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Chapter 2Atoms, Molecules and Ions

Preview:

• Fundamental Chemical Laws and Atom.

• Modern View of Atomic Structure, Molecules, and Ions.

• Periodic Table.

• Naming Simple compounds, Ionic compounds, Formula from names.

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The Early History of ChemistryThe Early History of Chemistry

Before 16th Century– Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold

17th Century– Robert Boyle: First “chemist” to perform quantitative experiments

18th Century– George Stahl: Phlogiston flows out of a burning material.– Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.”

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Law of Conservation of MassLaw of Conservation of Mass

Discovered by Antoine Lavoisier

Mass is neither created nor destroyed

Combustion involves oxygen, not phlogiston

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Other Fundamental Chemical LawsOther Fundamental Chemical Laws

A given compound always contains exactly the same proportion of elements by mass.

Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine: CCl4

Law of Definite Proportion

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Other Fundamental Chemical LawsOther Fundamental Chemical Laws

When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.

The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).

Law of Multiple Proportions

62.1

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Mass of Nitrogen That Combines

With 1 g Oxygen

Compound A 1.750 g

Compound B 0.8750 g

Compound C 0.4375 g

A/B = 1.750/0.8750 = 2/1B/C = 0.875/0.4375 = 2/1A/C = 1.750/0.4375 = 4/1

i.e. amount of nitrogen in A is twice that in B, etc.

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Dalton’s Atomic Theory (1808)Dalton’s Atomic Theory (1808)

Each element is made up of tiny particles called atoms.

The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

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Dalton’s Atomic Theory(continued)

Dalton’s Atomic Theory(continued)

Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.

Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

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Dalton's theory lead to:

• 1gm hydrogen + 8gm of oxygen water• he assumed that water formula is "OH" and the

mass of hydrogen is "1" and of oxygen is "8".• Using the same concepts, Dalton's proposed the

first table of atomic masses. It has been proved later that Dalton's table contain incorrect.

• Gay-lussac (1778-1850) found experimentally that:

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Figure 2.4: A representation of some of Gay-Lussac's experimental results on combining gas volumes.

Interpreted in 1811 by Avogadro

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Avogadro’s Hypothesis (1811)Avogadro’s Hypothesis (1811)

• 5 liters of oxygen

• 5 liters of nitrogen

• Same number of particles!

At the same temperature and pressure, equal At the same temperature and pressure, equal volumes of different volumes of different gasesgases contain the same contain the same number of particles.number of particles.

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Figure 2.5: A representation of combining gases at the molecular level. The spheres represent

atoms in the molecules.

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Early Experiments to Characterize the AtomEarly Experiments to

Characterize the Atom

J. J. Thomson - postulated the existence of electrons using cathode ray tubes.

• Measured charge /mass of e- • Received 1906 Nobel Prize in Physics

• e= -1.76 x 108 C/g

• m

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Figure 2.7: A cathode-ray tube. The fast-moving electrons excite the gas in the tube, causing a

glow between the electrodes.

182.2

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(Uranium compound)

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Figure 2.9: Thomson plum pudding model of the atom.

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Ernest Rutherford - explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.

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Figure 2.12: Rutherford's experiment on -particle bombardment of metal foil.

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Figure 2.13: (a) The expected results of the metal foil experiment if Thomson's model were

correct. (b) Actual results.

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Millikan oil drop experiment

• Millikan did another experiment to determine the mass of the –ve particles (electrons). The experiment used mainly to determine the magnitude of the electron charge and using e/m to get m- value.

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The Modern View of Atomic Structure

The Modern View of Atomic Structure

electrons

protons: found in the nucleus, they have a positive charge equal in magnitude to the electron’s negative charge.

neutrons: found in the nucleus, virtually same mass as a proton but no charge.

The The atomatom contains: contains:

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Figure 2.14: A nuclear atom viewed in cross section. Note that this drawing is not to scale.

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Figure 2.15: Two isotopes of sodium. Both have eleven protons and eleven electrons, but they

differ in the number of neutrons in their nuclei.

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The Mass and Charge of the Electron, Proton, and NeutronThe Mass and Charge of the

Electron, Proton, and Neutron

Particle Mass (kg) Charge

Electron 9.11 10 31 1

Proton 1.67 10 27 1+

Neutron 1.67 10 27 0

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Summary

• J.J. Thompson (1897) “cathode rays are electrons” (e–) and finds e/m ratio

• Robert Millikan (1909) measures e and hence melectron known at 9.1110-31 kg

• E. Rutherford (1906) bounces (He2+) off Au tissue proving protons (p+) in nucleus

• F.A. Aston (1919) “weighs” atomic ions• J. Chadwick (1939) observes neutrons (no

charge) by decomposition (to p+, e–, and ).

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The Chemists’ Shorthand: Atomic Symbols

The Chemists’ Shorthand: Atomic Symbols

K Element Symbol39

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Mass number

Atomic number

31

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Q6.In Rutherford’s experiment, most of the alpha particles directed at the thin metal foil,A) passed directly through the foil without being deflected.B) were reflected directly back from the foil.C) were deflected at a 90º angle.D) were absorbed by the foil.E) converted into high-energy gamma particles.

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Chemical BondsChemical Bonds

•The forces that hold atoms together in compounds. Covalent bonds result from atoms sharing electrons.

•Molecule: a collection of covalently-bonded atoms.

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The Chemists’ Shorthand:Formulas

The Chemists’ Shorthand:Formulas

• Chemical Formula:

• Symbols = types of atoms

• Subscripts = relative numbers of atoms

CO2

• Structural Formula:

• Individual bonds are shown by lines.

O=C=O

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Figure 2.16: The structural formula for methane.

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Figure 2.17: Space-filling model of methane. This type of model shows both the relative sizes of the atoms in

the molecule and their spatial relationships.

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Figure 2.18: Ball-and-stick model of methane.

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IonsIons

Cation: A positive ion

Mg2+, NH4+

Anion: A negative ion

Cl, SO42

Ionic Bonding: Force of attraction between oppositely charged ions.

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Periodic TablePeriodic TableElements classified by:

properties

atomic number

Groups (vertical)

1A = alkali metals

2A = alkaline earth metals

7A = halogens

8A = noble gases

Periods (horizontal)

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Earth’s Most Abundant Elements

Oxygen 0 46%

Silicon Si 27%

Aluminum Al 8%

Iron Fe 5%

Calcium Ca 3%

Sodium Na 2.8%

Potassium K 2.5%

Magnesium Mg 2.0%

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Figure 2.21: The Periodic Table.

http://center.acs.org/periodic/tools/pt.html

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http://www.elements-of-life.org/eol_index_flash.html

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Naming CompoundsNaming Compounds

1. Cation first, then anion

2. Monatomic cation = name of the element

Ca2+ = calcium ion

3. Monatomic anion = root + -ide

Cl = chloride

CaCl2 = calcium chlorideHI = hydrogen iodide

Binary Ionic Compounds:Binary Ionic Compounds:

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Figure 2.19: Sodium metal reacts with chlorine gas to form solid sodium chloride.

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Naming Compounds(continued)

Naming Compounds(continued)

metal forms more than one cation

use Roman numeral in name

PbCl2

Pb2+ is cation

PbCl2 = lead (II) chloride

Binary Ionic Compounds (Type II):Binary Ionic Compounds (Type II):

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FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride

FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride

Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide

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Figure 2.22: The common cations and anions

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Naming Compounds(continued)

Naming Compounds(continued)

Compounds between two nonmetals

First element in the formula is named first.

Second element is named as if it were an anion.

Use prefixes

Never use mono-

P2O5 = diphosphorus pentoxide

Binary compounds (Type III):Binary compounds (Type III):

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NF3 nitrogen trifluoride

SO2 sulfur dioxide

N2Cl4 dinitrogen tetrachloride

NO2 nitrogen dioxide

N2O dinitrogen monoxide

Molecular Compounds

2.7

TOXIC!

Laughing Gas

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Figure 2.23: A flowchart for naming binary compounds.

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Figure 2.24: Overall strategy for naming chemical compounds.

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Figure 2.25: A flowchart for naming acids. An acid is best considered as one or more H+ ions

attached to an anion.

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Naming Exercise

• Al2(S2O3)3

• P4O10

• Cu(NO2)2

• NaMnO4

• CS2

• Fe2(CrO4)3

• HCl (gas)

• PH4BrO2

• Aluminum thiosulfate

• Tetraphosphorous decaoxide

• Copper(II) nitrite

• Sodium permanganate

• Carbon disulfide

• Iron(III) chromate

• Hydrogen chloride

• Phosphonium bromite

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QUESTIONAccording to the law of multiple proportions: 1) if the same two elements form two different

compounds, they do so in the same ratio. 2) it is not possible for the same two elements

to form more than one compound. 3) the ratio of the masses of the elements in a

compound is always the same. 4) the total mass after a chemical change is

the same as before the change. 5) none of these

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ANSWER5) none of these Section 2.2 Fundamental Chemical Laws (p. 45) The Law of Proportions states that when two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are in ratios of small whole numbers.

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QUESTIONWhich of the following pairs of compounds can be used to illustrate the law of multiple proportions? 1) NH4 and NH4Cl 2) ZnO2 and ZnCl2 3) H2O and HCl 4) NO and NO2 5) CH4 and CO2

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ANSWER4) NO and NO2 Section 2.2 Fundamental Chemical Laws (p. 45) These are two compounds containing the same two elements, but in different ratios: 1:1 and 1:2.

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QUESTIONWhich of the following statements from Dalton’s atomic theory is no longer true, according to modern atomic theory? 1) Elements are made up of tiny particles called

atoms. 2) Atoms are not created or destroyed in chemical

reactions. 3) All atoms of a given element are identical. 4) Atoms are indivisible in chemical reactions. 5) All of these statements are true according to

modern atomic theory.

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ANSWER3) All atoms of a given element are identical. Section 2.3 Dalton’s Atomic Theory (p. 46) Since Dalton’s time we have learned that two atoms of the same element can have different numbers of neutrons. These are called isotopes.

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QUESTIONThe first people to attempt to explain why chemical changes occur were 1) alchemists. 2) metallurgists. 3) physicians. 4) physicists. 5) the Greeks.

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ANSWER5) the Greeks. Section 2.1 The Early History of Chemistry (p. 42) The origins of history, medicine, philosophy and mathematics can be traced back to the Greeks as well.

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QUESTIONThe first chemist to perform truly quantitative experiments was 1) Paracelsus. 2) Boyle. 3) Priestly. 4) Bauer. 5) Lavoisier.

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ANSWER2) Boyle. Section 2.1 The Early History of Chemistry (p. 42) These experiments included his work with the relationship between the pressure and volume of a gas.

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QUESTIONWhat element (in trace amounts) has been shown to help in protecting against heart disease and cancer? 1) Silicon 2) Oxygen 3) Selenium 4) Copper 5) Potassium

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ANSWER3) Selenium Section 2.3 Dalton’s Atomic Theory (p. 48) Selenium has been shown to be an important trace mineral for human beings. It appears to be an integral part to the structure of several enzymes.

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QUESTIONThe first scientist to show that atoms emit any negative particles was 1) J. J. Thomson. 2) Lord Kelvin. 3) Ernest Rutherford. 4) William Thomson. 5) John Dalton.

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ANSWER1) J. J. Thomson. Section 2.4 Early Experiments to Characterize the Atom (p. 49) The particles were studied by J.J. Thomson during his cathode ray tube experiments in the last part of the nineteenth century.

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QUESTIONThe scientist whose alpha-particle scattering experiment led him to conclude that the nucleus of an atom contains a dense center of positive charge is 1) J. J. Thomson. 2) Lord Kelvin. 3) Ernest Rutherford. 4) William Thomson. 5) John Dalton.

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ANSWER3) Ernest Rutherford. Section 2.4 Early Experiments to Characterize the Atom (p. 54) As the atomic number increases the number of neutrons of stable atoms increases at a higher rate.

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QUESTIONWhich of the following atomic symbols is incorrect?

1) 14

6C 4)

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19K

2) 37

17Cl 5)

14

8N

3) 32

15P

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ANSWER

5) 14

8N

Section 2.5 The Modern View of Atomic Structure: An Introduction (p. 54) The subscript is the atomic number. Nitrogen’s atomic number is always 7 no matter the isotope.

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QUESTION40

20Ca

2+ has

1) 20 protons, 20 neutrons, and 18 electrons. 2) 22 protons, 20 neutrons, and 20 electrons. 3) 20 protons, 22 neutrons, and 18 electrons. 4) 22 protons, 18 neutrons, and 18 electrons. 5) 20 protons, 20 neutrons, and 22 electrons.

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ANSWER1) 20 protons, 20 neutrons, and 18 electrons.

Section 2.5 The Modern View of Atomic Structure: An Introduction (p. 54)

The subscript is the number of protons. The superscript is the number of protons plus neutrons. Therefore the number of neutrons is 40 – 20. A neutral calcium atom has 20 electrons, but this is a +2 calcium ion, so it has 18.

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QUESTIONAll of the following are true except: 1) Ions are formed by adding electrons to a

neutral atom. 2) Ions are formed by changing the number of

protons in an atom’s nucleus. 3) Ions are formed by removing electrons from

a neutral atom. 4) An ion has a positive or negative charge. 5) Metals tend to form positive ions.

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ANSWER2) Ions are formed by changing the number of

protons in an atom’s nucleus. Section 2.6 Molecules and Ions (p. 56) The removal of a proton from the nucleus requires extraordinary amounts of energy. It will only happen during a nuclear reaction.

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QUESTIONWhich among the following represent a set of isotopes? Atomic nuclei containing:

a. 20 protons and 20 neutrons. b. 21 protons and 19 neutrons. c. 22 neutrons and 18 protons. d. 20 protons and 22 neutrons. e. 21 protons and 20 neutrons.

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QUESTION (continued)

1) a, b, c 2) c, d 3) a, e 4) a, d and b, e 5) No isotopes are indicated.

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ANSWER4) a, d and b, e Section 2.5 The Modern View of Atomic Structure: An Introduction (p. 54) Isotopes have the same number of protons and different numbers of neutrons, so they can be organized by atomic number.

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QUESTIONBy knowing the number of protons a neutral atom has, you should be able to determine 1) the number of neutrons in the neutral atom. 2) the number of electrons in the neutral atom. 3) the name of the atom. 4) two of these. 5) none of these

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ANSWER4) two of these. Section 2.5 The Modern View of Atomic Structure: An Introduction (p. 54) The number of electrons in a neutral atom is the same as the number of protons. The number of protons (atomic number) allows you to name the element by consulting the periodic table.

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QUESTIONThe average mass of a carbon atom is 12.011. Assuming you were able to pick up only one carbon unit, the chances that you would randomly get one with a mass of 12.011 is 1) 0%. 2) 0.011%. 3) about 12%. 4) 12.011%. 5) greater than 50%.

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ANSWER1) 0%. Section 3.1 Atomic Mass (p. 83) The 12.011 is an average of the isotopic masses. None of the isotopes has a mass of 12.011.

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QUESTIONHow many oxygen atoms are there in one formula unit of Ca3(PO4)2? 1) 2 2) 4 3) 6 4) 8 5) None of these

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ANSWER4) 8 Section 2.8 Naming Simple Compounds (p. 67) Remember to multiply the number of oxygen atoms in the parentheses by the subscript outside of the parentheses.

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QUESTIONWhich of the following are incorrectly paired? 1) Phosphorus, Pr 2) Palladium, Pd 3) Platinum, Pt 4) Lead, Pb 5) Potassium, K

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ANSWER1) Phosphorus, Pr Section 2.7 An Introduction to the Periodic Table (p. 60) Phosphorus has a symbol of P.

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QUESTIONAll of the following are characteristics of metals except: 1) good conductors of heat. 2) malleable. 3) ductile. 4) often lustrous. 5) tend to gain electrons in chemical reactions.

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ANSWER5) tend to gain electrons in chemical reactions. Section 2.7 An Introduction to the Periodic Table (p. 61) Metals have a low electron affinity and tend to lose electrons to nonmetals during the formation of ionic compounds.

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QUESTIONAll of the following are characteristics of nonmetals except: 1) poor conductors of electricity. 2) often bond to each other by forming

covalent bonds. 3) tend to form negative ions in chemical

reactions with metals. 4) appear in the upper left-hand corner of the

periodic table. 5) do not have a shiny (lustrous) appearance.

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ANSWER4) appear in the upper left-hand corner of the

periodic table. Section 2.7 An Introduction to the Periodic Table (p. 61) The nonmetals are found in the upper RIGHT-HAND corner of the periodic table.

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QUESTIONThe correct name for LiCl is: 1) lithium monochloride. 2) lithium (I) chloride. 3) monolithium chloride. 4) lithium chloride. 5) monolithium monochloride.

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ANSWER4) lithium chloride. Section 2.8 Naming Simple Compounds (p. 62) Lithium is a Group IA metal, so it always forms a +1 ion. Therefore, no roman numeral is necessary.

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QUESTIONThe correct name for FeO is: 1) iron oxide. 2) iron (II) oxide. 3) iron (III) oxide. 4) iron monoxide. 5) iron (I) oxide.

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ANSWER2) iron (II) oxide. Section 2.8 Naming Simple Compounds (p. 63) Iron is a transition metal that forms more than one type of ion. A roman numeral is needed to indicate which ion is present in the compound.