Chemistry Chapter 2

Atoms, Molecules and Ions

Dalton’s Atomic Theory

Lavosier laid down many of the foundations of modern chemistry.

John Dalton gave us the basic theory that all matter is composed of small particles called atoms.

Dalton’s Atomic Theory

The Postulates of Dalton’s Theory:– 1. All matter is composed of indivisible

atoms.*– 2. An element is a type of matter

composed of only 1 type of atom. Each atom of a given element has a characteristic mass.

• *recall we know that atoms can be broken down into p+, n0, and e-.

Dalton’s Atomic Theory

The Postulates of Dalton’s Theory:– 3. A compound is a type of matter composed of

atoms of 2 or more elements chemically combined in fixed proportions. The relative numbers of any 2 kinds of atoms occur in simple ratios.

– 4. A chemical reaction is a rearrangement of atoms in reacting substances yielding new products formed as a result of the reaction.

Dalton’s Atomic Theory

Atomic theory explains the difference between an element and a compound and provides some insight into 2 laws considered in Chapter 1:– The Law of Conservation of Mass– The Law of Definite Proportions (the

proportions of elements in a compound are always the same).

Law of Conservation of Mass

By Postulates 2 and 4…– By postulate 2, every atom has mass, and

by postulate 4 chemical reactions only rearrange atoms, therefore mass must be conserved.

Law of Definite Proportions

By Postulate 3, a compound is a type of matter containing atoms of 2 or more elements in definite proportions.

Theories

Good theories explain observations (facts & laws) and make predictions about new laws.

Thus Dalton observed the law of multiple proportions while thinking about his atomic theory.

This was an important step in convincing skeptics about the validity of his atomic theory.

The Law of Multiple Proportions

When two elements, A and B combine to form more than one compound, the masses of A and B will combine in simple ratios.

Consider CO2 and CO: – In one, 12g of C will combine with 16g of O

to give CO. – In the other, 12g of C will combine with 32g

of O to give CO2. • The oxygen masses combining with a fixed

mass of carbon are in the ratio of 16:32, or 1:2.

How Atomic Theory Fits In…

By applying atomic theory and using the calculated ratios, we can assume the compound having 16g of oxygen per 12g of carbon is CO, and the one having 32g of oxygen per 12g of carbon is CO2.

The Nucleus of an Atom

Contains most of the mass (p+ and n0) and is surrounded by the e-.

J.J. Thompson J.J. Thompson discovered the electron

in 1897 using a cathode ray tube (CRT).

A CRT is a sealed vacuum tube with an anode at one end and a cathode at the other. A high voltage source is hooked up to each end and the glass tube emits a green light.

The CRT Continued… Correct setup of the tube allows some of the rays

to pass through a hole at one end forming a beam. Beyond the hole are positively and negatively

charged plates. The cathode rays bend away from the -ve plate and toward the +ve plate and can be influenced by a magnet revealing their charge.

Earnst Rutherford

In 1911, Rutherford proposed a nuclear model of the atom.

He used data from his gold foil experiment to demonstrate this and to hypothesize that the atom was comprised mostly of empty space (i.e., the nucleus was very small).

The Gold Foil Experiment What Rutherford and others did was fire

-particles at metal foils and used a detector to show that about 7999/8000 passed directly through the foil.

The 1/8000 that didn’t get through were deflected at large angles.

The Gold Foil Experiment

The Atom

The atomic number of an atom is the number of p+ in the atom, and is the nuclear charge on that atom.

The mass number of an atom is the number of p+ + the number of n0 in the atom.

Isotopes

Isotopes are atoms of the same element with a different number of n0. They behave the same as any other atom in a chemical reaction and are used as tracers.

These give us the decimals associated with the atomic masses.

Isotopes

When we work with elements, we are working with a mixture of isotopes.

To determine the masses of each of the elements, a scale was devised based on the carbon-12 isotope. Carbon-12 was assigned a mass of exactly 12 amu.

A mass spectrometer is used to compare the mass of an atom to that of a C-12 atom.

Atomic Mass Units

One amu is a mass unit equal to 1/12 of the mass of a carbon-12 atom.

With our new scale, the atomic masses are represented on the periodic table as an average atomic mass for naturally occurring elements.

Determining Masses of Elements

We look at the fractional abundance.

Dimitri Mendeleev & Julius Lothar Meyer These 2 men independently discovered

that elements have similar properties and can be arranged into a table.

The only error these two made was that they arranged the elements by atomic masses.

http://upload.wikimedia.org/wikipedia/commons/1/1a/Lmeyer.jpg

Henry Mosely

Eventually, Henry Mosely discovered that the ordering of the atoms by mass created some problems. He found that when he rearranged them by atomic number he was able to eliminate these problems.

http://en.wikipedia.org/wiki/File:Henry_Moseley.jpg

Periods and Groups Periods are horizontal rows. Groups, columns, and families are what

we call the vertical columns.

Numbering the Columns

In North America, we number the columns using Roman Numerals & A’s and B’s.

In Europe, Roman Numerals are often used but some of the A’s and B’s get switched.

The IUPAC recommends numbering them 1-18 to eliminate the confusion.

Our Book

Our book uses the traditional North American convention.– A groups are the “main group,” or “representative”

elements.– B group elements are “transition elements.”

The 2 rows at the bottom are the Lanthanides and the Actinides. Collectively these are referred to as the Inner Transition elements.

Metals, Nonmetals, and Metalloids

Metals are good conductors and have shine and luster.

Nonmetals are gases or brittle solids. Metalloids are substances with

characteristics of both metals and nonmetals.

Chemical Formulas Chemical formulas use atomic symbols

to represent the elements in a compound.– NaCl-sodium chloride, no subscripts.– Al2O3-2 aluminums and 3 oxygens

Molecular formulas give the numbers of different atoms in a given element in a compound-H2O.

Structural formulas show how atoms are bonded together to form a molecule.

Some Vocabulary

Monomers vs. polymers Ions, anions, cations Ionic vs. covalent compounds

Ionic Substances

Ionic substances don’t contain molecules, thus we have to speak of the smallest unit of a substance-the formula unit; NaCl for example.

We also use a formula unit to describe the formulas of other compounds-ionic or not-e.g., H2O.

Electrically Neutral

All substances are electrically neutral, and we can use this fact to obtain the ionic charge on the chemical formula from the ionic charges of the ions.

Al2O3 --> Al3+ O2-

2(3+) 3(2-)

6+ 6- = 0

Organic Compounds

Organic compounds are ones which contain carbon, oxygen, and hydrogen.

Historically, organic compounds were though to only arise via a “vital force.”

Frederick Wöhler

In 1828, Wöhler disproved the notion of “vital force” when he synthesized urea from ammonia and cyanic acid.

The vitalists did not buy this claim after they discovered that the cyanic acid was produced from animal blood.

Mechanism Vs. Vitalism

The debate about such a “vital force” raged on for a few more years until it was discovered that organic compounds could be synthesized from substances strictly made in the laboratory.

Herman Kolbe synthesized acetic acid from inorganic substances obtained from pure elements.

Inorganic Compounds

Inorganic compounds are composed of elements other than carbon.

Some substances such as CO2, CO, CN, and CaCO3 are considered to be inorganic because they don’t contain hydrogen.

Naming Compounds

There is a process for naming compounds.

To name an ionic compound, give the name of the cation first, then give the name of the anion.– NaCl

• Na+ = Sodium• Cl- = Chlorine

– Name = Sodium Chloride

Predicting Charges on Monoatomic Ions 1. Most main group metallics have one

monoatomic cation with a charge equal to the group number:– Al, Group IIIA (13), Al3+

Predicting Charges on Monoatomic Ions 2. Some metallic ions in elements with a high

atomic number “break” the previous rule--they have more than one cation. They have a common cation with a charge equal to the group number minus 2, (In addition, they have a cation with a charge equal to the group number):– Pb, Group IVA (14), Pb2+, Pb4+

Predicting Charges on Monoatomic Ions 3. Most transition elements form more

than one monoatomic cation, and most have one ion with a charge of +2.– Fe2+, Fe3+

Predicting Charges on Monoatomic Ions 4. The charge on a monoatomic anion

for a nonmetallic main group element is the group number minus 8:– O, Group VIA, O2-

Rules for Naming Monoatomic Ions 1. Monoatomic cations are named after

the element if there is only one such ion. – Al3+ = aluminum ion– Na+ = sodium ion

Rules for Naming Monoatomic Ions 2. If there is more than one

monoatomic cation, the “stock system” is used which names the cation after the element followed by a Roman Numeral to indicate the charge.– Fe2+ = iron(II)– Fe3+ = iron(III)

Rules for Naming Monoatomic Ions In an older system* of nomenclature, -ous

and -ic are added to the stem name of the element.– -ous indicates the atom with the lower charge.

e.g., Cu+ = cuprous– -ic indicates the atom with a higher charge.

e.g., Cu2+ = cupric• *Often with this system, the Latin name of the element is

involved.

Rules for Naming Monoatomic Ions Monoatomic anions are named using

the stem name of the element followed by -ide.– Br- = bromide– Cl- = chloride

Rules for Naming Polyatomic Ions

Polyatomic ions are ions with more than one element chemically bonded and contains a charge.– NH4

+ = ammonium ion

There are no simple rules for writing polyatomics. Often there is a central element associated with oxygen called an oxoanion.– ClO2

-, for example

Usually the oxoanions have the name of the characteristic element with the suffix -ate or -ite.– The oxoanion with the greater number of

oxygens = -ate.– The one with the lesser number of oxygens

is -ite.• NO3

- = nitrate ion• NO2

- = nitrite ion

Rules for Naming Polyatomic Ions

When there are more than 2 oxoanions, the prefixes hypo- and per- are used.

A classic example:

Rules for Naming Polyatomic Ions

Hypo- ClO- hypochlorite

ClO2- -ite chlorite

ClO3- -ate chlorate

Per- ClO4- perchlorate

Binary Compounds

Binary compounds are composed of only 2 elements.

When composed of a metal and a nonmetal, these compounds are usually ionic and are named as such.– NaCl, KCl, MgCl2

Binary Compounds

Those binary compounds comprised of 2 nonmetals or a nonmetal and a metalloid are named using prefixes.– H2O, CO2, CO

Rules for Naming Binary Molecular Compounds 1. The compound name usually has the

elements in the order given in the formula.

2. Name the 1st element using the exact element name.

Rules for Naming Binary Molecular Compounds 3. Write the second element’s prefix

with the suffix -ide. 4. If there is more than one element,

add the Greek prefix to the stem of each element. In general, mono- isn’t used on the 1st element in the compound.

Rules for Naming Binary Molecular Compounds

Mono- 1 Di- 2 Tri- 3 Tetra- 4 Penta-5 Hexa- 6 Hepta-7 Octa- 8 Nona- 9 Deca- 10

H2O = dihydrogen monoxide

CO = carbon monoxide

CO2 = carbon dioxide

PCl5 = phosphorous pentachloride

Acids

In general (for now) an acid is a compound that donates a H+ in solution along with an anion.– HNO3 ↔ H+ + NO3

-

– HCl ↔ H+ + Cl-

HNO3 is an oxyacid--an acid containing H, O, and a central atom.

Acids The name of the oxoacid is easy. They

are related to the name of the oxoanion. By knowing the name of the oxoanion,

drop the – -ate and add -ic– -ite and add -ous

• HNO3 nitric acid

• H+ NO3- nitrite → nitric

• HNO2 nitrous acid

• H+ NO2- nitrite → nitrous

Acids

A small number of binary compounds containing hydrogen and a nonmetal give an acid solution when dissolved in water.

To name these solutions, add the prefix hydro- and add the suffix -ic to the nonmetal followed by the word acid.

Acids

The solution of the formula is denoted with an (aq).

HCl (aq) H+ Cl-Chlor

Hydro- chlor -ic + acid

Hydrochloric acid

Hydrates

Hydrates contain water molecules weakly bound in crystals.

Name them by naming the compound, adding a Greek prefix to the word hydrate.

CuSO4 • 5H2O (the “•” indicates hydrate)

Copper(II) Sulfate pentahydrate

Atom Rearrangement

Recall one of the aspects of Dalton’s atomic theory is that it explains a chemical reaction as a rearrangement of the atoms of the reacting substances in the formation of new compounds.

A chemical equation is a representation of this reaction:– 2Na(s) + Cl2(g) → 2NaCl(s)

reactants ↑ products “yields”

Phase Symbols

Useful phase indicators: (s), (l), (g), (aq), -heat or change, Pt-catalyst

Balancing Equations

It is important that we balance chemcial equations because according to the law of conservation of mass, we cannot creat nor destroy matter, we can only change its form.– 2H2(g) + O2(g) → 2H2O(l)

To balance these equations, you must choose coefficients that make the numbers of atoms on each side of the equation equal.