History of the Atom – Summary Sheet – ANSWER KEY Scientist Summary about Theory
Reasons for it’s failure
Democritus • first to proposed an atomic model
• More people believed Aristotle than Democritus
• all matter could be divided into smaller and smaller pieces until a single INDIVISIBLE particle was reached. • called this particle an ATOM • believed that different atoms are of different SIZES, have regular GEOMETRIC SHAPES and are in constant MOTION. • there is EMPTY space between atoms. Aristotle
• severely criticized Democritus’s theory, arguing that the idea of atoms in continuous motion in a void is illogical. • supported the FOUR ELEMENT theory of matter. (proposed by Empedocles a century earlier) • based on the idea that all matter is made up of FOUR basic substances: FIRE.
EARTH,
WATER,
AIR and
• each of these basic substances had different combinations of four specific qualities: dry, moist, cold, and hot • was the prevailing model for almost 2000 years, including the period of ALCHEMY in the Middle Ages
Dalton
• expanded upon the atomic theory proposed by Democritus. • consisted of the following statements. All matter is composed of tiny, indivisible particles called atoms. All atoms of an element have IDENTICAL properties. Atoms of different elements have different properties. Atoms of two or more elements can combine in constant RATIOS to form new substances. In chemical reactions, atoms join together or separate from each other but are not destroyed. • this theory explains of the law of CONSERVATION OF MASS • atoms are neither created nor destroyed in a chemical reaction. • they are only REARRANGED during chemical reactions, (i.e. you must end up with the same number and kinds of atoms after a chemical reaction as you had at the beginning.) • Therefore, there will be no change in mass during chemical reactions.
• the scientific revolution in physics and the new emphasis, in the 18th century, on quantitative studies showed that too many of the explanations and predictions using Aristotle’s theory were false. • Thomson’s experiments with cathode ray tubes demonstrated the existence of subatomic particles (i.e. particles that make up atoms) not just “solid indivisible particles”
History of the Atom – Summary Sheet – ANSWER KEY Scientist Summary about Theory
Reasons for it’s failure
• This theory explains the LAW OF CONSTANT COMPOSITION • atoms combine to form molecules in a fixed ratio in a given chemical reaction. • Since the atoms of an element have identical properties, such as mass, and combine in constant ratios, every compound must have a fixed, definite composition. Thomson
• performed experiments using CATHODE ray tubes • proposed that the cathode rays (the mysterious rays that where seen in a cathode ray tube) were subatomic particles • called the subatomic particles electrons. • that negatively charged electrons are distributed inside the atom, which is a positively charged sphere consisting mostly of empty space
Nagaoka
• represented the atom as a large, positively charged sphere surrounded by a ring of negative electrons
Rutherford
• famous GOLD FOIL or ALPHA PARTICLE experiment • “shot” alpha particles (small, positively charged particles produced by radioactive decay) through very thin pieces of gold foil. • Prediction – all the alpha particles would travel through (in a straight line) the foil largely unaffected by the atoms of gold. (based on Thomson’s model of the atom and the idea that the atom was composed mostly of empty space) • Result - most of the alpha particles did pass easily through the foil, a small percentage of particles was deflected at LARGE angles. • Conclusion - an atom must contain a positively charged CORE, which is surrounded by a predominantly empty space containing negative electrons • called the core the nucleus • coined the word proton for the smallest unit of positive charge in the nucleus.
• Rutherfords “gold foil” experiment disproved the model that the atoms was mostly empty space (either Thomson’s or Nagaoka’s model) • it could not explain the following. 1. if the nucleus of an atom contained several positive protons that repelled each other, how did it stay together? 2. why didn’t the negatively charged electrons rush toward and crash into the positively charged nucleus? 3. electrons in motion around the nucleus should continuously give off radiation in the form of light and one should observe a
History of the Atom – Summary Sheet – ANSWER KEY Scientist Summary about Theory
Reasons for it’s failure continuous spectrum
Chadwick
• showed that the atomic nuclei must contain HEAVEY neutral particles as well as positive particles.
N/A
• called neutral subatomic particles neutrons. Bohr
• developed to explain empirical evidence obtained from experiments involving hydrogen. 1. When hydrogen gas is HEATED, it emits a VIOLET light. 2. When this violet light is directed through a PRISIM, it separates into a series of coloured lines called a line spectrum. 3. Hydrogen produces a line spectrum consisting of four visible lines: red, blue-green, indigo, and violet (although the violet line may not always be visible.) • The same results can be obtained by passing electricity through a other gaseous element at low pressure: • The gas will emit light of only certain wavelengths, which we can see as LINES if the light is passed through a prism • These observations contrast (contradict) with what we see when we shine white light through a glass prism or a spectroscope: 1. In this case, we see a blended pattern of colour or a continuous spectrum. 2. The colours of the spectrum (plural: spectra) always appear in the same order: violet, indigo, blue, green, yellow, orange, and finally red. • Bohr proposed the following explanation for the line spectrum of hydrogen (also see below information on transitions) 1. When energy (HEAT or ELECTRICITY) is supplied to hydrogen atoms, EXCITED electrons gain a certain quantity of energy. 2. With this extra energy they jump from a lower energy level to a higher energy level. 3. As the electrons drop back to lower energy levels they RELEASE energy corresponding to a few precise wavelengths (i.e. the line spectrum for hydrogen) Bohr’s Main Ideas 1. same basic structure as Rutherford except for the following restriction on the movement of electrons 2. electrons within an atom can possess only certain discrete/distinct/unique energies, called ENERGY LEVELS. 3. electron energy is said to be QUANTIZED. (has a specific number attached to it)
• it could only explain the line spectrum of hydrogen. • it was not able to explain the more complex spectra of other elements.
History of the Atom – Summary Sheet – ANSWER KEY Scientist Summary about Theory
4. each energy level, is associated with a FIXED distance from the nucleus. 5. an electron with a particular energy travels along a THREE dimensional pathway called a SHELL or ORBIT. 6. shells are designated by the PRINCIPLE QUANTUM number, n, which can be any positive integer from 1 to infinity. 7. Historically, these shells have also been designated by the symbols K, L, M, N, Theory depended on these assumptions: 1. an electron can travel indefinitely within an energy level without LOSING energy; 2. the GREATER the distance between the nucleus of the atom and the energy level, the GREATER the energy required for an electron to travel in that energy level 3. an electron cannot exist BETWEEN orbits, but can move to a higher, unfilled orbit if it ABSORBS a specific quantity of energy, and to a lower, UNFILLED orbit if it LOSES energy. Transitions • if ADDITIONAL energy is supplied to an electron in a given energy level, then the electron can “jump” to a higher, UNFILLED energy level, farther away from the nucleus • this jump is called transition. • the quantity of energy (a QUANTUM) required to cause a transition is equivalent to the DIFFERENCE in energy between the energy levels. • when an electron DROPS or FALLS back to a lower energy level, it RELEASES an equivalent amount of energy. (observed as a line in the line spectrum for the element being observed) • when an electron is in the lowest energy level that it can occupy, it is said to be in its ground state • when all the electrons in an atom are in the lowest possible energy levels, the atom is said to be in its ground state. Why was this theory important? • suggested that the properties of the elements could be explained by the arrangement of electrons in orbits around the nucleus. hypothesized that the stable, unreactive nature of certain elements was due to filled outermost energy levels. • it explained both the stability of electrons in atoms and atomic emission spectra, and it enabled scientists to calculate the actual energies of the different energy levels within the hydrogen atom.
Reasons for it’s failure
