Unit 3: Atoms, Molecules, and Ions Development of Atomic Theory It took over 2000 years from the first proposal of an atomic universe before actual experimental evidence had been accumulated! Matter according to the Greek Philosophers BC: Empedocles supported by Aristotle Earth-Air-Fire-Water 400 BC: Zeno (also supported by Aristotle) Matter can be infinitely divided 400 BC: Leucippus BC: Democritus (student of Leucippus) Matter is made of indivisible particles called atoms Atomic Structure Revolution 1500 Robert Boyle 1500 Robert Boyle The Skeptical Chemist Lavoisier used an enclosed container to study chemical and physical changes. After very careful measurements, he concluded that mass is neither created nor destroyed during these processes. He performed detailed experiments with gases and began the break down of the Greek model of matter. 1700 Antoine Lavoisier Law of Conservation of Mass 1790 1800 Joseph Proust Law of Constant (Definite) Proportions Joseph Proust developed the Law of Constant (Definite) Proportions Compounds are composed of elements Proust determined that a given compound always contains exactly the same proportion of elements, by mass. For example, water is composed of a constant proportion of 1 part hydrogen to 8 parts oxygen, by mass. 1800 John Dalton Law of Multiple Proportions Proposed the Law of Multiple Proportions If two elements combine to form different compounds, the mass ratios of the two elements in the compounds can be expressed as a ratio with simple whole numbers. For example, hydrogen and oxygen react to make two different compounds: In water, 8 g of oxygen reacts with 1 g of hydrogen In hydrogen peroxide, 16 g of oxygen reacts with 1 g of hydrogen. The second mass ratio (16:1) is exactly double the first ratio (8:1) Since water is H 2 O, Hydrogen peroxide must be H 2 O 2 2.1 2 Law of Multiple Proportions, contd Copper forms two different oxides. An analysis of the two compounds shows that in the first compound 63.5 g of copper combines with 16.0 g of oxygen, and in the second compound, 10.0 g of copper combines with 5.04 g of oxygen. Show that the two substances obey the law of multiple proportions. Daltons Atomic Theory John Dalton proposed explanations for the laws of mass conservation, definite & multiple proportions: An element is composed of tiny particles called atoms. All atoms of a given element are identical and atoms of different elements are different. Atoms of different elements combine in ratios of small whole numbers when forming compounds. Chemical reactions only rearrange the way atoms are combined; the atoms themselves are not changed. 1897 J.J. Thomson cathode rays J.J. Thomson is studied mysterious cathode rays in a cathode ray tube (CRT). He discovered that these cathode rays carried a negative charge, and that they had mass His conclusion: cathode rays are really streams of negatively charged particles, electrons Cathode Ray Tubes Thomsons Conclusions Thomson saw that the cathode ray beam could be deflected using either an external magnetic or electric field Based on the direction of the beam deflection, he concluded it carried negative charge He determined the charge to mass ratio for an electron: e/m = x 10 8 C/g This is important because if either charge or mass can be determined then the other can be calculated. J.J. Thomson Thomsons Atomic Model Thomsons discovery showed that there was matter even smaller than atoms. After discovering the electron, J.J. Thomson proposed a plum pudding or raisin bun model of the atom He envisioned a positively charged dough with negatively charged electrons scattered throughout. 1909 Robert Millikan Determined the charge on a electron using an oil drop experiment. After he charge the oil drop then he balanced it between a negative and a positive plate. e= x C m= x g 1911 Earnest Rutherford Rutherford studied the new phenomenon of radioactivity, discovered by Henri Bequerel. In 1898, Rutherford discovered 3 different types of radioactivity: , , nuclear atom He later used the alpha particles in a famous experiment that ended with the discovery of the nuclear atom Next: Shockwave Animation Rutherfords Gold Foil Experiment The Gold Foil Experiment Thomsons Model? Nuclear Model? "It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you." - E. Rutherford Gold Foil Rutherfords Nuclear Model If an atom was as big as a football stadium, the nucleus would be the size of an ant on the 50-yard line! 1932 James Chadwick The last of the three main subatomic particles was discovered in 1932 by James Chadwick Worked with Rutherford looking for an uncharged subatomic particle, but failed neutron In 1932, he revisited some earlier experiments and was successful he discovered the neutron! Thanks, Mr. Chadwick! Three Important Subatomic Particles The electron was discovered first by Thomson it is negatively charged and the lightest of these three subatomic particles. The proton was deduced by Rutherford. It is positively charged and has a mass of approximately 1 amu. The neutron was the last particle to be discovered (by James Chadwick) in It has approximately the same mass as a proton, but is neutral. Important Vocabulary Atomic Number (Z) The number of protons in the nucleus of an atom Also equal to the number of electrons around the nucleus if the atom is neutral Determines the identity of an element Mass Number (A) The number of protons & neutrons (i.e. nucleons) within the nucleus of an atom Isotopes Atoms of the same element (with the same # of protons), with different numbers of neutrons (i.e. different mass numbers) Carbon-14 is an isotope that has 14 protons and neutrons Atomic Mass The mass of an atom. May be measured in grams More conveniently expressed in atomic mass units, amu Three Isotopes of Hydrogen Two Isotopes of Sodium Understanding Isotopes How many protons, electrons and neutrons are there in a neutral atom of phosphorus-32? 15 Protons (Z) 17 Neutrons (32 15) 15 Electrons (neutral) How many protons, electrons, and neutrons are in a neutral atom of potassium-39? 19 protons (Z) 20 neutrons (39 19) 19 electrons (neutral) The Mass Spectrometer An instrument that separates and analyzes particles based on their masses. Pictured below is the separation of neon gas into its three isotopes. The Mass Spectrum of Neon A computer displays a graph of Abundance vs Mass Number for neon gas The three peaks suggest three different isotopes: 20 Ne, 21 Ne and 22 Ne. The area under each peak represents the abundance or the fraction of the neon gas contributed by each isotope Average Atomic Mass & Natural Abundances Chromium exists as four stable isotopes. Use the information below to calculate the average atomic mass of chromium. MassAbundance amu4.35% amu83.79% amu9.50 % amu2.36% Naturally occurring lithium consists of two isotopes with atomic masses of and amu. The average atomic mass of lithium is amu. Calculate the natural abundance of each isotope. Ans. 7.42% & 92.58% Atomic Mass Units The atomic mass unit is DEFINED as one-twelfth the mass of the nucleus of an atom of carbon-12. A proton or neutron has a mass of approximately 1 amu 1 amu = x kg Originally defined with respect to hydrogen, and then oxygen. 1922 Niels Bohr Electrons closer to the nucleus have lower energy and are more stable due to strong electrostatic attraction with the nucleus. The letter n is used to designate energy levels (orbits), with lower whole number values of n representing lower energy orbits closer to the nucleus. The key to Bohrs Quantum Model was that electrons are restricted to certain allowed energy levels in atoms. Whereas Rutherford suggested electrons orbit the nucleus, Bohrs model required that they occupy only certain orbits! The Bohr Atom The first electron orbit holds a maximum of 2 electrons The second orbit holds a maximum of 8 electrons The third orbit may hold up to 18 electrons The n th orbit can hold up to 2n 2 electrons! Draw a Bohr diagram for Carbon-14 6 p 8 n Draw a Bohr diagram for Oxygen-15 8 p 7 n Evolution of the Atomic Model Bohrs model of the atom is very useful for understanding elementary concepts in bonding and chemical reactions However, it is fundamentally flawed and he soon helped replace it with the Quantum Mechanical Model of the atom which no longer views the electron as simply a particle, but acknowledges and depends on the wave-nature of the electron also. Energy Level Diagrams Very similar to Bohr diagrams, without the orbits The energy level diagram for neutral fluorine is __ 9p F 2 e - 7 e - The electrons in the highest energy level are called the VALENCE ELECTRONS so a neutral fluorine atom has 7 valence electrons! Ions During many chemical reactions, atoms gain or lose electrons. Since the # of electrons no longer equals the # of protons, these atoms must be charged. A charged atom is called an ion. Cations are positively charged ions they have lost electrons (and now have more protons than electrons) Anions are negatively charged ions they have gained electrons (and now have more electrons than protons) Remember ELECTRONS are gained and lost nothing happens to the protons! Predicting Ion Charges Atoms are most stable when their valence orbit (the outermost orbit) is FULL of electrons. The Noble Gas family has full valence energy levels and these elements are highly un-reactive. The other elements would like to achieve a Noble Gas electron configuration. For example, a fluorine atom gains 1 electron to become a fluoride anion, F - A sodium atom loses 1 electron to become a sodium cation, Na + F Na Predicting Ion Charges Count the valence electrons, add or subtract electrons (whichever is the smaller number) to achieve a full valence level ?? n/a Note that metals tend to lose electrons (become cations) While nonmentals gain electrons (become anions)! 1869 Dmitri Mendeleev In 1869 Mendeleev and Lothar Meyer (Germany) published nearly identical classification schemes for elements known to date. The periodic table is based on the similarity of properties and reactivities exhibited by certain elements. Later, Moseley (1912) established that each elements has a unique atomic number, which is how the modern periodic table is organized. Periodic Table: Metals & Nonmetals Metals Nonmetals What are the properties of metals and nonmetals? Where are the semimetals (metalloids)? Metals & Nonmetals Properties of Metals Lustrous (shiny) Malleable Ductile Conductors of heat & electricity Tend to lose electrons (become cations) Note: only Iron, Nickel, Cobalt are strongly magnetic metals Properties of Nonmetals Dull Brittle Insulators Tend to gain electrons (become anions) Note: Semimetals (metalloids) are elements with properties of BOTH metals and nonmetals. Representative, Transition, & Rare Earth Elements Periods: Across the Periodic Table 2nd Period 6th Period Groups: Down the Periodic Table Alkali Family: 1 e- in the valence shell Alkali Family: 1 e- in the valence shell Halogen Family: 7 e- in the valence shell Halogen Family: 7 e- in the valence shell Chemical Families of the Periodic Table Alkali Alkaline (earth) Transition Metals Noble Gas HalogenChalcogens