Atomic Theory 15,000 kilotons
Dismissed idea of the atom. Early Greeks Two schools of thought: Matter is made of indestructible particles called “atomos” Plato ( BC) Democritus (400 BC) Both theories lacked Scientific Evidence so idea of atom was lost for 2000 years
Dalton’s Theory Experimental evidence Scientific Laws Law of Conservation on Matter You can’t create or destroy atoms. Law of Definite Proportions Same compounds are same ratio by mass. Law of Multiple Proportions Different compounds with the same elements are whole number multiples of the atoms. Experiments Scientific Laws Atomic Theory (observations)(Patterns)(Explanations) John Dalton realized that there must be an atom as Democritus first proposed.
Summary for Dalton’s Atomic Theory Atoms are Tiny. Atoms of the same element are the same. Atoms of different elements are different. Atoms can’t be divided, created or destroyed. Atoms of different elements combine in simple whole-number ratios to form compounds.
The Atom The smallest unit of an element that retains the properties of that element. Building blocks for matter.
Electrical Nature of Matter Opposite charges attract each other. (+) (-) Like charges repel each other. (+) (+) (-) (-)
Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Crookes Tube
Cathode Rays (electrons) Observations: Cathode Ray Tube produces rays with constant charge to mass ratio. Conclusions and Hypotheses: Cathode rays (electrons) were found in all substances tested. Cathode rays (electrons) were attracted to the positive plate every time. Electrons are negatively charged. Neutral atoms are made up of equal amounts of (+) and (-) particles. All atoms contain tiny particles called electrons. The electron has a specific size and charge.
Thomson’s Atomic Model (1897) Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. The oil drop apparatus Mass of the electron is x kg That’s about 2000 times smaller than a proton!
Millikan’s Oil Drop Experiment (1909) Compared different masses and charges on an oil drop. Used electric field to determine charge. Used gravity to determine mass.
Inferences from studying properties of electrons Electrons have little mass. (about 2000 times less than hydrogen atom) Charge of e - is equal and opposite charge of proton. Since atoms are neutral, (# electrons = # protons)
Ernest Rutherford’s ( ) electrons embedded in a positive pudding. Where exactly are those electrons? Thomson’s Theory: “Plum Pudding” Shoot something at them to see where they are. Rutherford’s idea:
Rutherford’s has an idea… What if I shoot alpha radiation at gold atoms in gold foil?
Rutherford’s “Gold Foil Experiment” Alpha particles are helium nuclei, He 2+ Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded Radioactive source (+)
Rutherford’s Findings (1911) The atom is mostly empty space. The nucleus is dense. The nucleus is positively charged Electrons, e -, are moving large distances outside the nucleus. Observations: Most of the alpha particles passed right through Some alpha particles were deflected slightly VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions:
Rutherford’s Conclusion (1911)… Small, dense, positive nucleus. Equal amounts of (-) electrons at large distances outside the nucleus.
Neils Bohr’s Atomic model (1913) Small, dense, positive nucleus. Equal amounts of (-) electrons at specific orbits around the nucleus. This incorrect version of the atom is often used to represented atoms because it shows energy levels for electrons.
Current Atomic model Small, dense, positive nucleus. Equal amounts of (-) electrons occupy regions of space called “orbitals” in the electron cloud.orbitals
Modern Atomic Theory Atoms of the same element are chemically alike with a characteristic average mass which is unique to that element. Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! All matter is composed of atoms. Atoms of any one element differ in properties from atoms of another element The exact path of electrons are unknown and e - ’s are found in the electron cloud.
The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space “q” is a particle called a “quark”
About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark. Neutrons are made of one “up” quark and two “down” quarks. Quarks are held together by “gluons”
Gold foil experiment The Ato m Democritus Dalton Plato e - discovered There is an indestructible particle. “atomos” Dismissed idea of the atom. No experimental evidence There is an indestructible particle. “atom” Supported by evidence. Law of conservation of matter. Law of definite proportions. Law of multiple proportions. Thomson (Plum Pudding Model) Rutherford Finds dense positive nucleus. Atom is mostly space with e - moving very far from the nucleus. Bohr Places e - in orbits at fixed energy levels. Modern Theory e - are found based on probability. Still contains a dense positive nucleus.
Subatomic particles 1 amunucleus protonp+p+ n0n0 e-e- neutron electron nucleus Very far outside the nucleus or (e - cloud) amu 1 amu No charge 0 charge LocationMass (amu)
Subatomic particles 1 amunucleus protonp+p+ n0n0 e-e- neutron electron nucleus Very far outside the nucleus or (e - cloud) amu 1 amu No charge 0 charge LocationMass (amu)
Subatomic particles 1 amunucleus protonp+p+ n0n0 e-e- neutron electron nucleus Very far outside the nucleus or (e - cloud) amu 1 amu No charge 0 charge LocationMass (amu)