2 Composition of Atoms (see Table 4.1 on p 106) Protons: positive charge (+1), located in nucleus, are heavy, “p+”Neutrons: no charge(0), located in nucleus, are heavy, “n0”Electrons: negative charge (-1), located outside nucleus “electron cloud”, very light (1/1840 of a proton or neutron), “e-”
4 Charges in an AtomThe + charge on a proton is equal to the - charge on an electron.Atoms are neutral (have no overall charge)Key Concept: Therefore, the # of protons = # electrons in an atom.
5 Atomic numberLocated on periodic table, usually physically largest numberdetermines the identity of the atom.It tells us the number of protons in the atom.It also tells us the number of electrons (b/c an atom is neutral in charge.)Ex: atomic number of carbon, C = 6Question: how many p+s? How many e-s?Ex: p 111 Problem 4.1: Nitrogen has atomic number of 7. How many protons and electrons in a neutral atom of nitrogen?Slide 2.2
6 IsotopesThe number of neutrons can vary from atom to atom in an element.Atoms of the same element w/different #s of neutrons are called ISOTOPES.In order to know how many neutrons in an atom you must be told.Either you are told # of neutrons directly ORMass number
7 Isotope Symbols & Names Isotope Namecarbon -14CMass number146Atomic number
8 Mass Number The mass number tells you how much mass the atom has. Since p+ and n0 are the heavy parts,mass # = # of p+’s + n0’s.How to use this to determine composition of atomEx: p 112, # 4.1: identify p+, n0, e-A) beryllium-9:B) neon-20C) sodium-23 (sodium is Na)
10 QUESTION: If the mass number of a carbon atom is 14, How many protons?How many electrons?How many neutrons?LET’S PRACTICE!WhiteboardMarkerPaper towel
11 Atomic Mass Units Atoms are weighed in a.m.u. 1 a.m.u. is based on the mass of a Carbon-12 atom.it has 6 p+ and 6 n0,1 a.m.u = 1/12 the mass of a carbon-12 atom.
12 Atomic Mass(definition) Weighted average of all the isotopes of an element. See p 114 of text.calculating atomic massoften located below element symbol on periodic table.Usually a decimal
13 Relative Abundance & Atomic Mass Sample problem 4.2 p 117 of text Element X has 2 natural isotopes. The isotope with a mass of amu has a relative abundance of 19.91%. The isotope with a mass of amu has a relative abundance of 80.09%. Calculate the atomic mass of this element.
14 Analysis of ProblemIsotope: Relative Abundance 10X % 11X 80.09% Convert % to decimal values = =
15 SolutionTo find out how much each isotope contributes to the element’s atomic mass, multiply the isotope’s mass by its relative abundance amu x = amu amu x = amu
16 Solution ,cont.Now add these two numbers to find the atomic mass for the element, X.amu.Does this make sense? (Should your answer be closer to 10 or 11?)
17 Early Ideas About Matter Greek philosopher, Democritus (2500 y.a.) proposed matter was made of 4 elements: earth, air, fire, water. (PLEASE NOTE: None of these are elements!)He coined the word “atom” meaning “cannot be broken.”
18 “Modern” Atomic Theory Proust (1799) discovered water was always 11% hydrogen, 89% oxygen – law of definite proportionsCompounds’ components are always in a specific proportion by mass
19 Discovery of Atomic Structure 1800s- scientists thought atom was a tiny solid ball.THEN…JJ Thomson (1897) discovers the electron (e-)
20 JJ Thomson Cathode Ray Experiment Vacuum tube (no air inside) w/ electrode on each end, attached to a terminal.He send electricity through the tube and sawA bright ray travelling from the negative end (cathode) to the positive end (anode). “cathode rays”picture of cathode ray tubeCathode ray bends toward a positive end of a magnet.“ “ bends away from a negative end of a magnet.
21 Cathode Ray Experiment Conclusion:1. The cathode ray was actually ____ charged particles.2. The atom could not be ________ as scientists had thought, but must contain charged particles.
22 (Section 2):Electrons in Atoms Energy of ElectronsWhy electrons don’t crash into the nucleus: they have enough energy to keep them away.Why e-s (usually) don’t fly off of atoms: they have enough attraction to the nucleus to keep them in “orbit.”(Kind of like planets in orbit around the sun.)
23 Discovery of Protons & Neutrons, cont. In 1910 Thomson discovered that neon atoms have different masses.In 1932, James Chadwick confirms existence of the neutronConclusion: there must be another particle that has no charge, called a neutron.
24 Dalton’s Atomic Theory, cont. John Dalton ( )1. All matter is made of atoms2. Atoms are indestructible and can’t be divided3. All atoms of one element are exactly alike, but different from atoms of other elements.
25 Hypotheses, Theories, Laws (review) Hypothesis: testable prediction to explain an observationTheory: well tested explanation that explains many observations. May change over timeLaw: fact of nature observed so often it is accepted as truth. Doesn’t change.
26 Discovery of Protons & Neutrons Eugen Goldstein realized there was a second ray in the vacuum tube.It bent toward the – end of a magnet.It bent away from the + end of a magnet.Therefore, this ray was made of _____ charged particles.
27 Discovery of Nucleus1909-scientists now believe the atom is like chocolate chip cookie dough (see Fig 2.8 p 63 of text)1911-Rutherford’s Gold Foil ExperimentShot “alpha Particles” (helium nuclei) at gold foil.Hypothesis: they would pass through unaffected.Data: most did pass throughSome were deflectedOthers bounced straight back!
28 The Nuclear Model of the Atom Conclusion:1. Atoms are nearly all empty space!2. Atoms have small, densely packed central nucleus
29 Energy of Electrons (cont.) (Don’t write this!)DISCUSS WITH YOUR NEIGHBOR:You are an electron. If you have a lot of energy, will you stay close to the nucleus or will you move further from it?Answer: you may still stay in “orbit” but you will be able to move further away from the nucleus.
30 Bohr’s Model of Atom Neils Bohr studied w/Rutherford His model is also called the planetary modelHe discovered that e-s could only exist at certain distances from the nucleus.
31 Bohr’s Model of AtomSee p 75 of text: electron energy levels are like rungs of a ladder.LadderTo climb to a higher level, you can’t put your foot at any level,you must place it on a rungElectron energy levelse-s must move to higher or lower e.l.’s in specific intervals
32 Electron Cloud Model of Atom Electrons aren’t in perfect orbits.Energy levels are regions of space in which an e- is likely to be found most of the time.The area in which they move is like a cloud, an area of space surrounding the nucleus.
33 Electrons in Energy Levels Atoms are arranged in energy levels (e.l.’s), at different distances from nucleusClose to nucleus = low energyFar from nucleus = high energye-s in highest occupied level are “valence e-s”Only so many e-’s can fit in energy levelse-s fill lower e.l.’s before being located in higher e.l.’s*(* There are exceptions we will learn later!)
34 Electrons in Energy Levels Only so many e-’s can fit in energy levelsEnergy Level # of electrons1st 22nd 83rd *4th *
35 VALENCE ELECTRONS DETERMINE HOW ELEMENTS BEHAVE!!! KEY CONCEPT!!!VALENCE ELECTRONS DETERMINE HOW ELEMENTS BEHAVE!!!
36 Drawing Bohr ModelsLet’s practice drawing some atoms/ions In your teams, pick up enough of the following for your team: 1 white board per person 1 marker per person 1 paper towel per team (Please save a tree & share!)
37 Drawing Bohr Models Show # of protons and neutrons in the nucleus Draw e.l.’s and show each electron in the proper e.l.Ex: Bohr Model of BORON-11
38 Practice Hydrogen-2 (Practice together) Helium-4 Lithium-6 Beryllium-8 Carbon-12Magnesium-24
39 Lewis-Dot Diagrams Have 2 parts Chemical symbol of element Valence e-s, represented by dotsAre placed in one of four locationsAboveBelowRightleftAre not paired unless there is 1 e- in each location.Ex: Oxygen
40 Practice Lewis Dot Diagrams TEACHER DEMONSTRATIONHydrogenHeliumLithiumSTUDENT PRACTICEBerylliumBoronCarbon
41 PRACTICE WORKSHEETBohr ModelsLewis dot diagrams