Chapter 2 Atoms, Molecules, and Ions

Chemistry, The Central Science, 13th edition

Theodore L. Brown, H. Eugene LeMay, Jr., Bruce E. Bursten, Murphy, Woodward, and Stoltzfus.

2.1 The Atomic Theory of Matter

Greek Philosophers

Early philosophers debated the fundamental “stuff” of which the universe was made

Democritus’ idea of the world was based on “atomos” meaning indivisible or uncuttable

Later Plato and Aristotle formulated that there can be no uncuttable “things” and they successfully propagated their theory and therefor became the dominant theory for centuries

John Dalton

The atom reemerged in the 17th century

The ground work was laid by John Dalton when he noticed that elements that reacted with other elements to form new compounds

Dalton’s atomic theory was based on 4 postulates

Dalton’s Atomic Theory

Explains several laws that were known during Dalton’s time, including the law of constant composition.

In a given compound, the relative numbers and kinds of atoms are constant

The Law of Conservation of Mass

The total mass of materials present after a chemical reaction is the same as the total mass present before the reaction

The Law of Multiple Proportions

If two elements A and B combine to form more than one compound, the masses of B that can combine with given mass of A are in the ratio of small whole numbers

Dalton’s Atomic Theory

Dalton’s Theory Correct?

Give it some thought

Compound A contains 1.333 g of oxygen per gram of carbon, whereas compound B contains 2.666 g of oxygen per gram of carbon.

What chemical law does this data illustrate?

Law of Multiple Proportions

If compound A has an equal number of oxygen and carbon atoms, what can we conclude about the composition of compound B?

There are twice as many oxygen atoms in compound B than there is in compound A

2.2 The Discovery of the Atomic Structure

Subatomic Particles

Dalton based his theory on observations made in the laboratory

He and those who followed had no direct evidence of the atom

As time progressed, scientists began to probe the nature of matter and started to discover subatomic particles

We will see that the atom is composed in part by electrically charged particles

Keep in mind as we continue that same charges repel one another, whereas particles with unlike charges attract one another

Cathode Rays and Electrons (e-)

During the 1800’s scientists experimented (including Thomson) with evacuated glass tubes with electrodes inserted at both ends

Once a charge was applied a radiation between the electrodes was produced

Cathode rays emanated from the negative end and traveled to the positive end

The rays were unseen but caused certain materials to fluoresce

The rays were tested with magnets and electrically charged rods and it was found that the cathode rays were negatively charged

The identity of the new particle was the same regardless of the gas used

This new “thing” was called the electron

Thomson tested the beam and found that there was 1.76 x 108 coulombs per gram

Give it some thought

Thomson observed that the cathode rays produced in the cathode–ray tube behaved identically, regardless of the particular metal used as cathode. What is significance of this observation?

All atoms have the same subatomic particles called electrons!

Millikan Oil Drop Experiment

Once the charge-to-mass ratio of the electron was known, Milikan was then able to experimentally figure out the mass of the electron

Using an experiment similar to the one pictured to the right he calculated the mass of the electron to be 9.10 x 10-28 g by solving the charge of a single electron

This showed that the electron has a mass of about 2000 times less than hydrogen!

Radioactivity

In 1896, Henri Becquerel discovered that uranium emitted radiation.

Spontaneous emission is known as radioactivity

He concluded that the source or the radiation was the uranium atoms

Studies done by Ernest Rutherford showed that there were 3 types of radiation

Alpha (α)

Beta (β)

Gamma (γ)

Alpha and beta radiation was shown to be bent by an electric field (but in different directions) while gamma radiation was unaffected by it

α and β Rays

Both considered fast moving particles

Beta rays were shown to be the radioactive equivalent of cathode rays

Attracted to positively charged plates!

Charge of -1

The alpha particles were shown to have a positive charge

Charge of +2

Has a mass 7400 times that of an electron

Gamma radiation is high-energy radiation similar to X-rays and does not consist of particles and carries no charge

3 Types of Radiation

Thomson Model

Thomson in the early 1900’s reasoned that since the electrons are such a small portion of the mass of the atom that it must also only make up a small portion of the atom

Thomson proposed that that the electrons were embedded in the atom like raisons in pudding

The “pudding” having a positive charge

Rutherford Atomic Model

Rutherford was studying the angles at which alpha particles were being deflected

He discovered that most particles passed straight through the gold foil while only a small amount of the particles were deflected or even bounced back

Rutherford explained the result by postulating the nuclear model of the atom, which most of the mass of each gold atom resided in the nucleus

He postulated that the majority of the space in an atom was empty and that the charge of the nucleus was positive

Protons and Neutrons

Subsequent experiments led to the discovery of positive particles (protons) and neutral particles (neutrons) in the nucleus.

Protons were discovered in 1919 by Rutherford and neutrons in 1932 by British scientist James Chadwick (1891–1972). Thus, the atom is composed of electrons, protons, and neutrons.

Give it Some Thought

What happens to most of the alpha particles that strike the gold foil in Rutherford’s experiment?

They passed straight through

Why do they behave that way?

The atom is mostly empty space

2.3 The Modern View of Atomic Structure

The Modern View of Atomic Structure

The charge of an electron is -1.602 * 10-19 C.

The charge of a proton is opposite in sign but equal in magnitude to that of an electron: +1.602 * 10-19 C.

The quantity 1.602 * 10-19 C is called the electronic charge.

For convenience, the charges of atomic and subatomic particles are usually expressed as multiples of this charge rather than in coulombs.

The charge of an electron is 1- and that of a proton is 1+.

Neutrons are electrically neutral

Every atom has an equal number of electrons and protons, so atoms have no net electrical charge.

Atomic Diameter

Protons and neutrons reside in the center of the atom and take up only a small volume of the atom

Most atoms have diameters between 1 * 10-10 m 1100 pm2 and 5 * 10-10 m 1500 pm2. A convenient non–SI unit of length used for atomic dimensions is the angstrom 1A° 2, where 1 A° = 1 * 10-10 m. Thus, atoms have diameters of approximately 1 - 5 A° . The diameter of a chlorine atom, for example, is 200 pm, or 2.0 A° .

Give it Some Thought

If an atom has 15 protons, how many electrons does it have?

Phosphorous

Where do the protons reside in an atom?

The nucleus

Subatomic Particles

Sample Exercise 2.1

The diameter of a U.S. dime is 17.9 mm, and the diameter of a silver atom is 2.88 A° . How many silver atoms could be arranged side by side across the diameter of a dime?

Atomic Numbers, Mass Numbers, and Isotopes

The atoms of each element have a characteristic number of protons. The number of protons in an atom of any particular element is called that element’s atomic number.

Because an atom has no net electrical charge, the number of electrons it contains must equal the number of protons.

The atomic number is indicated by the subscript; the superscript, called, the mass number is the number of protons and neutrons

Isotopes

Isotopes differ by the number of neutrons but contain the same number of protons

2.4 Atomic Weights

Atomics Mass Scale

Scientists in the 19th century were aware that atoms of different elements have different masses

They knew that in water that there was 88.9 g of oxygen for every 11.1 grams of hydrogen in 100 grams of water

Today we can determine the masses of individual atoms with a high degree of accuracy.

The atomic mass unit is a convenient way when dealing with very small masses

i.e 1 mole of carbon twelve weights 12 amu

Atomic Weight

Atomic weight is the natural mixture of isotopes found in nature

To calculate the atomic weight of an element is to use the following equation:

Atomic Weight = Σ[(isotope mass) x (fractional isotope abundance)]

Σ sigma means over all masses

Give it Some Thought

A particular atom of chromium has a mass of 52.94 amu, whereas the atomic weight of chromium is given as 51.99 amu. Explain the difference in the two masses.

Mass Spectrometers

Some Pictures From Mr. Hunter’s Graduate Research

Actual Data Gathered by Mr. Hunter

Why are there bands over a certain mass range and not just a line?

The mass spectrometer is an energy filter that allows only stable trajectories of ions through.Therefore, all ions of the same mass have a range of energies (different velocity) coming from the ion source.

Book Work in Class

Pages 74-75

Answer only red questions for section 2.1-2.4

2.5 The Periodic Table

The Periodic Table

As the list of elements began to grow throughout history, certain elements were noticed to have similar traits as other elements.

For example, Na and K had similar properties while He and Ne also had similar properties to one another.

These similarities became known as periodic trends or families

Periodic TablePeriods= horizontal rowsFamilies (groups) = vertical columns

Common Families

Metals vs Nonmetals

Metals are found to the left of the stair-step lineNon-metals are found to the right of the stair-step lineMetalloids are found along the stair-step line

2.6 Molecules and Molecular

Compounds

Molecules and Molecular CompoundsAll of the pictured molecules are

in their chemical formulas exceptfor one

Oxygen and hydrogen exist as diatomic elements, i.e molecule

Molecules that contain more than one type of atomare called molecular compounds

Molecular and Empirical Formulas

Chemical formulas that indicated the exact number of atoms in a molecule are called molecular formulas

Chemical formulas that indicate the relative number of atoms in a molecule are called empirical formulas

Picturing Molecules

Structural formulas shows which atoms are attached to which

Atoms can be represented by their symbols and bonds by lines

This type of modeling does not show the actual geometry of the molecule

The Ball-and Stick model has the advantage of showing the actual geometry of the atoms in relation to one another

The space filling model shows the relative sizes of atoms and their geometries

Book Work

Page 75-76

Answer only red questions for section 2.5-2.6

2.7 Ions and Ionic Compounds

Ions and Ionic Compounds An ion is a charged particle

A cation is a positive particle (one or more less electrons)

An anion is a negative particle (one or more extra electrons)

Ions form because certain elements have a weak attraction to the outer electrons (valence electrons)

Other elements have a strong attraction for their electrons and surrounding elements electrons

The stronger pull can take the weaker element’s electron

This transfer of electrons causes ions to form

In addition to simple ions there are polyatomic ions such as ammonium and sulfate

These ions are made of at least 2 elements

Predicting Ionic Charges

Elements like to have a stable number of electrons.

The most stable elements on the periodic table are the noble gases because they have a full octet of electrons

Elements gain or lose electrons to get to this stable state

Nonmetals tend to gain electrons

Metals tend to lose electrons

Half Filled shells are more stable that partially filled shells

Meaning, electrons can move from one shell to another to stabilize the ion

This also means that s-orbital electrons can be taken instead of f- or d-shell electrons

Practice Problem

Predict the charge states of:

Fe

Sn

Sc

Ti

V

Ionic Compounds

Ionic compounds are a compounds made up of anions and cationsIonic compounds are generally metals and nonmetals.

Criss-Cross Method

Since there are not discrete ionic “molecules” we are only able to write empirical formulas for these compounds

The criss-cross method take the charges of each ion and puts it down at the subscript of the opposite ion

2.8 Naming Inorganic Compounds

Naming Inorganic Compounds

Chemical Nomenclature comes from the latin word nomen (name) and calare (to call)

The rules of chemical nomenclature are based on the division of substances into categories

Organic compounds contain carbon and hydrogen often with oxygen, nitrogen, and other elements

All other compounds are inorganic compounds

Names and Formulas of Ionic Compounds

Most metals that form cations can form different oxidation states. These are usually transition metals

Oxyanions- an anion containingone or more atoms bondedto another element (as in thesulfate and carbonate ions)

Oxyacids- an acid “see above“

Names and Formulas of Acids

Names and Formulas of Binary Molecular Compounds

The name of the element farther to the left in the periodic table (closest to the metals) is usually written first. An exception occurs when the compound contains oxygen and chlorine, bromine, or iodine (any halogen except fluorine), in which case oxygen is written last.

If both elements are in the same group, the one closer to the bottom of the table is

named first.

The name of the second element is given an -ide ending.

Greek prefixes indicate the number of atoms of each element.

(Exception: The prefix mono- is never used with the first element.) When the prefix ends in a or o and the name of the second element begins with a vowel, the a or o of the prefix is often dropped.

Give it Some thought

Is SOCl2 a binary compound?

Book Problems

Answer on red question for section 2.7-2.8

2.9 Some Simple Organic Compounds

Organic Chemistry

The study of compounds containing carbon is called organic chemistry

Organic chemistry is a very important branch of chemistry encompassing a large portion or what chemists do

Being able to name these compounds is very important for the standardization of chemistry experiments

Hydrocarbons are composed of only carbon and hydrogen

Alkanes are the simplest of hydrocarbons Alkanes always end in -ane

Some Derivatives of Alkanes

Functional groups can be added to organic molecules to change their chemical properties

Alcohols are made by adding an –OH group in place of a hydrogen

An alcohol has the ending –ol

Numbers are used to determine what carbon the functional group is attached

Always use numbers that make it the smallest possible number or smallest possible addend

Isomers

Isomers are compounds that have the same molecular formulas but a different chemical arrangement

The most common type of isomer are structural isomers

Other Functional Groups

There are hundreds of different functional groups in organic chemistry

Common functional groups are:

Halogens

Hydroxyl (-OH)

Carboxylic acid (-COOH)

Oxygen insertions

Carbonyl groups (-CO)

Alkane groups

Common Functional Groups

Give it some thought

Draw the structural formulas of two isomers of butane, C4H10