Chapter 2

Atoms, Ions, and Compounds

The Development of Earth

• Matter in the Universe condensed into planets.

• The planets closer to the sun have different chemical compositions than the rest of the universe. Many of the volatile chemicals were lost

from these planets.

Composition of Planets

The Composition of Earth

Compounds and Earth’s Early Atmosphere

• Carbon Dioxide

• Water

• Sulfur Dioxide

• Sulfur Trioxide

• Nitrogen Oxide

• Nitrogen Dioxide

The Composition of Compounds

• The law of multiple proportions states that the masses of element Y that combine with a fixed mass of elements X to form two or more different compounds are in the ratios of small whole numbers.

• Examples: NO, NO2, N2O, N2O5, etc.

Atomic Structure

• J. J. Thomson discovered the subatomic particle know as the electron.

• Thomson used cathode ray tube.

Mass of An Electron• Robert Millikan

determined the mass of an electron with his oil-droplet experiment.

Units Numeric value

Unified Atomic Mass Unit 5.4868 E- 4 u (Da)

Kilogram (SI) 9.109 E -31 kg

other 0.511 MeV/C2

Thomson Model of the Atom• J. J. Thomson’s Plum-pudding model (PPM) of the atom.

Radioactivity and the Nuclear Atom

• Henri Becquerel discovered that some samples produced invisible radiation.

• One type is a steam of beta () particles (high energy electrons).

• A second type consists of alpha () particles, which have a +2 charge and a mass equivalent to that of a helium nucleus (4He+2).

Rutherford’s Experiment

• Rutherford and two of his students bombarded a thin foil of gold with particles to test Thomson’s model of the atom.

• PPM would predict that the -particles should travel through the foil without significant deflections, but the results falsified the hypotheses in the PPM.

Rutherford’s Experiment

The Nuclear Atom

• The nucleus of an atom contains all the positive charge and nearly all the mass in an atom.

• The nucleus is about 1/10,000 the size of the atom.

• A proton is a positively charged subatomic particle present in the nucleus of an atom.

The Third Subatomic Particle

• A neutron (10n) is an electrically neutral or

uncharged subatomic particle found in the nucleus of an atom.

• Atomic Mass Units (amu) comprise a relative scale used to express the masses of atoms and subatomic particles.

• The scale is based on the definition that the mass of 1 atom of carbon with 6 protons and 6 neutrons in its nucleus is exactly 12 amu.

Particle amu grams Charge

Neutron 1.00867 1.67494E-24 0

Proton 1.00728 1.67263E-24 +1

Electron 5.48580E-4 9.10939E-28 -1

10n 1

1p + 0-1e

BigBang

The neutron to proton ratio is critical in determining the stability of a nucleus.

Elements above the belt of stability undergo -decay and elements below the belt of stability undergo positron emission or electron capture.

Sec. 2.2: Fusion processes in stars have been shown to form nuclei up to 26 protons and 30 neutrons (56

26Fe).

Supernova

SN 1054 produced the Crab Nebula

Supernova produce temperatures (energies) capable of synthesizing elements through Z=94

Decay % Natural Name Symbol Mass (amu) Mode Half-Life Abundance Oxygen-13 13O 13.02810 0e 8.9 ms ---- 8 1 Oxygen-14 14O 14.008595 0e 70.6 s --- 8 1 Oxygen-15 15O 15.003065 0e 122 s --- 8 1 Oxygen-16 16O 15.994915 --- --- 99.76 8 Oxygen-17 17O 16.999131 --- --- 0.04 8 Oxygen-18 18O 17.999160 --- --- 0.20 8 Oxygen-19 19O 19.003577 0e 26.9 s --- 8 -1 Oxygen-20 20O 20.004075 0e 13.5 s --- 8 -1 Oxygen-21 21O 21.008730 0e ≈3 s --- 8 -1

Oxygen isotopes and decay modes

Aston’s Experiment

• Francis W. Aston built a postive-ray analyzer.

• Data from neon gas samples demonstrated that two different kinds of atoms or isotopes existed.

• Isotopes are atoms of an element whose nuclei have the same number of protons but different numbers of neutrons.

Aston’s Positive-ray Analyzer

Now these instruments are called “mass spectrometers”.

Symbols of Isotopes

• Each element consists of atoms with the same number of protons in the nucleus. This number is called atomic number (Z).

• Protons and neutrons in atomic nuclei are called nucleons.

• The mass number (A) is the total number nucleons in one atom of an element.

Writing the Symbol of an Isotope

XAZ

• A is the mass number

• Z is the atomic number

• X is the atomic symbol

• n is the charge

n±

Average Atomic Masses

• A mass spectrometer is an instrument that measures precise masses and relative amounts of ions of atoms and molecule.

• The natural abundance of an isotope is its relative proportion among all the isotopes found a natural sample.

• The average atomic mass of an element is calculated by multiplying the natural abundance of each isotope by its exact mass in atomic mass units and then summing these products.

Example

Average atomic mass of neon = (19.9924 x 0.904838) + (20.99395 x 0.002696) + (21.9914 x 0.092465) = 20.1797 amu

Isotope Mass (amu) Natural abundance (%)

Neon-20 19.9924 90.4838

Neon-21 20.99395 0.2696

Neon-22 21.9914 9.2465

Neon is found in three isotopes in nature.

Exact Mass of MgO (amu) Natural Abundance (%)

39.9872 78.70

40.9886 10.13

41.9846 11.17

Chemistry of Mars The 1997 mission to Mars included a small robot, the Sojourner, which analyzed the composition of Martian rocks. Magnesium oxide from a boulder dubbed “Barnacle Bill” was analyzed and found to have the following isotopic composition:If essentially all of the oxygen in the Martian MgO sample is oxygen-16 (which has an exact mass of 15.9948)

Exact Mass of Mg (amu) Relative Abundance Weighted mass

23.9924 0.7870 18.88

24.9938 0.1013 2.532

25.9898 0.1117 2.903ave. mass of Mg = 24.32

Symbol Exact Mass (amu) Natural

Abundance (%)

36Ar 35.96755 0.337

38Ar 37.96272 0.063

40Ar ? 99.60

Average 39.948

Answer: 39.96 amu

Another Isotope Problem…Using the following table of abundances and masses of the three naturally occurring argon isotopes, calculate the exact mass of 40Ar.

Mendeleev’s Periodic Table

• Dmitrii I. Mendeleev arranged elements in the periodic table by their chemical and physical properties.

• He left open spaces in his periodic table to account for elements not yet discovered.

The Modern Periodic Table

• The modern periodic table is also based on a classification of elements in terms of their physical and chemical properties.

• The horizontal rows are called periods.• Columns contain elements of the same family

or group.• Transition metals are the elements in group 3

through 12 in the periodic table.

Groups of Elements

• Group 1 contains the alkali metals.

• Group 2 contains the alkaline earth metals.

• Group 17 contains the halogens.

Broad Categories of Elements

• Metals are elements on the left-hand side of the table.

Metals are shiny solids that conduct heat and electricity well and are malleable and ductile.

• Nonmetals have properties opposite to those of the metals and are on the right side of table

• Metalloids are the elements between the metals and nonmetals.

Continued

• Main group elements or representative elements are the elements in groups 1,2 and 13 through 18.

• The noble gases are the elements in Group 18.

Kinds of Compounds

• Molecular Compounds are composed of atoms held together by covalent bonds.

• Covalent bonds are shared pairs of electrons that chemically bond atoms together.

• Ionic Compounds are composed of positively and negatively charged ions that are held together by electrostatic attraction.

• Ions with negative charge are called anions.• Ions with positive charge are called cations.

Most common charges on ions. Note elements within a Group (column) typically have the same charge. Are these charges consistent with the most stable electron configurations?

Continued

• Molecular compounds are made of nonmetals

• Ionic compounds are made of a metal and a nonmetal.

• Metal form cations and nonmetals form anions.

Naming Compounds

• Binary Molecular Compounds Compounds consisting of two nonmetals

• First element in the formula is named first.• Second element is named by changing the elemental

name ending to ide.• Use prefixes to identify quantity of atoms.

Mono-1, di-2, tri-3, tetra-4, penta-5, hexa-6, hepta-7, octa-8, nona-9, deca-10

• Never use the prefix mono-

P2O5 = diphosphorus pentoxide

Compounds and Earth’s Early Atmosphere

• Carbon Dioxide

• Water (dihydrogen monoxide)

• Sulfur Dioxide

• Sulfur Trioxide

• Nitrogen Oxide

• Nitrogen Dioxide

Naming…Binary molecular compounds

CO

CO2

NO

NO3

SO2

SO3

Practice

Name the following compounds or give

the correct chemical formula.1. Tetraphosphorus decoxide

2. CCl43. P2N5

4. Sulfur trioxide

Binary Ionic Compounds• Binary ionic compounds consist of

cations (usually metals) and anions (usually nonmetals).

Binary Ionic Compounds

• The cation is named first using the elemental name. If the metal can form cations with different

charges then a Roman numeral is added to indicate the charge of the cation.

• The anion is named with the ide ending.

• The formulas for ionic compounds must always be neutral.

Practice

Write the name or chemical formula for the following compounds.

1. NaCl

2. CrCl33. Zinc nitride

4. Copper(I) oxide

Polyatomic Ions

• Acetate C2H3O2-

• Carbonate CO32-

• Perchlorate ClO4-

• Nitrate NO3-

• Sulfate SO42-

• ChromateCrO42-

• Examples from Table 2.3 on page 64

Practice

Write the names or chemical formulas for the following compounds.

1. Cr(ClO4)3

2. NH4NO3

3. Lithium bicarbonate

4. Calcium hypobromite

Nomenclature

Give the chemical names for the following ionic compounds:

a. NiCO3

b. NaCN

c. LiHCO3

d. Ca(ClO)2

Give the formula and charge of the oxoanion of each of the following

a. Potassium tellurite b. sodium arsenate

c. Calcium selenite d. potassium chlorate

Naming Binary Acids

• Binary acids contain hydrogen and a halogen atom.

• The names of these acids are contain the halogen base name with the prefix, “hydro,” and suffix, “ic,” and the word acid.

• Example HBr - hydrobromic acid

Oxy Anions & Related Acids

• ClO- Hypochlorite HClO Hypochlorous acid

• ClO2- Chlorite HClO2 Chlorous acid

• ClO3- Chlorate HClO3 Chloric acid

• ClO4- Perchlorate HClO4 Perchloric acid

Anion Anion Name Acid Acid Name

Nucleosynthesis• After the creation of the universe, energy had

transformed into matter in accordance with Einstein’s equation E = mc2.

• The first types of matter to form were the smallest fundamental particles: electrons, and quarks.

• Quarks are particles that combine to form neutrons and protons.

• Nucleosynthesis is the fusing of fundamental and subatomic particles to create atomic nuclei.

Fusion of Hydrogen and the Mass Defect

protons

Positrons

42He

4 11H 4

2He + 01e

• 2 neutrons + 2 protons:

2 x 1.67494E-24 + 2 x 1.67263E-24 = 6.69513E-24 g

• Subtract the actual mass of the nucleus:

6.69513E-24 – 6.64465E-24 = 5.048E-24 g

• This is the mass defect.

• E = (m)c2

• (5.048E-29 kg) x (2.998E8 m/s)2 = 4.537E-12 kg (m/s)2

• or 4.537E-12 J (Joules) = Binding energy for helium-4

For the reaction: H2(g) + ½ O2(g) → H2O(l)

the energy released is 4.7E-19 J per H2 molecule.

Binding energy of the 42He nucleus

The mass of a stable nucleus is always less than the mass of the individual particles that make up the nucleus.This is known as the mass defect (m). The larger the mass defect the stronger the energy that binds the nuclear particles together.

The binding energy (E) can be calculated by substituting the mass defect into Einstein's equation for the relationship between mass and energy: E = mc2 to become E = (m)c2

Formation of Nuclides

11H + 0

1n 12H

11H is a proton and 0

1n is a neutron

2 12H 2

4He

Binding Energy, Problem 12.

What is the Binding Energy of 50Ti? The mass of 50Ti is 49.9448 amu.

Answer: 7.0173E-11 J

Binding Energy, Problem 12.

What is the Binding Energy of 50Ti? The mass of 50Ti is 49.9448 amu.

Giant Stars with core temperatures ~1E6 K can build elements by fusion of -particles

This fusion process can “build” to an element with 26 protons and 30 neutrons (56Fe)

Beyond this point the fusion process consumes more energy than is gained by binding.

56Fe + 4He + E → 60Ni

Whereas

52Cr + 4He → 56Fe + E

Sec. 2.2: Fusion processes in stars have been shown to form nuclei up to 26 protons and 30 neutrons (56

26Fe).

Heavier elements are made only in the hottest stars where free neutrons exist and can collide with a nucleus and increase its mass – neutron capture. This is followed by -decay.

In this example cadmium-110 absorbs 5 neutrons then undergoes -decay.

-Decay Process in carbon-14 nuclides (t½ = 5730 yr).

The “free” electron emitted is called a -particle

Beta-decay produces a nucleus with one more proton…

Paths of Radioactive Decay

View animated explanations of alpha decay, beta decay, positron emission, and electron capture.

»PC version

Radioactive Decay Modes Tutorial

The neutron to proton ratio is critical in determining the stability of a nucleus.

Elements above the belt of stability undergo -decay and elements below the belt of stability undergo positron emission or electron capture.

Decay % Natural Name Symbol Mass (amu) Mode Half-Life Abundance Oxygen-13 13O 13.02810 0e 8.9 ms ---- 8 1 Oxygen-14 14O 14.008595 0e 70.6 s --- 8 1 Oxygen-15 15O 15.003065 0e 122 s --- 8 1 Oxygen-16 16O 15.994915 --- --- 99.76 8 Oxygen-17 17O 16.999131 --- --- 0.04 8 Oxygen-18 18O 17.999160 --- --- 0.20 8 Oxygen-19 19O 19.003577 0e 26.9 s --- 8 -1 Oxygen-20 20O 20.004075 0e 13.5 s --- 8 -1 Oxygen-21 21O 21.008730 0e ≈3 s --- 8 -1

Oxygen isotopes and decay modes

•32P

•10C

•50Ti

•19Ne

Refer to the belt of stability and predict the modes of radioactive decay for these isotopes

Problem 24

Write a balanced nuclear equation for:

. emission by 28Mg

. emission by 255Lr

c. Electron capture by 129Cs

d. Positron emission by 25Al

A Ton of TNT

For some reason, the comparison unit for nuclear explosions which became most popular was the "ton of TNT". A nominal energy release for a ton of TNT can be extracted from general statements about nuclear weapons. One of those is "one kilogram of mass converted to energy is equivalent to about 22 megatons of TNT". From the Einstein equation, the conversion is

mc2 = (1kg)c2 = 9E16 Joules = 22 megatons TNT

mc2 = (0.001kg)c2 = 9E13 J = 22 kilotons

This is consistent with the oft-quoted statement that the 20 kiloton Hiroshima bomb converted about 1 gram of mass to energy.

ChemTour: Rutherford Experiment

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This recreates Rutherford’s gold foil experiment, which led to the discovery of the atomic nucleus.

Terms

• Molecular formulas describe the exact number and type of atoms present in one molecule of a compound.

• An empirical formula gives the simplest whole-number ratio of elements in a compound.

• The formula unit of an ionic compound is the smallest electrically neutral unit within the crystal of the compound.

ChemTour: Millikan Oil-Drop Experiment

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This ChemTour recreates the experimental procedure used by Millikan to determine the change of an electron.

ChemTour: Cathode-Ray Tube

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This ChemTour explores the effects of magnetic and electric fields and cathode rays.

ChemTour: Synthesis of Elements

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This ChemTour animates the neutron capture process and explains how elements are synthesized in stars.

ChemTour: NaCL Formation

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