Unit 3 Atomic Structure.ppt - Unit 3 Atomic Structure Defining the Atom The Greek philosopher Democritus(460 B.C 370 B.C was among the first to suggest

Unit 3 Atomic Structure.ppt - Unit 3 Atomic Structure...

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Unformatted text preview: Unit 3 “Atomic Structure” Defining the Atom The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (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 chemical behavior, and was not based on the scientific method – but just philosophy Dalton’s Atomic Theory (experiment based!) John Dalton (1766 – 1844) 1) All elements are composed of tiny indivisible particles called atoms 2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element. 3) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties 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, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes Structure of the Nuclear Atom One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles: Electrons, protons, and neutrons are examples of these fundamental particles There are many other types of particles, but we will study these three Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron Modern Cathode Ray Tubes Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 9.11 x 10-28 Electron cloud Proton (p+) +1 1.67 x 10-24 Nucleus Neutron (no) 0 1.67 x 10-24 Nucleus Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded The Rutherford Atomic Model Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” The nucleus is composed of protons and neutrons (they make the nucleus!) The electrons distributed around the nucleus, and occupy most of the volume His model was called a “nuclear model” Atomic Number Atoms are composed of identical protons, neutrons, and electrons How then are atoms of one element different from another element? Elements are different because they contain different numbers of PROTONS The “atomic number” of an element is the number of protons in the nucleus # protons in an atom = # electrons Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon 6 6 Phosphorus 15 15 Gold 79 79 Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 Nuclide Oxygen - 18 Arsenic 75 Phosphorus - 31 p+ n0 e- Mass # 8 10 8 18 33 42 33 75 15 16 15 31 Complete Symbols Contain the symbol of the element, the mass number and the atomic number. Mass Superscript → number Subscript → Atomic number X Symbols Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number 80 35 Br Isotopes Dalton was wrong about all elements of the same type being identical Atoms of the same element can have different numbers of neutrons. Thus, different mass numbers. These are called isotopes. Isotopes We can also put the mass number after the name of the element: carbon-12 carbon-14 Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Isotope Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium) Protons Electrons Neutrons 1 1 0 1 1 1 1 1 2 Nucleus Isotopes Atomic Mass How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms. We are more concerned with the average atomic mass. This is based on the abundance (percentage) of each variety of that element in nature. We don’t use grams for this mass because the numbers would be too small. Measuring Atomic Mass Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity. Each isotope has its own atomic mass, thus we determine the average from percent abundance. To calculate the average: Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu) Atomic Masses Atomic mass is the average of all the naturally occurring isotopes of that (mass ) x (% as a decimal) Isotope Mass % in nature element. Carbon-12 12 98.89% 12 x .9889 =11.87 Carbon-13 13 1.11% 13 x .0111 = 0.14 Carbon-14 14 0.01% 14 x .0001 = . 0014 Carbon = 11.87 + 0.14 + .0014 =12.011 The Periodic Table: A Preview A “periodic table” is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties The periodic table allows you to easily compare the properties of one element to another The Periodic Table: A Preview Each horizontal row (there are 7 of them) is called a period Each vertical column is called a group, or family Elements in a group have similar chemical and physical properties Identified with a number and either an “A” or “B” More presented in Chapter 6 ...
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