Chemistry is the study of matter, its properties and its changes. The idea that matter is made up of particles is over 2000 years old, but the idea of using properties of these particles to explain observable characteristics of matter has more recent origins. In ancient Greece, it was proposed that matter is composed of particles of four elements (earth, air, water, and fire) and that these particles are in continual motion. The idea that particles could explain properties of matter was not used for about 2000 years. In the late 1600s the properties of air were attributed to its particulate nature; however, these particles were not thought to be fundamental. Instead, it was thought that they could change into other particles with different properties.

In the late 1700s solid evidence about the nature of matter, gained through quantitative scientific experiments, accumulated. Such evidence included the finding that during a chemical reaction matter was conserved. In the early 1800s a theory was proposed to explain these experimental facts. In this theory, atoms were hard, indivisible spheres of different sizes and they combined in simple whole-number ratios to form compounds. The further treatment of particles of matter as hard spheres in continual motion resulted in the 1800s in the kinetic molecular theory of matter, which was used to explain the properties of gases.

In the late 1800s evidence was discovered that particles of matter could not be considered hard spheres; instead, particles were found to have an internal structure. The development of cathode ray tubes, and subsequent experiments with them in the 1860s, led to the proposal that small, negatively charged particles-electrons-are part of the internal structure of atoms. In the early 1900s, to explain the results of the "gold foil experiment," a small, dense nucleus was proposed to be at the center of the atom with electrons moving about in the empty space surrounding the nucleus. Around this time, energy was proposed to exist in small, indivisible packets called quanta. This theory was used to develop a model of the atom which had a central nucleus surrounded by shells of electrons. The model was successful in explaining the spectra of the hydrogen atom and was used to explain aspects of chemical bonding. Additional experiments with radioactivity provided evidence that atomic nuclei contained protons and neutrons.

Further investigation into the nature of the electron determined that it has wave-like properties. This feature was incorporated into the wave-mechanical model of the atom, our most sophisticated model, and is necessary to explain the spectra of multi-electron atoms.

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