Chem 1.1-2.PDF

e hapt*r 'l: $ti"u*t*rs *nd Fr*p*dies o{ Atom* ^^^^^,i I i" _->5L11 1 I Atomic Structure introduction 3anples of matter can be broadly classified into one of two categories: pure substances or mixtures. Pure substances contain only one chemical species (...

e hapt*r 'l: $ti"u*t*rs *nd Fr*p*dies o{ Atom* ^^^^^,i I i" _->5L11 1 I Atomic Structure introduction 3anples of matter can be broadly classified into one of two categories: pure substances or mixtures. Pure substances contain only one chemical species (ie, either an element or a compound) that has a : srrnct identity. An element is a substance that cannot be separated into simpler substances by :-emical interactions, and a compound consists of two or more chemically bonded elements that form a -:,,,,substance with a new identity that can be separated back into its component elements by chemical =dil5. : s.lmmary of the classifications of matter is shown in Figure 1.1. I Matter I I I Mixture Pure Substance. Two or more types of substances. One type of substance. Variable composition $eparation by. Fixed composition physical processes. Can be separated. Cannot be separated by physical methods by physical methods Heterogeneous Hoinogeneous Compound Element Mixture Mixture Two or more chemically Matter that cannot Icnemical Varying particle Uniform particle bonded elements that reactions be separated into f distribution distribution cannot be separated simpler substances throughout mixture throughout mixture by physical means by chemical means *ft &'# ,ffie *@*@ fi s e-# ffiu ffiffi b"t **-ft '-a 4-.9 frt s *"ffi "ww # + %s-E + "ry Q.# 4s lEe s"sk W * #6l"s sw J\e Sand (SiOr) & water Aqueous bromine Water (HrO) Zincmetal Figure 1.1 Classifications of matter.  Chapter 1: $tructure and Properties of Atcms Elements are composed of atoms, the smallest units into which a sample of matter can be divided without changing the sample's identity or properties. As such, the elemental level is the limit to which matter can be chemically separated. Each element is made up of its own unique type of atom, but all atoms share the same structural characteristics' protons All atoms consist of a central nucleus containing a mixture of positively charged particles called particles and chargeless particles called neutrons, and the nucleus is surrounded by negatively charged called electrons. The various elements differ in the number of protons in their respective nuclei. A model of an atom of the element carbon is shown in Figure 1.2. 6 protons 6 electrons Figure 1.2 Model of a carbon atom. 1.1.01 Atomic Number and Mass Number The atomic number of an atom refers to the number of protons in its nucleus and determines the protons). etemental identity of the atom (ie, all atoms of the same element have the same number of Atoms with different numbers of protons (ie, different atomic numbers) are different elements wtth different properties. to The mass number of an atom indicates the sum of the number of protons and neutrons contributing of neutrr:ns in an atom can be determined by the total mass of the nucleus. As such, the nurnher subtracting the atomic number from the mass number. Number of neutrons = Mass number - Atomic number Any atom in its e/ementat state is electrically neutral (ie, no net charge) because the number of negatively chirged electrons (e-) is equal to the number of positively charged protons. An example of the number of of particle in a carbon atom with a mass number of 13 is shown in Figure 1.3. "".tityp"  Chapter 1: $tructure and Properties of Atoms 7 neutrons 6 protons 6 electrons 6 protons (+) 6 electrons (-) 0 (no net charge) @ @. Figure 1.3 Model of a carbon atom in its elemental state with a balance of positive and negative charges. An atom in its elemental state that gains or loses electrons becomes an atomic ion (ie, an atom with a net charge). A neutral atom acquires a net charge of +1 for each electron lost and a net charge of -1 for each electron gained because the numbers of protons and electrons become unequal. For example, the number of each type of particle in an atomic ion formed from a fluorine atom with a mass number of 19 is shown in Figure 1.4. lon formation is discussed further in Lesson 2.1. w w @ & qii& &* & w w {* & 6,& w w w tu 4& *}j$ Neutral fluorine atom (F) Charged fluoride ion (F-) Frr:tons. s Protcr"rs: I Neutrons: 1n Neutrons: 1n [!ri:r*rsl; t *ln::'l':i:ln- 1 r"l Figure 1.4 Model of a fluorine atom converted to an atomic ion from its elemental state.  *hapt*r 1: $iructur* and Frnpertics of At*n'ls #mrn*mgrt #[\*m'fi; 'i. t many neutrons and Atoms of calcium (ca) ionize during reactions to form ca2* ions. How electrons are in a single Ca2* ion that has a mass number of 42? S'::!s*$.$mlt Note: The appendix contains the answer. 1.1.42lsotopes determine the elemental identity The number of neutrons contributes to the mass of an atom but does not of protons) that have different of an atom. lsl,;:[r:11,3..r are atoms of the sarne element (ie, same number by their mass number (the numbers of neutrons in their nuclei. For this reason, isotopes are identified of the elemental name (eg, zinc- sum of the number of protons and neutrons), which appears at the end 65Zn)' 65) or placed as a superscript to the upper left of the element symbol (eg' below the mass number (eg' ln nirnlsar- iloiniir:ir, the atomic number is sometimes included as a subscript because all atoms of a given element have the same ![zn); however, the atomic number is often omitted the name (or symbol) of an element' number of protons (eg, all zinc atoms have 30 protons). By knowing on the $eriorjir-; i;rhlc' the number of protons can be determined by referencing the atomic number The percentage of each Most naturally occurring elements exist as a mixture of different isotopes. an element is determined by the natural abundance of each isotope found in an ave"rage sample of present on Earth). The distribution of tsotopes can be found by isotope (ie, the relative amounts naturally analyzing a pure elemental sample using ini:irrji $i.ii,ti,lii.iiijrtil'ir. The mass spectrum of a pure sample of elemental zinc is shown in Figure 1.5. -60 ;< o aF40 C f (o o20 fit 6 t0 62 64 66 68 70 Atomic mass (amu) of pure elemental zinc' Figure 1.5 Distribution of isotopes in the mass spectrum of an average sample configuration, not the number of The chemical behavior of atoms is determined primarily by their electron properties (eg, bonding, neutrons. lsotopes of the same element have nearly identical chemical in their physicalproperties (eg, reactivity) because they have the same electron configuration but differ of atoms are examined in density, mass). The reasons why electrons largely determine the reactivity Lesson 2.1.  Chapter 1: Structure and Properties of Atoms 1.1.03 Atomic Mass Because all atoms contain protons and neutrons, the masses of different isotopes can be conveniently measured and compared by the atomic mass unit (amu), which is defined as one twelfth the mass of a carbon-12 atom. However, the atomic mass of a single isotope cannot be used in chemical calculations for samples used in a lab because most samples of substances contain a mixture of isotopes. Instead, lab measurements use the atomic mass (atomic weight), which is an average of the masses of each isotope proportionally weighted to its natural abundance. These weighted averages are listed below each element symbol on the periodic table. An example of the weighted average for the isotopes in a sample of zinc is shown with its mass spectrum in Figure 1.6. Average 65.38 amu G' 60 ;o d All frav c o5o o20 'g -g o s0 Average atomic 62 64 66 68 70 mas$ of isotopes isotoper (weighted) hted) Atomic mass (amu) Figure 1.6 Average mass of isotopes in an average sample of pure elemental zinc.  Chapter 1: $tructure and Properties of Atc #mmcept 6heck 1.f, 100 s c) 80 o C, (U 60 c :f o$ 40 o o 20 o) t 0 190 192 194 Atomic mass (amu) pure sample of iridium Based on the masses of the isotopes and their natural abundance in a shown in the mass spectrum above, calculate the average atomic mass of iridium' "$oluntieim Note: The appendix contains the answer. r Chapter 1: $tructure and Properties of Atoms Lesson 1.2 Nuclear Decay The nucleons (protons and neutrons) in an atomic nucleus are held together by a close-range attraction called the strong nuclear force. However, the positively charged protons also repel each other according to Coulomb's law. As such, the nucleus remains intact by achieving a balance between the competing attractive and repulsive forces between the particles. Neutrons assist this balance of forces by separating and distributing the protons within the nucleus. As a result, the stability of the nucleus depends, in part, on the number of nucleons and the ratio of neutrons to protons (ie, the n0/p* ratio). A nucleus is most stable when the no/pt ratio is around 1. In larger nucleiwith a greater number of nucleons, no/pt ratios that deviate from 1 are less stable and have a less effective balance between the competing nuclbar forces. lf the repulsive forces are not sufficiently balanced by the attractive forces, a nucleus can eject nuclear particles in a process called radioactive decay. The emission of radiation (ie, nuclear particles andior high-energy photons) allows an unstable nucleus to achieve a more stable configuration by releasing energy and fragmenting into smaller nuclei with more favorable no/p* ratios. Several types of radioactive decay are known, but the three most common types are alpha decay, beta decay, and gamma emission. )uring alpha decay, an unstable nucleus ejects an alpha (o) particle consisting of 2 protons and 2 :eutrons (ie, a helium-4 nucleus without its electrons). Accordingly, the resulting nucleus formed ':llowing an alpha decay has a mass number that is 4 units less and an atomic number that is 2 units less :ran the nucleus that underwent the decay. An example of alpha decay is shown in Figure 1.7. "'tr '.,-.-@ '*]*6. q:h Alpha particle '33 Pu fHe Eigure 1.7 Alpha decay of plutonium-239. -,:ha particles are the least energetic and most massive of the common radioactive emissions. As such, alpha particle can be easily stopped by collisions with other matter. Alpha particles ultimately acquire =- ;::trons from the surrounding environment to form helium atoms. As a result, natural underground :ecosits of helium gas are produced over time by the radioactive decay of elements such as uranium.  *hapl*r 'i: $tru*tur* *rld $3rr:p*rti*s *{ Ai, fl)mmme*p,+t {il h*fi }*i'1. i.i lf actinium-227 is formed by the alpha decay of element X, what is the identity, atomic number, and mass number of element X? "1sq3[iltImrl Note: The appendix contains the answer' 1.2:02 Beta Decay, Positron Emission, and Electron capture Radioactivity involving beta (B) decay occurs in one of three forms: B- decay (electron emission), B* decay (posiiron emission), and electron capture. In all three forms of beta decay, the nra";n nurnhcr ,"r"lni unchanged butthe ill.riiriir; friii.i"itri:f either increases ordecreases by 1 unit, depending on the type of emission. In B- decay, a neutron converts to a nbclear proton and ejects a high-speed electron (represented symbolical-ly as e-, -!e, or -?p) and an antineutrino (represented symbolically as te or $v"), which increases the atomic number by 1 unit. For example, the 9- decay of potassium-40 yields the results shown in Figure 1.8. 13 r- 23ca +_le+6ve n n- t!,,, ;.,64*u"'&'"1 , n,' ',y B- decay Yrt Electron &c Antineutrino Figure 1.g B- decay of potassium-40 showing the emission of a beta particle (electron) and an antineutrino. In B* decay, which is commonly called positron emission, a nuclear proton converts to a neutron (t opposite of B- decay) and ejects a positron (represented symbolically as e*, or *!e, *?P) and a neutri (represented symbolically as ve or flv"), which decreases the atomic number by unit. For example, '1 B* decay of potassium-40 yields the results shown in Figure '1'9'  Chaptcr '1: $trunture and Properties sf Atoms 13f-13nr* *?e+3ve B* decay @ Positron ?3x 13nr # Neutrino : Figure 1.9 B* decay of potassium-4O showing the emission of a positron (antielectron) and a neutrino. ln electron capture, a nuclear proton captures an electron orbiting near the nucleus and the combined particles convert to a neutron without emitting a particle. Like B. decay, this process decreases the atomic number by 1 unit. For example, the nuclear decay of potassium-4O by electron capture yields the results shown in Figure 1.10. i3 t< * -?e - 13Ar i3r 13nr Figure 1.10 Radioactive decay of potassium-40 by electron capture. Although neutrinos and antineutrinos are present in B* decay and B- decay, respectively, these essentially massless particles are not always explicitly stated when discussing beta decay in a chemistry context. Some sources choose to omit neutrinos and antineutrinos from nuclear equations to give abbreviated statements of the nuclear conversions that focus on the particles that are most relevant to chemistrv.  Chapter 1: $tructure and Pr"opertins of Atoms ffi s.)n'!cerlt ffi heclq ,|.4 (23Na or Magnesium-23 has been determined to undergo beta decay. Which of the two isotopes 23Al) detected in the magnesium sample would provide evidence of B- decay? ,ffiq)fru-ril;!ffifl'l Note: The appendix contains the answer. 1.2.03 Gamma Emission releases During gamma (y) decay, an unstable nucleus in an excited state (ie, a higher-energy state) excess energy by emitting a gamma ray (represented symbolically as y,3y, or hv), which is a high' (ie, packet). Because only energy is released in a gamma energy ptroton an erl,,rclronlapnetir;',,vrlur: emisJion, the number of protons and neutrons in the nucleus remains the same, and the isotopic and elemental identityof the nucleus is unchangbO. Rn exampleof gammaemission isshown in Figure 1.1'1. Excited nuclear state y ray 103 46 Pd ,?3 po Figure 1.11 Gamma emission by an atom of palladium-'|03' more The rates at which different radioisotopes decay vary significantly because some nuclei are much stable than others. The relative stability of an isotope can be assessed from its half'life, which is defined as the time required for half a sample of an isotope to decay. A very short half-life indicates an unstable very rapidly whereas a longer half-life indicates a more stable isotope that decays isotope that decays more slowly. The half-life of an isotope is an intrinsrc property that does not change over time. passes, as illustrated The amount of a radioisotope in a sample decreases by half with each half-life that by the diagram in Figure 1.12. This decay causes the fraction of the initial amount of the radioisotope remaining in the sample to become exponentially smaller, and eventually approach zero.  Chapter 1: $tructure and Properties of Atoms t t,9u090e t') :E o E E 112 thalf-lifegTg$|W@eflry c o C) o tL 1t4 1/8 half-lives ry fuh-Half decay Time Figure 1.12 Radioactive decay plot and half-life diagram. The fraction of radioisoto'pe remaining after n half-lives can be calculated as: Fraction remaining =-1rL After one half-life (n = 1),)or 50"/o of the original substance remains; after two half-lives (n = 2),lor 25o/, f,or 12.5% remains; and so on. Accordingly, the activity of the remains; after three half-lives (n = 3), sample decreases as time passes. The activity of a radioisotope refers to the number of decay events per unit of time. An older unit of activity called the curie (Ci) was initially developed based on the,number of decays per second detected from 1 g of the radium-226 isotope: L curie (Ci) = 3'7 x L010 decays/second However, the preferred Sl unit of activity is the becquerel (Bq): 1, becquerel (Bq) = 1 decaY/second The activity of a radioisotope is proportional to both the amount of decaying radioisotope (ie, more atoms yield a higher activity) and to the half-life of the radioisotope (ie, a shorter half-life gives more activity). Activity will decrease by 50% with each successive half-life that passes. Activity = (Fraction remaining) x (lnitial activity) Activity = $, (lnitial activity) When interpreting radioactive decay data, the half-life of a sample can be read directly from a radioactive decay plot corresponding to the time at which 50% of the initial amount has decayed. lf the decay data is  Chapt*r 1: Str*ctur* and Fr*p*r{i*s *f At*ffis between two data presented in a table, the half-life can be roughly estimated by finding a time interval points that gives an approximately 50% reduction in the amount of the sample or its activity' ffimm*mpf ffifums$q $.ffi activity shown in the The radioactive decay of a sample of rhodium-'l06 was found to have the table below. Time (s) Activity (Bq) 15 5,303 ^^ JU 3 750 45 '. 2,652 60 1,875 75 1,326 90 938 What is th"e approximate half-life of 106Rh, and what will be the activity of the sample after three half-lives?.${:}lLr{[*rl Note: The appendix contains the answer' 14