Early ideas about atoms
All matter is made of An atom is the simplest particle of an element that can exist on its own in a stable environment.
The word 鈥渁tom鈥 is derived from the ancient Greek word atomos 鈥 鈥渟omething that cannot be divided鈥.
An early An equation, diagram or analogy that helps explain a scientific idea. of the atom was the The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons in it.. It imagined that an atom was a sphere of positive charge, with negatively-charged Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. embedded in it.
Ernest Rutherford disproved this model and suggested that an atom had a positive The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. at its centre, orbited by negatively-charged electrons.
Then, in 1932, James Chadwick discovered the Uncharged subatomic particle, with a mass of 1 relative to a proton. 鈥 a particle with no charge in the nucleus of the atom. The modern idea of the atom was complete.
The modern atomic model
Today, scientists use an atomic model that has a central, positively-charged nucleus that contains:
- positively-charged Subatomic particle with a positive charge and a relative mass of 1.
- neutral neutrons
Negatively-charged electrons orbit the nucleus in shells.
An atom鈥檚 nucleus gives it most of its mass.