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1Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 2Atoms and thePeriodic TableKent L. McCorkleCosumnes River CollegeSacramento, CACopyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2Subatomic Particles and Atomic Structure 2.2In the late 1800’s, many scientists were doing research involving radiation, the emission and transmission of energy in the form of waves.They commonly used a cathode ray tube, which consists of two metal plates sealed inside a glass tube from which most of the air has been evacuated.
3Subatomic Particles and Atomic Structure When metal plates are connected to a high-voltage source, the negatively charged plate, or cathode, emits an invisible ray.The cathode ray is drawn to the anode where it passes through a small hole.Although invisible, the path is revealed when the ray strikes a phosphor-coated surface producing a bright light.
4Subatomic Particles and Atomic Structure Researches discovered that like charges repel each other, and opposite charges attract one another.J. J. Thomson ( ) noted the rays were repelled by a plate bearing a negative charge, and attracted to a plate bearing a positive charge.
5Subatomic Particles and Atomic Structure This prompted him to propose the rays were actually a stream of negatively charged particles.These negatively charged particles are called electrons.By varying the electric field and measuring the degree of deflection of cathode rays, Thomson determined the charge-to-mass ratio of electrons to be 1.76×108 C/g. (C is coulomb, the derived SI unit of electric charge.)
6Subatomic Particles and Atomic Structure R. A. Millikan ( ) determined the charge on an electron by examining the motion of tiny oil drops.The charge was determined to be ×10-19 C.
7Subatomic Particles and Atomic Structure Knowing the charge, he was then able to use Thomson’s charge-to-mass ratio to determine the mass of an electron.
8Subatomic Particles and Atomic Structure Wilhelm Rontgen ( ) discovered X-rays. They were not deflected by magnetic or electric fields, so they could not consist of charged particles.Antoine Becquerel ( ) discovered radioactivity, the spontaneous emission of radiation.Radioactive substances, such as uranium, can produce three types of radiation.
9Subatomic Particles and Atomic Structure Alpha (α) rays consist of positively charged particles, called α particles.Beta (β) rays, or β particles, are electrons so they are deflected away from the negatively charged plate.Gamma (γ) rays, like X-rays, have no charge and are unaffected by external electric or magnetic fields.
10Subatomic Particles and Atomic Structure Ernest Rutherford used α particles to prove the structure of atoms.The majority of particles penetrated the gold foil undeflected.Sometimes, α particles were deflected at a large angle.Sometimes, α particles bounced back in the direction from which they had come.
11Subatomic Particles and Atomic Structure Rutherford proposed a new model for the atom:Positive charge is concentrated in the nucleus.The nucleus accounts for most of an atom’s mass and is an extremely dense central core within the atom.A typical atomic radius is about 100 pmA typical nucleus has a radius of about 5×10–3 pm1 pm = 1×10–12 m
12Atoms First2.1An atom is the smallest quantity of matter that still retains the properties of matter.An element is a substance that cannot be broken down into two or more simpler substances by any means.Examples: gold, oxygen, heliumAtoms can also be divided smaller and smaller until eventually only a single atom remains. Dividing it any smaller would give pieces that are no longer an atom.A DVD collection can be separated into smaller numbers until you have just one DVD left. But a single DVD cannot be separated into smaller pieces that are still DVDs.
13What are atoms made of?Protons: positively charged particles that are housed in the nucleusof an atom and have significant massNeutrons: neutral particles that are housed in the nucleus. They act tohold the protons in place since like charges repel eachother. Neutrons have significant massElectrons have negligible mass, have a negative charge and are allowedto roam freely in the electron cloud so they take up significantvolume in the atomTaylor 2010
14Atomic Number, Mass Number, and Isotopes 2.3All atoms can be identified by the number of protons and neutrons they contain.The atomic number (Z) is the number of protons in the nucleus.Atoms are neutral, so it’s also the number of electrons.Protons determine the identity of an element. For example, nitrogen’s atomic number is 7, so every nitrogen has 7 protons.The mass number (A) is the total number of protons and neutrons.Protons and neutrons are collectively referred to as nucleons.Mass number(number of protons + neutrons)Elemental symbolAtomic number(number of protons)
15Atomic Number, Mass Number, and Isotopes Most elements have two or more isotopes, atoms that have the same atomic number (Z) but different mass numbers (A).1 proton0 neutrons1 proton1 neutron1 proton2 neutronsIsotopes of the same element exhibit similar chemical properties, forming the same types of compounds and displaying similar reactivities.
16Worked Example 2.1Determine the numbers of protons, neutrons, and electrons in each of the following species: (a) Cl, (b) Cl, (c) K, and (d) carbon-14.35173741Strategy Recall the superscript denotes the mass number (A) and the subscript denotes the atomic number (Z). If no subscript is shown, the atomic number can be deduced from the elemental symbol or name. Atoms are neutral so the number of electrons equals the number of protons.SolutionZ = 17, so 17 protonsA = 35, so = 18 neutrons# of electrons = # of protons, so 17 electronsElement is chlorine again, so Z must be 17; 17 protonsA = 37, so = 20 neutrons17 protons, so 17 electrons
17Worked Example 2.1 (cont.)Determine the numbers of protons, neutrons, and electrons in each of the following species: (a) Cl, (b) Cl, (c) K, and (d) carbon-14.35173741Strategy Recall the superscript denotes the mass number (A) and the subscript denotes the atomic number (Z). If no subscript is shown, the atomic number can be deduced from the elemental symbol or name. Atoms are neutral so the number of electrons equals the number of protons.Solution(c) Potassium’s atomic number is 19, so 19 protonsA = 41, so = 22 neutrons# of electrons = # of protons, so 19 electrons
18Worked Example 2.1 (cont.)Determine the numbers of protons, neutrons, and electrons in each of the following species: (a) Cl, (b) Cl, (c) K, and (d) carbon-14.35173741Strategy Recall the superscript denotes the mass number (A) and the subscript denotes the atomic number (Z). If no subscript is shown, the atomic number can be deduced from the elemental symbol or name. Atoms are neutral so the number of electrons equals the number of protons.Solution(d) Carbon-14 can also be represented as CCarbon’s atomic number is 6, so 6 protonsA = 14, so = 8 neutrons6 protons, so 6 electrons14Think About It Verify that the number of protons and the number of neutrons for each example sum to the mass number that is given. In part (a), there are 17 protons and 18 neutrons, which sum to give a mass number of 35, the value given in the problem. In part (b), 17 protons + 20 neutrons = 37. In part (c), protons + 22 neutrons = 41. In part (d), 6 protons + 8 neutrons = 14.
19Do You Understand Isotopes? How many protons, neutrons, and electrons are inC146?6 protons, 8 (14 - 6) neutrons, 6 electronsHow many protons, neutrons, and electrons are inC116?6 protons, 5 (11 - 6) neutrons, 6 electrons2.3
20Ions Atoms are neutral—meaning that the number of protons is equal to the number of electronsIf an atom loses or gains electrons the atom isNo longer neutral but has a charge.
21cation – ion with a positive charge An ion is an atom, or group of atoms, that has a net positive or negative charge.cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.Na11 protons11 electronsNa+11 protons10 electronsanion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.Cl-17 protons18 electronsCl17 protons17 electrons
22A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3-A polyatomic ion contains more than one atomOH-, CN-, NH4+, NO3-
23Nuclear Symbol of Ions X A C Z X = element symbol A = atomic number Z = mass numberC = charge of IonNumber of Protons = ZNumbers of Neutrons = A – ZNumber of electrons = Z - C
25Do You Understand Ions? Al Se How many protons, neutrons and electrons are in ?Al27133+Protons = 13Neutrons = = 14Electrons = 13 – 3 = 10How many protons, neutrons and electrons are in ?Se78342-Protons = 34Neutrons = 78 – 34 = 44Electrons = 34 – (-2) = 36Taylor 2012
26Atomic mass is the mass of an atom in atomic mass units (amu) Micro Worldatoms & moleculesMacro WorldgramsAtomic mass is the mass of an atom in atomic mass units (amu)By definition:1 atom 12C “weighs” 12 amuOn this scale1H = amu16O = amu3.1
271 amu = 1/12 the mass of a carbon-12 atom 2.4Average Atomic MassAtomic mass is the mass of an atom in atomic mass units (amu).1 amu = 1/12 the mass of a carbon-12 atomThe average atomic mass on the periodic table represents the average mass of the naturally occurring mixture of isotopes.Average mass (C) = (0.9893)( amu) + (0.0107)( amu)IsotopeIsotopic mass (amu)Natural abundance (%)12C98.9313C1.07= amu
28Worked Example 2.2Oxygen is the most abundant element in both Earth’s crust and the human body. The atomic masses of its three stable isotopes, O ( percent), O ( percent), O (0.205 percent), are , , and amu, respectively. Calculate the average atomic mass of oxygen using the relative abundances given in parentheses.168178188Strategy Each isotope contributes to the average atomic mass based on its relative abundance. Multiplying the mass of each isotope by its fractional abundance (percent value divided by 100) will give its contribution to the average atomic mass.Solution( )( amu) + ( )( amu) + ( )( amu)= amuThink About It The average atomic mass should be closest to the atomic mass of the most abundant isotope (in this case, oxygen-16) and, to four significant figures, should be the same number that appears in the periodic table on the inside front cover of your textbook (in this case, amu).
29Average atomic mass of lithium: Natural lithium is:7.42% 6Li (6.015 amu)92.58% 7Li (7.016 amu)Average atomic mass of lithium:(7.42 x 6.015) + (92.58 x 7.016)100= amu3.1
31Dozen = 12 Names associated with an amount Pair = 2 Can you think of any more?????
32The Mole….. The mole (mol) is the amount of a substance that contains as many elementary entities as thereare atoms in exactly grams of 12C1 mol = NA = x 1023Avogadro’s number (NA)
33Molar mass is the mass of 1 mole of in grams marbles atoms eggsshoesMolar mass is the mass of 1 mole of in gramsmarblesatoms1 mole 12C atoms = x 1023 atoms = g1 12C atom = amu1 mole 12C atoms = g 12C1 mole lithium atoms = g of LiFor any elementatomic mass (amu) = molar mass (grams/mol)3.2
342.6The Mole and Molar MassThe mole is defined as the amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12.This experimentally determined number is called Avogadro’s number (NA).We normally round this to 6.022×1023.1 mole = 6.022×1023, just like 1 dozen = 12 or 1 gross = 144.NA = x 1023
35Worked Example 2.3Calcium is the most abundant metal in the human body. A typical human body contains roughly 30 moles of calcium. Determine (a) the number of Ca atoms in moles of calcium and (b) the number of moles of calcium in a sample containing 1.00×1020 Ca atoms.Strategy Use Avogardo’s constant, 1 mole = 6.022×1023, to convert from moles to atoms and from atoms to moles.Solution30.00 mol Ca ×1.00×1020 Ca atoms ×6.022×1023 Ca atoms1 mol Ca= 1.807×1025 Ca atoms1 mol Ca6.022×1023 Ca atoms= 1.66×10-4 mol CaThink About It Make sure that units cancel properly in each solution and that the result makes sense. In part (a), for example, the number of moles (30) is greater than one, so the number of atoms is greater than Avogadro’s number. In part (b), the number of atoms (1×1020) is less than Avogadro’s number, so there is less than a mole of substance.
36Salt (Sodium Chloride) The MoleOne mole each of some familiar substances:Helium (in balloon)WaterSugar (Sucrose)AluminumCopperSalt (Sodium Chloride)
37Molar MassThe molar mass of a substance is the mass in grams of one mole of the substance.By definition, the mass of a mole of carbon-12 is exactly 12 g.Mass of 1 carbon-12 atom: exactly 12 amuMass of 1 mole of carbon-12: exactly 12 gAlthough molar mass specifies the mass of one mole, making the units (g), we usually express molar masses in units of grams per mole (g/mol) to facilitate cancellation of units in calculations.
38Worked Example 2.4Determine (a) the number of moles of C in g of carbon, (b) the number of moles of He in g of helium, and (c) the number of moles of Na in g of sodium.Think About It Always double-check unit cancellations in problems such as these–errors are common when molar mass is used as a conversion factor. Also make sure that the results make sense. For example, in the case of part (c), a mass smaller than the molar mass corresponds to less than a mole.Strategy Molar mass of an element is numerically equal to its average atomic mass. Use the molar mass for each element to convert from mass to moles.Setup (a) The molar mass of carbon is g/mol. (b) The molar mass of helium is g/mol. (c) The molar mass of sodium is g/mol.Solution25.00 g C ×10.50 g He ×15.75 g Na ×1 mol C12.01 g C= mol C1 mol He4.003 g He= mol He1 mol Na22.99 g Na= mol Na
39Interconverting Mass, Moles, and Number of Atoms Molar mass is the conversion factor from mass to moles, and vice versa.Avogadro’s constant converts from moles to atoms.
40Worked Example 2.5Determine (a) the number of C atoms in g of carbon, and (b) the mass of helium that contains 6.89×1018 He atoms.Strategy Use the conversions depicted in the previous slide to convert (a) from grams to moles to atoms and (b) from atoms to moles to grams.Think About It A ballpark estimate can help you prevent common errors. For example, the mass in part (a) is smaller than the molar mass of carbon. Therefore, you should expect a number of atoms smaller than Avogadro’s number. Likewise, the number of atoms in part (b) is smaller than Avogadro’s number. Therefore, you should expect a mass of helium smaller than the molar mass of helium.Setup (a) The molar mass of carbon is g/mol. (b) The molar mass of helium is g/mol. NA = 6.022×1023Solution0.515 g C ×6.89×1018 He atoms ×1 mol C12.01 g C6.022 ×1023 C atoms1 mol C×= 2.58×1022 C atoms1 mol He6.022 ×1023 He atoms4.003 g He1 mol He×= 4.58×10-5 g He
41How many atoms are in 0.551 g of potassium (K) ? 1 mol K = 39.10 g K 1 mol K = x 1023 atoms K1 mol K39.10 g Kxx6.022 x 1023 atoms K1 mol K=0.551 g K8.49 x 1021 atoms K
42What is the mass of 1.21 1020 atoms of sulfur (in mg) ?