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1.1 Nature of Chemistry and 1.2 Atomic Structure • Empirical knowledge: based on observable phenomena that is observed directly and is capable of being experimented on. • Theoretical knowledge is created to explain observations based on ideas. It is an attempt to rationalize things we cannot see. • Theories are constantly being revised to make them fit the evidence that is being evaluated. • Theories aim to describe, explain, and predict in the most concise manner possible. IUPAC • International Union Pure and Applied Chemistry – Established in 1919 – Role is to help regulate standards and procedures in chemistry Early Models of Matter Democritus • matter can be divided until you arrive at the smallest piece possible • called it the atom • atoms are in constant motion • empty space between atoms Aristotle • thought Democritus was out to lunch! • believed all matter was derived of four substance: earth, water, fire and air • idea lasted for 2000 years John Dalton • “pool ball” model • all matter is composed of tiny indivisible spheres called atoms • all atoms of an element have the same properties • atoms of different elements will have different properties • atoms of two or more elements can combine in constant ratios to form new compounds • atoms cannot be created, destroyed or subdivided in a chemical change J. J Thomson • “raisin bun” model • first to come up with the idea that an atom consisted of positive and negative parts • negatives were embedded in a positive sphere Hantaro Nagaoka • He proposed a “Saturian or planetary model" of the atom (1904). • The model was based on principles used to explain the stability of Saturn’s rings. – the rings are stable because the planet they orbit is very, very massive. Hantaro Nagaoka -Predictions – a very massive nucleus (analogous to a massive planet) – electrons revolved around the nucleus, bound by electrostatic forces (analogous to the rings revolving around Saturn, being bound by gravitational forces) Ernest Rutherford • “beehive” model • gold foil experiment • proved that the positive part of an atom was at the centre • there was a lot of empty space in an atom • that some electrons could be found amongst the empty space James Chadwick • proved the existence of neutrons • neutrons are heavy particles with no charge Neils Bohr (1885- 1962) Danish Physicist Bohr’s Model • Based on the element Hydrogen • electrons orbit the nucleus at discrete energy levels • these energy levels are at a fixed distance from the nucleus • the electron travels along a three dimensional path called an orbit • these energy shells are designated by the principal quantum number n • n=1,2,3 … • electrons can transition between levels • in order to jump it must receive that specific amount of energy that would put it to the next energy level • when an electron loses its gained energy, it falls back to its original orbit “ground state” Assumptions • an electron can travel indefinitely within an energy level without losing energy • that 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 • that 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 Experimental Evidence • The basis for Bohr’s Theory came from observing line spectra • line spectrum: a pattern of distinct lines, each of which corresponds to light of a single wavelength, produced when light consisting of only a few distinct wavelengths passes through a prism or spectroscope • How spectroscopy works! • This is the bright line spectrum for hydrogen Particle Location Charge Mass Symbo l Electron Orbiting the nucleus Negative 9.11 × 10-31 kilograms e- Proton Nucleus Positive 1.67 × 10-27 kilograms p+ Neutron Nucleus Neutral 1.67 × 10-27 kilograms n0 Atomic Number and Mass Number What is an atom's atomic number (Z)? • number of protons in the nucleus • each element has a unique number that identifies the number of protons • Oxygen atoms contain 8 protons and has an atomic number of 8 What is an atom’s mass number (A)? • derived from the number of protons and neutrons in an atoms nucleus. • (N) = A- Z The Mass of an Atom • The mass of an atom is very small. Scientists compare the mass of an atom to that of a standard atom. • As the mass of an atom is compared to that of another reference atom it is referred to as a relative atomic mass (Ar). • The reference atom used to compare all other atomic masses is the stable and abundant isotope of carbon . . . carbon-12. • The C-12 atom is said to be 12 unified atomic mass units (u). Consequently, one atomic mass unit is equivalent to the mass of a nucleon (proton or neutron). • Therefore, the atomic mass unit is defined as 1/12 the mass of a carbon-12 atom