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4.1 – Studying Atoms Science Depends on Evidence Remember, we know what we know based on the EVIDENCE that we have collected over many, many years. As new evidence comes along, our knowledge changes. Old ideas are thrown out New ideas are tested, and if the evidence is there, accepted Science is not really a quest to be “right”, but to get less and less “wrong”. Ancient Greek Models of Atoms Democritus (460-370 B.C.E.) Idea: Matter is made up of tiny individual particles called atomos. Atomos are: Solid, homogeneous, indestructible, and indivisible. Different kinds of atomos have different sizes and shapes. Size, shape, and movement of atomos determine the properties of matter. Evidence: None Aristotle (384-322 B.C.E.) Idea: Matter is made up of earth, fire, air, and water (the “four elements”). Evidence: None Dalton’s Atomic Theory (19th Century, 1800s) Idea: Dalton’s Atomic Theory All elements are composed of atoms. All atoms of the same element have the same mass, and atoms of different elements have different masses. Compounds contain atoms of more than one element. In a particular compound, atoms of different elements always combine in the same way. Atoms are pictured as solid, homogeneous, indestructible spheres. Evidence: Evidence provided by other scientists from all over the world (ex: Lavoisier) A given compound always forms from the same mass ratio of components. Ex: 1000 g of magnesium oxide always forms from 397 g of oxygen and 603 g magnesium Ex: 1000 g of water always forms from 889 g of oxygen and 111 g of hydrogen Matter is never created or destroyed in a chemical reaction. Ex: Combine 100 g of stuff, and you will get 100 g of new stuff All atoms of an element appeared to have the same mass. Thomson’s Model of the Atom (Late 19th-Early 20th Century, 1850) Evidence: Used a cathode ray tube to experiment on matter (different metals) and electricity. Noted that the tube had a glowing beam. Charged plates and magnets outside the tube would cause the beam to bend (repelled by negative plate, attracted by positive plate). The same charged particles came from every type of tested metal plates. Beam must be negatively charged particles that were much lighter than the lightest atom known (hydrogen). Ideas: Atoms are NOT solid, indestructible spheres. Atoms are a smeared-out positive charge with negatively charged particles imbedded inside. Atom is like a plum pudding or chocolate chip cookie Rutherford’s Atomic Theory (1911) Evidence: Used alpha particles to probe the inside of the atom Expected almost all alpha particles to burrow through smeared out positive charge with some small deflection. Most particles passed through, but there were more deflections than expected Some of the deflections were over 90o. This was like shooting bullets at a piece of toilet paper and having some of the bullets bounce off. Ideas: Atom is NOT a smeared out positive charge. Atom contains a super-small, super-dense positively charged nucleus (high mass, low volume). Negative charges are somewhere outside the nucleus. Atom is over 99% empty space 4.2 – The Structure of the Atom Properties of Subatomic Particles Protons 1+ charge Found in nucleus of atom 2nd particle discovered Electrons 1- charge; very light Found outside the nucleus 1st particle discovered Neutron No charge; slightly heavier than proton Found in nucleus 3rd particle discovered Isotopes Discovery: Atoms of the same element (same number of protons) that have different number of neutrons and different mass numbers. Examples: Hydrogen (1 proton): Hydrogen-1, Hydrogen-2, Hydrogen-3 Carbon (6 protons): Carbon-12, Carbon-14 Oxygen (8 protons): Oxygen -16, Oxygen-17, Oxygen-18 Isotopes disproved that atoms of the same element always had the same mass. Atomic and Mass Number Number of protons in the nucleus = atomic number Atomic number identifies element Number of electrons in atom = number of protons (neutral atom only) Number of protons + neutrons = mass number Mass number based on mass of carbon-12 isotope Number of neutrons = mass number – atomic number Example: Lithium has an atomic number 0f 3 and an atomic mass of 7… 3 protons 3 electrons 7 – 3 = 4 neutrons 4.3 – Modern Atomic Theory Bohr’s Model of the Atom Ideas: Like Rutherford’s model, except electrons: Exist on specific energy levels Energy levels can be imagined like orbits Electrons can jump from orbit to orbit by either absorbing or emitting a specific energy photon This photon would have a specific energy and specific color, if visible. Evidence: Excited elements would give off specific colors of lights. The spectrum of the element would only contain specific colors that would correspond to the energy levels in the atom. Sample Evidence Electron Cloud Model (Atom Gets Weird) Heisenburg showed that precise electron orbits are impossible to describe. Electrons actually move in a less predictably precise way. Schrodinger treated electrons as waves and assigned them probabilities. Electron cloud model describes possible/probable locations of the electron. Orbits replaced by clouds that contain orbitals. Can be visualized as a spinning fan. Atomic Orbitals Orbitals are regions of space where an electron is likely to be found. Each orbital can hold 2 electrons. Each energy level has at least one sublevel and orbital. An electron cloud is the combination of all orbitals in an atom. Energy Level Sublevels Present Sublevel # of orbitals s 1 1 s p 3 2 s, p d 5 3 s, p, d f 7 4 and higher s, p, d, f Probability Models of Orbitals