Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Introduction to Atomic and Molecular Structure Chem 101 Lecture 9/9/03 After a few thousand years of observing matter, folks learned: • Some substances cannot be broken down into other substances by chemical means (elements). • Elements can be combined to form substances with new properties. • The total mass of materials present after a chemical reaction is the same as the mass before the reaction (Law of Conservation of Mass). John Dalton proposed four postulates central to modern chemical thinking • Each element is composed of extremely small particles called atoms. • All atoms of a given element are identical; atoms of different elements are different. • Atoms of an element are not changed into different types of atoms by chemical reactions. • Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms. 1 Differences in properties determined by differences in atoms • But, what are atoms? What makes them different from each other? – Through careful quantitative studies, chemists determined that elements have unique atomic masses. – But, what are atoms made of? The electron was the first subatomic particle discovered • Discovered by Thomson in 1897 using a cathode ray tube. Thomson and his Tube • Thomson concluded that the “ray” consisted of small, negatively charged particles (now called “electrons”) •From the deflections, measured the mass to charge ratio of an electron (e/m = 1.76 x 108 C/g) 2 Thomson’s “plum pudding” model of the atom “electrons” (raisins) Positive charge (pudding) Structure of atom revealed by the Rutherford Foil Experiment The Modern View of the Atom • A positively charge nucleus containing protons and neutrons. • The nucleus is surrounded by a cloud of rapidly moving electrons. • The total chare of the nucleus = the total charge of the electrons (i.e., # protons = # electrons). 3 Atoms can be visualized using atomic force microscopy Carbon atoms on a layer of graphite What’s different about atoms from different elements ? • Different elements have different numbers of protons in their atoms. • Atomic number: number of protons in the atom; different elements have different atomic numbers. • A neutral atom (i.e., one with no net charge) will have the same number of electrons as protons). • Most chemical properties are determined by the number and arrangement of electrons (more on this later). What about the neutrons? • Two atoms may have the same number of protons in their nucleus (same atomic number), but different numbers of neutrons: these are called isotopes. • The mass number of an atom is equal to the total number of protons + neutrons. Mass number X Atomic number 4 Atoms can gain or lose electrons to form ions • If an atom loses an electron, it is left with a net positive charge (called a cation). • If an atom gains an electron, it is left with a net negative charge (called an anion). • Number of electrons = atomic number - charge Atoms can be combined by forming chemical bonds • Atoms interact by forming chemical bonds. • Covalent bonds are formed when atoms share electrons; the resulting collection of atoms is called a molecule. H2O -- Water Methanol Cisplatin: PtH6N2Cl2 Charged species interact via electrostatic attraction/repulsion Attraction Repulsion • Positive and negative charges are attracted by the Coulomb force(opposite charges attract, like charges repel). • Interactions given by Coulomb’s Law: Fµ -Q Q r 1 2 2 1, 2 5 Ionic bonds involve the forces of attraction between oppositely charged ions Sodium chloride : NaCl We’ve raised a bunch of questions (which we’ll answer in the next couple weeks): • How is the structure of an atom (number of electrons and protons, etc.) related to the properties of the element? • Why do some atoms tend to form positive ions, others negative ions, and some no ions at all? • What does it really mean to say that atoms “share” electrons? Where we’ve been so far... • Elements consist of atoms. • Elements can be distinguished by differences in the number of protons, electrons, and neutrons in their atoms. • Protons are heavy, positively charged particles located in the nucleus. • Electrons are light, negatively charged particles surrounding the nucleus. • Questions: How are the electrons organized in the atom? Why do some atoms lose electrons to form cations, while others gain electrons to form anions? 6