An Introduction to Atomic Theory. VCE Chemistry Unit 1: The Big Ideas of Chemistry Area of Study 1 The Periodic Table

Transcription

1 An Introduction to Atomic Theory VCE Chemistry Unit 1: The Big Ideas of Chemistry Area of Study 1 The Periodic Table

2 From Democritus to Dalton Two thousand years ago, Democritus proposed that matter consisted of very small indivisible particles called atoms In 1661 Robert Boyle wrote a paper formally describing the term, element in his paper, The Sceptical Chymist.

3 Priestley and Lavoisier Joseph Priestley in 1775 first stated that air is not an elementary substance, but a composition," or mixture, of gases. Among them was the colourless and highly reactive gas he called "dephlogisticated air, Antoine Lavoisier later named it oxygen Priestley invented carbonated water and the rubber eraser Antoine Lavoisier also made a number of notable contributions to chemistry... Defined the law of the conservation of mass Was instrumental in devising the modern system of chemical nomenclature Wrote a paper describing the first list of elements (substances that could not be broken down further), including oxygen, nitrogen, hydrogen, phosphorus, mercury zinc and sulfur, however it did also include light and caloric

4 1. John Dalton John Dalton ( ) was the scientist responsible for the basis of modern atomic theory. On the basis of experimental facts in 1805, he proposed that: Matter is composed of very small spherical particles called atoms which are indivisible and cannot be created or destroyed (Law of Conservation of Mass) Atoms of a particular element are identical in weight and have identical properties. Different elements have atoms differing in weight. Atoms combine to form compounds in definite ratios of whole numbers (Law of Definite Proportions).

5 2. JJ Thomson J.J. Thomson discovered the electron in 1897 & incorporated the electron into Dalton s model. He proposed that the electrons were distributed throughout the atom like plums in a pudding.

6 3. Ernest Rutherford Rutherford devised a set of experiments to test Thomson s plum-pudding model in This involved firing a beam of positively charged a-particles at a very thin sheet of gold foil and predicting that all of the a-particles would pass through the foil.

7 Ernest Rutherford and the Nuclear Atom Rutherford discovered that while most a-particles passed straight through the gold atoms, a few were deflected at very large angles. This meant that a-particles were hitting something with high positive charge density. He proposed the existence of a very small dense positive nucleus with electrons orbiting like planets around the sun. Most of the atom was made of empty space.

8 4. Niels Bohr and Orbiting Electrons Rutherford s model was flawed because his theory could not explain the emission spectra (light) of elements. Niels Bohr proposed that the electrons moved in circular orbits with fixed energies. Emission spectra arise from excited electrons falling back from occupied higher into empty lower energy levels. They release energies of specific frequency and wavelength (photons) which when passed through a prism are shown as coloured lines on a black background. The lines indicate that exact quanta (amount) of energy involved and show that electrons can only occupy quite specific regions of space.

9 Rutherford and Bohr Atomic Models One area of negative electrons Positive Nucleus Electrons in different energy level - shells

10 5. The Schrödinger Model This model is also known as the quantum mechanical model. In 1926, Schrödinger proposed electrons behaved like waves around the nucleus because they were moving so fast. Electrons are not restricted to a given orbit but behave as negative charge clouds restricted to certain regions of space called atomic orbitals.

11 6. James Chadwick The Neutron In 1932, James Chadwick bombarded beryllium with a-particles and found a beam of electrically neutral particles with masses similar to protons. Chadwick called the particles neutrons. Differing weights of isotopes of a particular element could now be explained by the presence of differing numbers of neutrons.

12 Fredrick Soddy & Isotopes Frederick Soddy found that lead coming from the radioactive decay of 238 U had an RAM (relative atomic mass) of 206 whereas lead coming from the radioactive decay of 232 Th had an RAM of 208. He found two different isotopes of lead, Pb and Pb. Isotopes of elements differ from each other by the number of neutrons.

13 In Summary.. Chemist Dalton Thomson Rutherford Bohr Shrodinger Chadwick Discovery Matter is made of indivisible particles Plum pudding model Atoms were positive balls studded with negative electrons The atom was mostly empty space with positive nucleus and electrons orbiting nucleus Electrons exist in energy orbitals Electrons behave as cloud of negative charge - orbitals Neutron

14 Ions Ions differ from the standard atom by a gain or loss of electrons Positively charged ions (those that have lost electrons) are called CATIONS Negatively charged ions (those that have gained electrons) are called ANIONS