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Chapter 2

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University of Waterloo
CHEM 120
Carey Bissonnette

Chem 120 Chapter 2 Section 1 Law of conservation of mass Law of constant composition: all samples of a compound have the same composition – the same proportions by mass of the constituent elements Dalton’s Atomic Theory 1. Each chemical element is composed of minute, indivisible particles called atoms. Atoms can neither be created or destroyed during a chemical change 2. All atoms of an element are alike in mass (weight) and other properties, but the atoms of one element are different from those of all other elements 3. In each of their compounds, different elements combine in a simple numerical ratio: for example, one atom of A to one of B (AB) or one atom of A to two of B (A2 ) Law of multiple proportions: If two elements form more than a single compound, the masses of one element combined with a fixed mass of the second are in the ratio of small whole numbers Section 2 All object of matter are made up of charged particles Electrons (cathode rays – independent -19 - -1.6022 x 10 C - mass = 9.1094 Ion: charged atom X-Rays and Radioactivity Alpha particles: carry two fundamental units of positive charge and have essentially the same mass as helium atoms Beta particles: negatively charged particles produced by changes occurring within the nuclei of radioactive atoms and have the same properties as electrons Gamma rays: not made up of particles; it is electromagnetic radiation of extremely high penetrating power The Nuclear Atom 1. Most of the mass and all of the positive charge of an atom are centered in a very small region called the nucleus, remainder of the atom is mostly empty space 2. The magnitude of the positive charge is different for different atoms and is approximately one half the atomic weight of the element 3. There are as many electrons outside the nucleus as there are units of positive charge on the nucleus. The atom as a whole is electrically neutral Protons: positively charged particles in nuclei Neutrons: neutral particles Atomic number (Z): no. of protons Mass number (A): no. of protons and neutrons Neutron number: A-Z Atomic mass unit: 1/12 of the mass of the atom ---- carbon-12 Elements in periodic table have atomic number: 1 – 112, 114 and 116 Chemical symbols: abbreviations of the name Section 4 Atoms coming from same element have same atomic no. Atoms with same no. of protons come from same element Elements beyond uranium (Z=92) must be synthesized in particle accelerators , doesn’t occur naturally First 111 elements internationally agreed Contrary to Dalton’s theory: atoms in the same element don’t necessarily all have same mass J.J. Thomson measured mass-to-charge ratio of positive ions formed from neon atoms, found out that atoms can have different mass. - isotopes: atoms that have same atomic no. (Z) but different mass (A) - look at notes pg.2 - mass numbers of isotopes incorporated into names of elements (e.g. Notes pg.2) - some elements consist of just a single type of atom, don’t have naturally occurring isotopes (e.g. aluminum -27) Ions Atoms that lose/gain electrons through chemical reaction form ions. Isotopic mass A small portion of the mass of protons and neutrons is converted to energy and released (nuclear binding energy) See notes for mass spectrometer Actual masses of individual atoms (u) are never whole no.s (except for carbon-12) Ratio of the mass of O to C = 1.33291, so mass of oxygen-16 atoms = 1.33291 x 12u = 15.9949u = close to 16 Section 5 Atomic mass (weight) of element: average of the isotopic masses, weighted according to the naturally occurring abundances of the
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