Chapter 2: The Components of Matter
Chapter 2: The Components of Matter CHEM-UA 125
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Chapter 2: The Components of Matter 2.1 - Elements, Compounds, and Mixtures: An Atomic Overview Matter can be classiﬁed into 3 types-elements, compounds, and mixtures Elements and compounds = substances Substances: Matter with a ﬁxed composition Mixtures [variable composition] are NOT substances Element: The simplest type of matter with unique physical and chemical properties contains only one kind of atom - properties of atoms are unique cannot be broken down occur in molecular form Molecule: an independent structure of two or more atoms bonded together Diatomic molecule= two-atom molecules Compound: consists of two or more diﬀerent elements that are bonded chemically 2 deﬁning factors of a compound: The elements are present in ﬁxed parts by mass [ﬁxed mass ratio] because each unit of the compound consists of a ﬁxed number of atoms of each element A compound’s properties are diﬀerent from the properties of its elements A compound can be broken down into simpler substances Mixture: Consists of two or more substances [elements and /or compounds] that are physically intermingled, Not chemically combined Components can vary in their parts by mass Not a substance Retains many of its properties of its components Can be separated into their components by physical changes 2.2 The Observations That Led To An Atomic View of Matter Any model of the composition of matter deﬁned by: The Law of Mass Conservation The Law of Deﬁnite (or Constant) Composition The Law of Multiple Proportions Mass Conservation The Law of Mass Conservation: The total mass of substances does not change during a chemical reactionNumber of s ubstances and properties change Total amount of matter = constant Matter cannot be created nor destroyed Mass before and after a reaction is not exactly the same A very small amount of mass may be converted to energy Mass is considered as conserved Definite Composition The Law of Deﬁnite (or Constant) Composition: Not matter what its source, a particular compound is composed of the same elements in the same parts (fraction) by mass Fraction by Mass (mass fraction): The part of the compound’s mass that each element contributes mass of fraction = (mass of element X in compound A)/(mass of compound A) Percent by Mass (Mass Percent, Mass %): Is the fraction by mass expressed as a percentage Mass Percentage= Mass Fraction x 100 Each element in compound has a ﬁxed mass of fraction and mass percentage Mass of Element in Sample= (mass of compound in sample) x (mass of element in compound / mass of compound) Multiple Proportions The Law of Multiple Proportions: If elements A and B react to form two compounds, the diﬀerent masses of B that combine with a ﬁxed mass of A can be expressed as a ratio of small whole numbers In diﬀerent compounds of the same elements, the masses of one element that combin e with a ﬁxed mass of the other can be expressed as a ratio of small whole numbers 2.3 Dalton’s Atomic Theory Postulates of the Atomic Theory Dalton’s atomic theory explained the mass laws by proposing all that matter consists of indivisible, unchangeable atoms of ﬁxed, unique masses John Dalton’s atomic theory: 1. All matter consists of atoms, tiny indivisible units of an element that cannot be created or destroyed 2. Atoms of one element cannot be converted into atoms of another element in chemical reaction, the atoms of the original substance recombine to form diﬀerent substances 3. Atoms of an element are identical in mass and other properties and are diﬀerent from atoms of any other element 4. Compounds result from the chemical combination of a speciﬁc ratio of atoms of diﬀerent elements How the Theory Explains the Mass Laws Mass Conservation Postulate 1 and Postulate 2: mass is conserved during a reaction because the atoms retain their identities but are combined diﬀerently Deﬁnite Composition: Postulate 4 and Postulate 3:Each compound has a ﬁxed mass fraction of each of its elements because it is composed of a ﬁxed number of each type of atom Multiple Proportions: Postulate 1 and 3: Diﬀerent compounds of the same elements exhibit multiple proportions because they each consist of whole atoms 2.4 The Observations That Led To The Nuclear Atom Model Discovery of the Electron and Its Properties Electric current could decompose certain compounds into their elements Cathode Rays originate at the negative electrode (cathode) and move to the positive electrode (a node) Consist of negatively charged particles found in all matter rays become visible as their particles collide with the few remaining gas molecule s in the evacuated tube cathode ray particles = electrons Mass and Charge of the Electron: Two classic experiments and their conclusions reveal ed the mass and charge of the electron 1. Mass/charge ratio: 1897 - J.J. Thomson measured the ratio of the mass of a cathode ray particl e to its charge compared the value found with the mass/charge ratio for the lightest charge d particle in solution Estimated the cathode ray particle weighed less than 1/1000 as much as hy drogen (the lightest atom) Discovered that atoms contain even smaller particles 2. Charge 1909 - Robert Millikan measured charge of the electron observed the movement of tiny oil droplets in an apparatus that contained electrically charged plates and an x-ray source X-rays knocked electrons from gas molecules in the air within the apparatus, and the electrons stuck to an oil droplet falling through a hole in a positively charged plate Electrical ﬁeld oﬀ - measured the mass of negative droplet Adjusting ﬁeld’s strength - made droplet slow and hang suspended, measured total charge Electron’s charge = -1.602X10^(-19) C (Coulomb - the SI unit for charge) Conclusion: Calculating the electron’s mass: The electron’s mass/charge rat io, multiplied by the value for the electron’s charge gives the electron’s mas s = 9.109X10^(-28)g Atomic Number, Mass Number, and Atomic Symbol Discovery of the Atomic Nucleus Thompson’s “plum- pudding” model - a spherical atom composed of diﬀuse, positively charged matter wit h electrons embedded in it like “raisins in a plum pudding” used to explain why matter is has a neutral charge when electrons in an atom are negatively charged 1910: Ernest Rutherford tested Thompson’s model: Experimental Design: Tiny, dense, positively charged alpha particles emitted from radium are aimed at gold foil. A circular, zinc-sulﬁde screen registers the deﬂection (scattering angle) of the alpha particle emerging from the foil by emitting light ﬂashes when the particles strike it Hypothesis and expected results: Rutherford expected only minor, if any, deﬂections of the alpha particles because they should act as bullets and go right through the gold atoms Actual results: Initial results were consistent with hypothesis, but then the data showed very few alpha particles deﬂected at all and only 1 in 20,000 deﬂected by more than 90 degrees Rutherford’s Conclusion: The few alpha particles were being repelled by something small, dense, and positive within the gold atoms. An atom is mostly space occupied by electrons. In the center is a tiny region, nucleus, that contains all the positive particles lay and essentially all the mass of the atom. 1932 James Chadwick discovered the neutron - an uncharged dense particle that also resides in the nucleus 2.5 The Atomic Theory Today Structure of the Atom Atom: an electrically neutral, spherical entity composed of a positively charged central nucleus surrounded by one or more negatively charged electrons electrons move rapidly An atomic nucleus consists of protons and neutrons exception: hydrogen nucleus, consists of only protons Proton (p+) has a positive charge Neutron (n^0) has no charge Electron (e-) has a negative charge An atom is neutral because the number of protons in the nucleus equals the number of electrons surrounding the nucleus Atomic Number, Mass Number, and Atomic Symbol Atomic Number (Z) = the number of protons in the nucleus of each of its atoms all atoms of an element have the same atomic number the atomic number of each element is diﬀerent from that of any other element Mass number (A) = the total number of protons and neutrons in the nucleus of an atom Atomic Symbol (element symbol) based on its English, Latin, or Greek name often written with the Atomic symbol is the atomic number (Z) as the left subscript and the mass number (A) as a left superscript Isotopes All atoms of an element have the same atomic number, but not the same mass number Isotopes of an element are atoms that have diﬀerent numbers of neutrons and therefore diﬀerent mass numbers all isotopes of an element have nearly identical chemical behavior Atomic Masses of the Elements The mass of an atom is measured relative to the mass of an atomic standard atomic mass unit = dalton (Da) Finding Atomic Mass from Isotopic Composition Mass Spectrometry: Determines the isotopic composition of an element measures the relative masses and abundances of atomic -scale particles very precisely ex. 1. Measuring the mass rate of 28Si to 12C mass of 28Si atom / Mass of 12C standard = 2.331411 2. Find the Isotopic mass Isotopic mass of 28Si = measured mass ratio X mass of 12C = 2.331411 X 12amu = 27.97693 amu Atomic mass = the average of the masses of an element’s naturally occurring isotopes The Atomic-Mass Interval An atomic-mass Interval is used for 10 element with exceptionally large variations in Isotopic composition H, Li, B, C, N, O, Si, S, Cl, Th ex. H = atomic-mass interval of 1.00784; 1.00811 -> the mass is greater than or equal to 1.00784 and less than or equal to 1.00811 The mass of any given isotope of an element is constant, but the proportion of isotopes varies form source to source Important for very precise work 2.6 Elements: A Firt Look AT The Periodic Table 118 Diﬀerent Elements Organization of the Periodic Table Each box in Periodic table: an element atomic number atomic symbol atomic mass The boxes lie from left to right, in order of increasing atomic number Boxes are arranged into a grid of Periods (rows) and groups (columns) Period a number from 1 - 7 Inner Transition elements [the lanthanides and the actinides] ﬁt between the elements in Group 3B and Group 4B & are placed between the main body of the table Group a number from 1-8 and either letter A or B 8 A groups contain the main-group elements 10 B groups [btwn Groups 2A and 3A], contain the transition elements Classifying the Elements the “staircase” line that runs from the top of Group 3A to the bottom of Group 6A is a dividing line Metals - large lower-left portion of the table conduct heat and electricity well Nonmetals - small upper-right portion of the table generally gases or dull, brittle solids at room temperature [bromine is the only liquid] conduct heat and electricity poorly Metalloids (semimetals) - lie along the staircase line have properties of both metals and nonmetals 2 Major points: In general elements in a group have similar chemical properties and elements in a period have diﬀerent chemical properties Despite this classiﬁcation of three types of elements, in reality, there is a gradation in properties from left to right and top to bottom Group 1A except for hydrogen, consists of the alkali metals highly reactive elements Group 2A consists of the alkaline earth metals highly reactive elements Group 7A halogens highly reactive nonmetals Group 8A relatively unreactive nonmetals Groups 3A to 6A are often named for the ﬁrst element int he group ex. Group 6A is the oxygen family 2.7 Compounds: Introduction To Bonding elements typically are combined with other elements Elements combine in two general ways, both involving the electrons of the atoms of interacting elements 1. Transferring Electrons from one element to another to form Ionic Compounds 2. Sharing Electrons between atoms of diﬀerent elements to form Covalent Compounds Both combinations result in Chemical Bonds the forces that hold the atoms together in a compound The Formation of Ionic Compounds Ions: charges particles that form when an atom (or small group of atoms) gains or loses one or more electrons Binary Ionic Compound: an ionic compound composed of two elements typically forms when a metal reacts with a nonmetal Each metal atom loses one or more electrons and become a cation - a positively charged ion Each nonmetal atom gains one or more of the electrons lost by the metal atom and becomes an anion - a negatively charged ion Essentially: the metal atoms transfer electrons to the nonmetal atoms Monatomic Ion: a cation or anion derived from a single atom Polyatomic Ions: derived from a small group of atoms The Case of Sodium Chloride All binary ionic compounds are solid arrays of oppositely charges ions Even the tiniest visible grain of table salt contains an enormous number of sodium and chloride ions Coulomb’s Law Coulomb’s Law: two particles is directly proportional to the product of the charges and inversely proportional to the distance between them Ions with higher charges attract (or repel) each other more strongly than ions with lower charges Smaller ions attract (or repel) each other more strongly than larger ions, because the charges are closer to each other Predicting the Number of Electrons Lost or Gained Ionic compounds are neutral because they contain equal numbers of positive and negative charges Metal atoms lose electrons and nonmetal atoms gain electrons to form ions with the same number of electrons as in an atom of the nearest noble gas When an element located near a noble gas forms a monatomic ion: Metals lose electrons Group 1A - lose one electron Group 2A- lose two electrons Aluminum in Group 3A- lose three Nonmetals gain electrons Group 7A - gain one electron O and S in Group 6A- gain two electrons N in Group 5A - gain thee electrons The Formation of Covalent Substances Covalent substances form when atoms of elements share electrons, which usually occurs between nonmetals Covalent Bonding in Elements and Some Simple Compounds Atoms of diﬀerent elements share electrons to form the molecules of a covalent compound ex. a sample of hydrogen ﬂuoride consist of molecules in which one H atom forms a covalent bond with one F atom Distinguishing the Entities in Covalent and Ionic Substances Most covalent substances consists of molecules no molecules in an ionic compound The nature of the particles attracting each other in covalent and in ionic substances is fundamentally diﬀerent Covalent bonds = the mutual attraction between two positively charged nuclei and two negatively charged electrons that reside between them Ionic bonding = the mutual attraction between positive and negative ions Polyatomic Ions: Covalent Bonds Within Ions Polyatomic Ions: two or more atoms bonded covalently and have a net positive or negative charge In many reactions, the polyatomic ion stays together as a unit 2.8 Compounds: Formulas, Names, and Masses Binary Ionic Compounds For all ionic compounds, names and formulas give the positive ion (cation) ﬁrst and the negative ion (anion) second For all binary ionic compounds, the name of the cation is the name of the metal, and the name of the anion has the suﬃx -ide added tot he root of the name of the nonmetal ex. the anion formed from bromine is named bromide (from + ide) Compounds of Elements That Form One Ion Monastic ions of elements int he same main group have the same ionic charge alkali metals = 1+ halogens = 1 - and so forth Cations - ion charge equals A-group number ex. Na in Group 1A = Na+ Anions- ion charge equals A group number minus 8 ex. S in group 6A = S2- the A-group monatomic ions in Table 2.3 = the same number of electrons as an atom of the nearest noble gas Formula unit: the relative numbers of cations and anions in the compound ex. Ca2+ ions and Br- ion = two Br- balance each Ca2+ = CaBr2, not Ca2Br The subscripts refers tot he element symbol preceding it The subscript 1 is understood from the presence of the element symbol alone ( that is, we do not write Ca1Br2) The charge (without the sign) of one ion become the subscript of the other ex. Ca2+ Br- = Ca Br2 Reduce the subscripts to the smallest whole numbers that retain the ratio of ions Compounds of Metals That Form More Than One Ion If element forms more than one ion, they include a roman numeral within parentheses immediately after the metal ion’s name to indicate its ionic charge The Latin root of the metal is follow day either of two suﬃxes: -ous for the ion with the lower charge -ic for the ion with the higher charge Compounds That Contain Polyatomic Ions Polyatomic ion stays together as a charged unit When two or more of the same polyatomic ion are present in the formula unit, that ion appears in parentheses with the subscript written outside Families of Oxyanions Oxyanions: those in which an element, usually a nonmetal, is bonded to one or more oxygen atoms With two oxyanions in the family the ion with more O atoms take the nonmetal root and the suﬃx -ate The ion with fewer O atoms takes the nonmetal root and the suﬃx -ite With four oxyanions in the family The ion with most O atoms has the preﬁx per-, the nonmetal root and the suﬃx - ate The ion with one fewer O atoms has just the root and the suﬃx -ate The ion with two fewer O atoms has just the root and the suﬃx -ite The ion with least (three fewer) O atoms has the preﬁx hypo-, the root, and the suﬃx -ite Hydrated Ionic Compounds Hydrates: have a speciﬁc number of water molecules in each formula unit Shown after a centered dot int he formula and named with a Greek numerical preﬁx before the word hydrate ex. magnesium sulfate heptahydrate Acid Names from Anion Names Two common types of acids are binary acids and oxyacids Binary Acid solution: form when certain gaseous compounds dissolve in water Preﬁx hydro- + nonmetal root + suﬃx -ic (cation) or -ide (anion) + separate word acid Oxyacid: names are similar to those of the oxyanions, except for two suﬃx changes -ate in the anion becomes -ic in the acid -its in the anion becomes -ious in the acid The preﬁxes hypo- and per- are retained Binary Covalent Compounds Binary covalent compounds: typically formed by the combination of two non-metals The element with the lower group number int he periodic table come ﬁrst in the name. The element with the higher group number comes second and is named with its root and the suﬃx -ide If both elements are int he same group, the one with the higher period number is names ﬁrst Covalent compounds use Greek numerical preﬁxes to indicate the number of atoms of each element the ﬁrst element in the name has a preﬁx only when more than one atom of it is present; the second element usually has a preﬁx The Simplest Organic Compounds: Straight-Chain Alkanes organic compounds typically have complex structures that consist of chains, branches, and/or rings of carbon atoms that are also bonded to hydrogen atoms and, often, to atoms of oxygen, nitrogen, and a few other elements Hydrocarbons: the simplest type of organic compound contain only carbon and hydrogen Alkanes: the simplest type of hydrocarbon Straight-Chain Alkanes: The simplest alkanes to name because the carbon chains have no branches Alkanes are names with a root, based on the number of C atoms int he chain followed by the suﬃx -ane Molecular Masses from Chemical Formulas Molecular mass (molecular weight) of a formula unit of the compound as the sum of the atomic masses Molecular mass = sum of atomic masses Ionic compounds don’t consist of molecules, so the mass of a formula unit is called the Formula Mass To calculate the formula mass of a compound with polyatomic ion the number of atoms of each element inside the parentheses is multiplied by the subscript outside the parentheses Representing Molecules with Formulas and Models Molecular formula uses element symbols and numerical subscripts to give the actual number of atoms of each element in a molecule of the compound Structural Formula shows the relative placement and connections of atoms in the molecule uses symbols for the atoms and wither a pair of dots or a line to show the bonds between the atoms A space - ﬁlling model an accurately scaled-up image of the molecule shows the relative sizes of the atoms, the relative distances between the nuclei, and the angles between he bonds Bonds are not shown - can be diﬃcult to see each atomic a complex molecule 2.9 Mixtures: Classiﬁcation and Separation Two broad classes of mixtures: heterogeneous mixtures and homogeneous mixtures (or solution) Heterogeneous Mixture: one or more visible boundaries between the components composition is not uniform - varies from one region to another in some, boundaries can be seen only with a microscope Homogeneous misture (or solution): no visible boundaries because the components are individual atoms, ions, or molecules composition is uniform Solutions occur in all three physical states: gas, solid, and liquid Aqueous Solution: solutions in water Mixtures diﬀer from compounds in three major ways: 1. The proportions of the components can vary 2. The individual properties of the components are observance 3. The components can be separated by physical means