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Democritus • a Greek philosopher (450 BC) • Small indivisible units of matter • Called them “Atomos” Atom • Smallest particle of an element that retains the chemical identity of that element Comparison •World population: 7,000,000,000 •Atoms in Cu penny: 29,000,000,000,000,000,000,000 Molecule • Smallest particle of a compound that retains the chemical identity of the compound Law of Definite Proportions (Proust) •a compound always contains the same elements in the same proportions by mass Percent by mass % by mass = 100• element mass compound mass Problem: •a 78.0 g sample of an unknown compound contains 12.4 g. hydrogen. What is the percent by mass of hydrogen? 100 (12.4 / 78.0) = 15.9% Problem #2 •If 3.5 g of X reacts with 10.5 g of Y to form XY, what is the percent by mass of X? … of Y? 25% X and 75% Y Law of Multiple Proportions (Dalton) •Elements that combine to form multiple compounds combine in specific whole number ratios for each compound. Dalton’s Atomic Theory • All matter is made up of extremely small particles called atoms. • All atoms of the same element are exactly alike (mass, chemical behavior,…) but they differ from atoms of other elements. • Atoms cannot be divided, created, nor destroyed. Dalton’s Atomic Theory -cont. • Different atoms combine in whole number ratios to form compounds. • Chemical reactions cause atoms to separate, combine or rearrange. Electron •Named for “elektron”, an amber fossilized tree sap •First discovered subatomic particles. Thomson discovered the negatively charged particles we now call electrons. Thomson’s Atom •“plum pudding” model pudding: + charge raisins: - charge Millikan’s Oil Drop Experiment Rutherford’s Gold Foil Experiment Nucleus •core or center of the atom Rutherford’s Atom •small central positive nucleus •negative charges in empty space around nucleus. Subatomic Particles Symbol Location Charge electron proton neutron Mass Subatomic Particles Symbol Location Charge electron e− proton p+ neutron n Mass Subatomic Particles Symbol Location Charge − empty space electron e proton p+ nucleus neutron n nucleus Mass Subatomic Particles Symbol Location Charge − empty space −1 electron e proton p+ nucleus +1 neutron n nucleus 0 Mass Subatomic Particles Symbol Location Charge Mass empty space −1 0.0005 amu − electron e proton p+ nucleus +1 1 amu neutron n nucleus 0 1 amu Atoms are identified by number of protons Strong nuclear force •holds the atom’s nucleus together (protons + neutrons) •One of 4 basic forces in nature Review #1: Who first came up with the idea that matter is made up of atoms? Democritus Review #2: Dalton said matter is made up of atoms which cannot be? divided, created, destroyed Review #3: Is Dalton’s model a law or a theory? theory Review #4 •What changes did Thomson make to the atomic model of Dalton’s from about100 years earlier? #4 •Subatomic particles (protons & electrons) •Plum pudding model Review #5 •What changes did Rutherford make to Thomson’s atomic model? #5 •Atoms are mainly empty space •Nucleus of protons & neutrons •Electrons in empty space around nucleus Subatomic Particles • Protons • Neutrons • Electrons Atomic number (Z) • the number of protons in an atom • written just above the chemical symbol on the periodic table Atoms are neutral •Total # protons = total # electrons Mass number • the number of protons plus neutrons in an atom 14 N 7 or N-14 “14” is mass #, “7” is atomic # 15 N 7 N-16 Atomic mass unit •unit of mass for atoms •1 amu = 1/12th mass of C-12 atom •protons & neutrons are appx. 1 amu electrons are 0 amu Ion: atom with an electrical charge Anion: more electrons than protons: - charge Cation: more protons than electrons: + charge 25 2+ Mg 12 179 4+ Hf 72 37 − Cl 17 Isotopes –Atoms of the same element that have different numbers of neutrons. Iron-56 Fe-56 What has: 5 protons 6 neutrons Atomic Mass • average mass of an element’s atoms •averaged according to their relative abundance •sum of rel. abundances x isotope mass Atomic mass =? 93.12% K-39 6.88% K-41 (0.9312•39) + (0.0688•41) = 39.14 amu Find atomic #, mass #, # p+, # n, & # e- for each: ۰ Ni-58 ۰ Cl-36 ۰ at.#40 & 51 n ۰ at.#47 & mass #107 ۰ 30 p+ & 35 n ۰ mass #52 & 24 e- Why is: Atomic mass = 35.453 for Cl? Shouldn’t it be a whole number?