History of the Atom The Model of the Atom

Part I: The Road to Modern Atomic Theory Early Theories - 400 B.C. common Greek theory was that all matter consisted of four "elements"—earth, air, fire, and water. Democritus (460-360 BC) = a philosopher who theorized that all matter was made of indivisible particles (atomos). First to use the word atom his theory was based on logical reasoning, not data. philosophers/scientists of this time period did not think doing experiments was necessary—you could reach the truth by pure logical reasoning.

Dalton, John—“Solid marble” model Part II: Early Atomic Models Dalton, John—“Solid marble” model English schoolteacher (1808), proposed explanation for the 3 laws above thought elements were composed of atoms, which were indivisible

Dalton, John—“Solid marble” model Part II: Early Atomic Models Dalton, John—“Solid marble” model English schoolteacher (1808), thought elements were composed of atoms, which were indivisible Dalton’s Atomic Theory: (five parts) all matter is composed of very tiny particles called atoms atoms of a given element are identical in size/mass/other properties; other elements’ atoms are different atoms cannot be subdivided, created, or destroyed in chem rxn, atoms are combined/separated/rearranged atoms of diff. elements combine in small, whole-# ratios to form compounds Even after 2000 years of the atomic concept, the structure of the atom was still unknown. at this point, the atom was still thought to be the smallest unit of matter.

Thomson, Joseph John—“Plum Pudding” model (or “cookie dough”) English physicist (1897), performed cathode-ray tube experiments discovered the electron and its charge cathode-ray tube: glass vacuum tube (attached to a voltage source) through which electricity flows, producing a glow. electrons travel towards anode (opposite charges attract) paddle wheel placement shows that particles were moving, had mass, and had a negative charge

this model shows the electrons on the outside of the positively-charged atom (if there were negative parts to atoms, then there must be positive parts as well to balance it, because most matter is neutral).

Rutherford, Ernest—“Electron cloud, positive nucleus” model (1911) performed the gold-foil experiment, leading to the discovery of the positively-charged nucleus. experiment = bombard a thin gold foil with alpha-particles (He+). they did this assuming that the charge and mass within each atom was uniformly distributed throughout the atom, and expected the alpha particles to go right through. most did, but some (1/8000) actually bounced back toward the particle source!

Rutherford concluded that the reason some particles bounced back was that there was a very dense positively-charged area that occupied only a very small amount of space within the core of the atom (nucleus). to explain this, Rutherford proposed a new model for the atom. He imagined the atom as a miniature solar system with a nucleus as the sun and electrons orbiting like planets. although the nucleus was only one million millionth the volume of the atom, it had over 99.9% of the mass.

Bohr, Neils—“Quantized energy level” model Danish physicist (1913), new model that explained why negative electrons do not fall in towards the positive nucleus Bohr used Rutherford's ideas to explain the behavior of the simplest atom with one electron—hydrogen. he suggested that an electron was restricted to certain allowed orbits round the nucleus. if it jumped from one orbit to another, the atom emitted or absorbed light. thus, electrons only exist in specific energy levels (orbits around the nucleus) similar to the rungs of a ladder. these levels reflect quantum amounts of energy that are required to move electrons from one level to another

Modern Atomic Model e- in the modern model are arranged in “orbitals,” not orbits (Bohr model). Orbitals are cloud-like regions around the nucleus of the atom in which 1 or 2 electrons are most likely to be found. They are arranged about the nucleus on the x, y, and z axis.

Part III: The Dual Wave-Particle Nature of Light the modern model assumes the n0 and p+ still reside in the nucleus, and the e- reside in “orbitals,” or electron clouds (regions of space in which there exists a high probability of finding 1 or 2 e-). To explain, let’s start from the beginning: we know that e- can be excited, or caused to jump to higher-energy orbits from their lower-energy ground states, with the introduction of energy, usually in the form of electricity (think of the glow in a CRT). further experiments were performed in the early 1900s concerning the photoelectric effect. this effect was produced when e- were emitted as certain frequencies of visible light shone on the surface of the metal (think photovoltaic cells).

but why only certain frequencies but why only certain frequencies? Scientists could not answer this question until they began to think of light as having a dual wave-particle nature, rather than existing only as a wave. waves can contain energy at any level within the spectrum; particles only carry energy with them in small, specific amounts, called quanta. this explained why only certain frequencies of light would cause the electrons to escape the surface of the metal during the photoelectric experiments—the light was acting as a particle, not a wave, in this case.

As seen in the figure here, when hydrogen’s e- are excited, they move to higher-energy orbits. Falling from these orbits to their original ground state causes them to emit photons of light. if this light passes through a prism, it is split into its com- ponent wavelengths (and colors). Each element has its own distinct line emission spectrum. the energy of each emitted photon corresponds to a particular frequency of visible light (see diagram above). this worked nicely for hydrogen, but no so much with other elements!