Presentation on theme: "Chapter 5 – Early Atomic Theory & Structure"— Presentation transcript:
1Chapter 5 – Early Atomic Theory & Structure 5.1 Early Thoughts5.6 Subatomic Parts of the Atom5.7 The Nuclear Atom5.2 Dalton's Model of the Atom5.8 General Arrangement of Subatomic Particles5.3 Composition of Compounds5.9 Atomic Numbers of the Elements5.4 The Nature of Electric Charge5.5 Discovery of Ions5.10 Isotopes of the Elements5.11 Atomic Mass
2Early ThoughtsThe earliest models of the atom were developed by the ancient Greek philosophers.Empedocles stated that matter was made of 4 elements: earth, air, fire, and water.Democritus (about B.C.) thought that all forms of matter were divisible into tiny indivisible particles. He called them “atoms” from the Greek “atomos” indivisible.
3Early ThoughtsAristotle ( B.C.) rejected the theory of Democritus and advanced the Empedoclean theory.Aristotle’s influence dominated the thinking of scientists and philosophers until the beginning of the 17th century.
4Dalton’s Model of the Atom 2000 years after Aristotle, John Dalton an English schoolmaster, proposed his model of the atom–which was based on experimentation.
5Dalton’s Atomic Theory Elements are composed of minute indivisible particles called atoms.Atoms under special circumstances can be decomposed.Modern research has demonstrated that atoms are composed of subatomic particles.Atoms of the same element are alike in mass and size.Atoms of different elements have different masses and sizes.Chemical compounds are formed by the union of two or atoms of different elements.
6Dalton’s Atomic Theory Atoms combine to form compounds in simple numerical ratios, such as one to one , two to two, two to three, and so on.Atoms of two elements may combine in different ratios to form more than one compound.
7Dalton’s atoms were individual particles. Atoms of each element are alike in mass and size.5.1
8Dalton’s atoms were individual particles. Atoms of different elements are not alike in mass and size.5.1
9Daltons atoms combine in specific ratios to form compounds.
10The Law of Definite Composition Composition of CompoundsThe Law of Definite CompositionA compound always contains two or more elements combined in a definite proportion by mass.
11Water always contains the same two elements: hydrogen and oxygen. Composition of WaterWater always contains the same two elements: hydrogen and oxygen.The percent by mass of hydrogen in water is 11.2%.The percent by mass of oxygen in water is 88.8%.Water always has these percentages. If the percentages were different the compound would not be water.
12The Law of Multiple Proportions Composition of CompoundsThe Law of Multiple ProportionsAtoms of two or more elements may combine in different ratios to produce more than one compound.
13Composition of Hydrogen Peroxide Hydrogen peroxide always contains the same two elements: hydrogen and oxygen.The percent by mass of hydrogen in hydrogen peroxide is 5.9%.The percent by mass of oxygen in hydrogen peroxide is 94.1%.Hydrogen peroxide always has these percentages. If the percentages were different the compound would not be hydrogen peroxide.
14Combining Masses of Hydrogen and Oxygen Mass Hydrogen(g)Mass Oxygen(g)Water1.08.0Hydrogen Peroxide16.0Hydrogen peroxide has twice as much oxygen (by mass) as does water.
15Combining Ratios of Hydrogen and Oxygen Hydrogen peroxide has twice as many oxygens per hydrogen atom as does water.The formula for water is H2O.The formula for hydrogen peroxide is H2O2.
17The Nature of Electric Charge Properties of Electric ChargeCharge may be of two types: positive and negative.Unlike charges attract (positive attracts negative), and like charges repel (negative repels negative and positive repels positive).Charge may be transferred from one object to another, by contact or induction.The less the distance between two charges, the greater the force of attraction between unlike charges (or repulsion between identical charges).q1 and q2 are charges, r is the distance between charges and k is a constant.
18Discovery of IonsMichael Faraday discovered that certain substances when dissolved in water conducted an electric current.He found that atoms of some elements moved to the cathode (negative electrode) and some moved to the anode (positive electrode).He concluded they were electrically charged and called them ions (Greek wanderer).
19Discovery of Ions NaCl → Na+ + Cl- Δ Svante Arrhenius reasoned that an ion is an atom (or a group of atoms) carrying a positive or negative electric charge.Arrhenius accounted for the electrical conduction of molten sodium chloride (NaCl) by proposing that melted NaCl dissociated into the charged ions Na+ and Cl-.NaCl → Na+ + Cl-Δ
20Discovery of Ions NaCl → Na+ + Cl- In the melt the positive Na+ ions moved to the cathode (negative electrode). Thus positive ions are called cations.In the melt the negative Cl- ions moved to the anode (positive electrode). Thus negative ions are called anions.
21The diameter of an atom is 0.1 to 0.5 nm. Subatomic Parts of the AtomThis is 1 to 5 ten billionths of a meter.The diameter of an atom is 0.1 to 0.5 nm.If the diameter of this dot is 1 mm, then 10 million hydrogen atoms would form a line across the dot.Even smaller particles than atoms exist. These are called subatomic particles.
22Subatomic Particle - Electron In 1875 Sir William Crookes invented the Crookes tube.Crookes tubes experiments led the way to an understanding of the subatomic structure of the atom.Crookes tube emissions are called cathode rays.
23Subatomic Particle - Electron In 1897 Sir Joseph Thompson demonstrated that cathode rays:travel in straight lines.are negative in charge.are deflected by electric and magnetic fields.produce sharp shadowsare capable of moving a small paddle wheel.
24Subatomic Particle - Electron This was the discovery of the fundamental unit of charge – the electron.
25Subatomic Particle - Proton Eugen Goldstein, a German physicist, first observed protons in 1886:Thompson determined the proton’s characteristics.Thompson showed that atoms contained both positive and negative charges.This disproved the Dalton model of the atom which held that atoms were indivisible.
26Subatomic Particle - Neutron James Chadwick discovered the neutron in 1932.Its actual mass is slightly greater than the mass of a proton.
28IonsPositive ions were explained by assuming that a neutral atom loses electrons.Negative ions were explained by assuming that extra electrons can be added to atoms.
29When one or more electrons are lost from an atom, a cation is formed. 5.4
30When one or more electrons are added to a neutral atom, an anion is formed. 5.4
31The Nuclear Atom Radioactivity was discovered by Becquerel in 1896. Radioactive elements spontaneously emit alpha particles, beta particles and gamma rays from their nuclei.By 1907 Rutherford found that alpha particles emitted by certain radioactive elements were helium nuclei.
32The Rutherford Experiment Rutherford in 1911 performed experiments that shot a stream of alpha particles at a gold foil.Most of the alpha particles passed through the foil with little or no deflection.He found that a few were deflected at large angles and some alpha particles even bounced back.
34The Rutherford Experiment An electron with a mass of 1/1837 amu could not have deflected an alpha particle with a mass of 4 amu.Rutherford knew that like charges repel.Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume. He called this mass the nucleus.
35The Rutherford Experiment If a positive alpha particle approached close enough to the positive mass it was deflected.Most of the alpha particles passed through the gold foil. This led Rutherford to conclude that a gold atom was mostly empty space.
36The Rutherford Experiment Because alpha particles have relatively high masses, the extent of the deflections led Rutherford to conclude that the nucleus was very heavy and dense.
37The Rutherford Experiment ScatteringDeflectionDeflection and scattering of alpha particles by positive gold nuclei.5.5
38General Arrangement of Subatomic Particles Rutherford’s experiment showed that an atom had a dense, positively charged nucleus.Chadwick’s work in 1932 demonstrated the atom contains neutrons.Rutherford also noted that light, negatively charged electrons were present in an atom and offset the positive nuclear charge.
39General Arrangement of Subatomic Particles Rutherford put forward a model of the atom in which a dense, positively charged nucleus is located at the atom’s center.The negative electrons surround the nucleus.The nucleus contains protons and neutrons
41Atomic Numbers of the Elements The atomic number of an element is equal to the number of protons in the nucleus of that element.The atomic number of an atom determines which element the atom is.
42Atomic Numbers of the Elements Every atom with an atomic number of 1 is a hydrogen atom. Every hydrogen atom contains 1 proton in its nucleus.
43atomic number1HEvery atom with an atomic number of 1 is a hydrogen atom.1 proton in the nucleus
44Atomic Numbers of the Elements Every atom with an atomic number of 6 is a carbon atom. Every carbon atom contains 6 protons in its nucleus.
45atomic number6CEvery atom with an atomic number of 6 is a carbon atom.6 protons in the nucleus
46atomic number92UEvery atom with an atomic number of 92 is a uranium atom.92 protons in the nucleus
47Atomic Numbers of the Elements Atoms of the same element can have different masses.They always have the same number of protons, but they can have different numbers of neutrons in their nuclei.The difference in the number of neutrons accounts for the difference in mass.These are isotopes of the same element.
48Atomic Numbers of the Elements Isotopes of the Same Element HaveEqual numbers of protonsDifferent numbers of neutrons
55Hydrogen has three isotopes 1 proton0 neutrons1 proton1 neutron1 proton2 neutrons
56Examples of Isotopes Element Protons Electrons Neutrons Symbol HydrogenHydrogenHydrogenUraniumUraniumChlorineChlorine
57Atomic MassThe mass of a single atom is too small to measure on a balance.Using a mass spectrometer, the mass of the hydrogen atom was determined.
58A Modern Mass Spectrometer Positive ions formed from sample.Electrical field at slits accelerates positive ions.Deflection of positive ions occurs at magnetic field.From the intensity and positions of the lines on the mass spectrogram, the different isotopes and their relative amounts can be determined.A mass spectrogram is recorded.5.8
59A typical reading from a mass spectrometer A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the abundance (%) given.5.9
60Atomic MassUsing a mass spectrometer, the mass of one hydrogen atom was determined to be x g.To overcome this problem of such a small mass relative atomic masses using “atomic mass units” was devised to express the masses of elements using simple numbers.
61Atomic MassThe standard to which the masses of all other atoms are compared to was chosen to be the most abundant isotope of carbon – carbon 12.A mass of exactly 12 atomic mass units (amu) was assigned to carbon 12.1 amu is defined as exactly equal to the mass of a carbon-12 atom1 amu = x g
63Average Relative Atomic Mass Most elements occur as mixtures of isotopes.Isotopes of the same element have different masses.The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to the mass of carbon-12 (exactly …amu).
64Average atomic mass (amu) To calculate the atomic mass multiply the atomic mass of each isotope by its percent abundance and add the results.IsotopeIsotopic mass (amu)Abundance (%)Average atomic mass (amu)69.0930.91( amu)=43.48 amu( amu)=20.07 amu63.55 amu
65Relationship Between Mass Number and Atomic Number The mass number minus the atomic number equals the number of neutrons in the nucleus.mass numberatomic numbermass numberatomic numbernumber of neutrons-=62109-47=
66Concepts Dalton’s Atomic Mode; Law of Definite Composition Law of Multiple ProportionsThree principle subatomic particlesThomson Model of the AtomRutherford alpha-scattering experimentAtomic Number, Mass number, number of neutrons, number of Protons, Number of ElectronsThree Isotopes of HydrogenAverage Atomic Mass of an Element