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Unit Two-Atomic Theory Notes Defining the Atom The Greek philosopher ________________(460 B.C. – 370 B.C.) was among the first to suggest the existence of ____________(from the Greek word “atomos”) He believed that atoms were indivisible and indestructible. His ideas did agree with later scientific theory, but did not explain______________ ______________, and was not based on the _______________ _______________– just philosophy-not experiment based Dalton’s Atomic Theory (experiment based!) 1) All elements are composed of _______ ________________ ___________called atoms. 2) _________of the same element are ______________. Atoms of any one element are different from those of any other element. 3) Atoms of different elements __________in simple ________-number ratios to form _____________ compounds. 4) In chemical reactions, atoms are_________________, ________________, or ____________– but never changed into atoms of another element. Sizing up the Atom ___________ are able to be ______________into smaller and smaller particles – these are the atoms, and they still have_____________ of that element. If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long. Despite their small size, ______________atoms are ______________with instruments such as scanning electron microscopes. Discovery of the Proton 1886-Eugene Goldstein observed what is now called the ___________-particles with a __________ charge, and a relative mass of 1 (or 1840 times that of an electron). Discovery of the Electron In 1897, J.J. Thomson used a _______ _____ _______ to deduce the presence of a negatively charged particle: the __________. Page 1 of 5 Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a ____________ charged “pudding,” thus it was called the “plum pudding” model. Plum Pudding Discovery of the Neutron 1932-James Chadwick confirmed the existence of the ___________ - a particle with no charge, but a mass nearly equal to a proton. Subatomic Particles Particle Charge Mass Location Ernest Rutherford’s Gold Foil Experiment – 1911 Alpha particles are ________ _________- The _________ particles were fired at a thin sheet of ______ foil. Particles that hit on the detecting screen (film) are recorded. Example Demo Rutherford’s Findings Most of the particles passed right through. A few particles were ____________. VERY FEW were greatly deflected. Conclusion The nucleus is __________. The nucleus is _________. Page 2 of 5 The nucleus is ____________ charged. The Rutherford Atomic Model The atom is mostly ________space. All the positive _________, and almost all the _______is concentrated in a small area in the center. He called this a “_______________”. The nucleus is composed of ___________ and ___________(they make the nucleus!). The __________ is held together by a _________ _________ ________. The electrons are distributed around the ____________, and occupy most of the volume. Atomic Number Atoms are composed of ______________ protons, neutrons, and electrons. How then are atoms of one element different from another element? Elements are different because they contain different numbers of ____________. The “_________ __________” of an element is the number of protons in the nucleus. # __________in an atom = # __________ ___________ ___________of an element is the number of _________ in the _________. of each atom of that element. Element # of Protons Atomic # Carbon Phosphorus Gold Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Nuclide P+ N0 eMass # Oxygen-18 Arsenic-75 Phosphorus-31 Isotopic Notation-Complete Symbols Contain the symbol of the element, the mass number and the atomic number. Example: Find each of these: number of protons= number of neutrons= number of electrons= Atomic number= Mass Number= Page 3 of 5 If an element has an atomic number of 34 and a mass number of 78, what is the: number of protons= number of neutrons= number of electrons= complete symbol= If an element has 91 protons and 140 neutrons what is the Atomic number= Mass number= number of electrons= complete symbol= If an element has 78 electrons and 117 neutrons what is the Atomic number= Mass number= number of protons= complete symbol= Isotopes Frederick Soddy proposed the idea of isotopes in 1912. ____________ are atoms of the same element having different masses, due to the varying numbers of neutrons. Atoms of the same element can have different numbers of _________. This is how the same element can have different mass numbers. We can also put the mass number after the name of the element: o carbon-12 o carbon-14 o uranium-235 Elements occur in nature as _____________ of isotopes. Isotope Protons Electrons Neutrons Nucleus Diagram 0 1 2 Atomic Mass How heavy is an atom of oxygen? It depends, because there are different isotopes of oxygen atoms. We are more concerned with the average atomic mass. Page 4 of 5 This is based on the abundance (___________) of each isotope of that element in nature. Measuring Atomic Mass Instead of grams, the unit we use is the _________ ________ ________(amu) We don’t use grams for this mass because the numbers would be too small. It is defined as one-twelfth the mass of a carbon-12 atom. Each ____________has its own________ _______, thus we determine the average from percent abundance. Atomic mass is the ____________of all the naturally occurring isotopes of that element. Isotope Carbon-12 Carbon-13 Carbon-14 Page 5 of 5 Symbol Composition of the nucleus % in nature 98.89% 1.11%