively-charged particle that orbits the nucleus of an atom.lements and Compounds. 1  Element-An element is a substance composed of the same type of atoms (e.g. gold Au, oxygen O 2). 2  Compound-A compound is a substance made of more than one type of atom (e.g. water H 2 O, carbon dioxide CO 2).