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Chemistry • What is chemistry? – Chemistry is the study of matter, what it is made of, and how it changes. • What is matter? – Matter is anything that… • Has mass. • Takes up space. What Is Matter? • Pretty much everything – Element • A pure substance that cannot be broken down to anything simpler by chemical or physical means. – Compound • A pure substance that cannot be broken down to anything simpler by physical means. – Mixture • Not a pure substance. Can be broken down by physical means. What Is Matter? • Is there a smallest particle of matter? – Philosophers and scientists have pondered this question since about 400 B.C. • Our current theory: – An atom is the smallest particle of an element that retains all the properties of the element. – History of the modern atomic theory. The History of the Atom How I Have Been Misunderstood Who First Thought of Me? • Democritus • 400 BC • Greek Philosopher credited with coming up with the idea of a single indivisible atom that was the smallest particle for all matter. Have I been famous since then? • No • Why not? • Nobody believed Democritus. He could not prove his idea. • Who didn’t believe him? • Other famous Greek dead dudes like… History of the Atomic Theory • By 1790, more accurate measurement lead to three major laws of nature: – Law of Conservation of Mass – Law of Definite Proportions (Constant Composition)-mass ratio – Law of Multiple Proportions (small whole numbers) So what happened? • The idea of the atom wallowed in obscurity for about 2000 years – not completely forgotten, but not believed, until…1808 John Dalton’s Atomic Theory • 1808 – All matter is composed of small particles called atoms. – Atoms of a given element are identical in size, mass, and properties; atoms of different elements are different in size, mass, properties. – Atoms cannot be subdivided, created, or destroyed (law of conservation of mass). John Dalton’s Atomic Theory – Atoms of different elements combine in simple whole number ratios to form chemical compounds (Law of definite proportions, Law of multiple proportions). – In chemical reactions, atoms are combined, separated, or rearranged (law of conservation of mass). Main Parts of Daltons Theory • All things are made of atoms. • All atoms of the same element have the same size, mass, properties. • Atoms can’t be created subdivided or destroyed. • Atoms can combine or rearrange to form compounds. Modern Atomic Theory • All things are made of atoms. • All atoms of the same element have the same size, mass, properties. • Atoms can’t be created subdivided or destroyed. • Atoms can combine or rearrange to form compounds. How did we find out about the parts? • JJ Thomson - 1897 discovered • ELECTRONS • Plum pudding model Did we keep that model? • No • Ernie Rutherford – 1911 gold foil experiment discovered • NUCLEUS Why don’t the electrons get sucked into the nucleus? • Electrons have their own energy – Neils Bohr • Only can absorb or give off energy in specific amounts called quanta Parts of the Atom • Information on the Periodic Table tells you about the number of each of the different parts of any given element. --------------------- Atomic Number • The atomic number is the number of protons in a an atom. – This determines what the element is. More or less protons would change it into a different completely element. – The number of protons = the total of the positive charges for an atom – BUT • An Atom is neutral, so there can’t be an overall positive or negative charge. – Therefore, the number of protons also tells you … …the total of the negative charges for the atom . Atomic Mass • For a single atom: – Atomic mass = # of neutrons + # of protons in the atom. • For a bunch of atoms = average mass of the atoms (protons and neutrons are the parts with mass). • Recall that atomic number tells you the number of protons. Use Atomic Number and Atomic Mass together to determine… … the total number of neutrons in the atom.