CHEM 1411 - General Chemistry I

Course Lecture Notes

Summer 2007

Instructor: Dr. Shawn Amorde

Web page:

Atoms, Molecules, and Ions

Text Chapters (2.2, 2.4,-2.7, 2.9, 5.7, 5.9)

Recommended Problems: (1, 3, 24, 28, 29, 32, 36, 38, 40,

50, 51, 52, 56, 64, 76, 78, 81, 90, 102, 104)

I. Fundamentals of Matter

A. Elements

B. Atoms

C. Isotopes

D. Atomic Weights

E. The Mole

F. The Periodic Table

G. Compounds, Molecules, and Molecular Formulas

H. Ions

I. Molar Mass, Molecular Weight, and Formula Weight

J. Determining Chemical Formulas

K. Chemical Analysis

Elements

Elements are the fundamental substances of chemistry!! Today there are 109 elements, 88 naturally occurring…..Although new are being discovered and add to the periodic table.

Elements can not be converted or decomposed into another substance by any form of energy we deal with in this course!! No Element can be converted into another Element!! (Barring extraordinary heat such as on the surface of the sun and nuclear reactions)

The elements are grouped into the Periodic Table of Elements. We will be using this table almost everyday in this course!! The Periodic Table is one of the single most important tools in chemistry, and we will learn to use it in many ways.

The elements are listed in the periodic table by symbol, some of the common symbols, C, O, H,….and so forth you should recognize.

Atoms

Atoms, which were once thought to be the ultimate invisible particles that make up all of matter, are among the fundamental particles of the science of chemistry. The idea of the atom originated in the early 1800’s with Dalton’s Atomic Theory and has been since proven.

An atom is the smallest particle of an element that we can still identify as that element.

Atoms are made up of three elementary particles, protons, electrons, and neutrons.

Protons, exist in the nucleus with a mass of 1.673 X 10^-24g, they are positively charged (+1) each. They are huge compared to electrons!!

Neutrons, also exist in the nucleus, but are neutral. They are very close in mass to the protons, 1.675 X 10^-24g.

Electrons, exist “in orbit” around the nucleus, they are negatively charged (-1) each, and are tiny compared to protons and neutrons (1/1835 proton mass).

· Charges review:

· Positive and Negative attract!!

· Positive and Positive repel each other!!

· Negative and Negative repel each other!!

· Like the poles of a magnet

Atomic structure

Each element has a fixed number of electrons, protons, and neutrons!! The number of protons and electrons is always the same!!

This is the first arrangement for the periodic table.

The total number of protons and neutrons is the atomic mass number (atomic weight)

The total number of protons in an atom determines what element the atom is, this number is the atomic number.

The periodic table lists these.

Monoatomic molecules exist in nature as there single atoms appear on the Periodic table, some elements however, exist in nature as a diatomic or polyatomic molecules, for example, oxygen exists as O₂ in nature, so does hydrogen, H₂, nitrogen, N₂, chlorine, Cl₂.

Ions

Atoms are neutral, that is they carry no net charge. That means the number of protons and electrons is equal in every element.

The movement of electrons is the basis for all chemical bonding and interactions.

This is why the identity of an element is established by the number of protons and not electrons.

Lets look at N, atomic number 7, atomic weight 14.

What if N loses an electron? The overall charge is now out of balance. There are 7 protons and only 6 electrons, that is 7 – 6 = +1 charge.

Ions are atoms or groups of atoms that carry an electrical charge. (Flashlight experiment here?) Ion is a word derived from a Greek word meaning “to go”. In 1834 Michael Faraday used the term ion to describe the chemical particles moving from one electrical pole to another. He divided the charged particles into two categories:

Anions, the negatively charged particles that move to the positively charged pole (the anode);

Cations, the positively charged particles that move to the negatively charged pole (the cathode).

We use the same terms today….only we now understand ions also are at rest, as in NaCl.

Isotopes

Atoms with the same number of protons and electrons, but a different number of neutrons are called isotopes. For example, lets look at chlorine, Cl.

The atomic weight is an average of the isotopes found for each element.

Remember :

29 atomic number

Cu atomic symbol

63.546 atomic weight

The Periodic Table

The elements are arranged first, by their atomic numbers. Second, they are arranged in rows and columns in such a way that elements with similar properties are in vertical columns. These columns refer to a “group” of elements. Each group has an accepted name for the elements within that group.

The horizontal rows are called “periods”.

There are three general groupings, metals, nonmetals, and metalloids.

The groups from left right are as follows;

Group 1A: Alkali Metals

Ancient Arab alchemists studied many natural substances and noticed the ashes of certain plants gave their hands a slippery feel, al-qali, it is now known these ashes contained several of the group 1A elements.

Group 2A: Alkaline Earth Metals

These elements are named for their alkaline properties when dissolved in water.

Group 3B-8B: Transition Metals

All of the elements in this group are metals and some are the most abundant on earth.

Group 3A: Has no specific name, but contains Al, the most abundant metal in the earth’s crust.

Group 4A: Has no specific name, but is incredibly important in this course. Carbon is found in the form of graphite and diamonds, or in molecular form in all life on the planet. Silcon is the second most abundant element in the earth’s crust, i.e. sand, glass, quartz, clay…etc.

Group 5A: No common name for this group either. Nitrogen comprises ¾ of the earth’s atmosphere. Phosphorus is and essential element in our bodies.

Group 6A: Oxygen!! Essential for all life on this planet. Sulfur has been known since ancient times as brimstone. Sulfuric acid is produced in the largest amounts of any chemical in the chemical industry.

Group 7A: Halogens

The name for these elements comes from the Greek words hals for salt and genes for forming. The most commonly know salt here is NaCl, table salt.

Group 8A: The Rare Gases, Noble Gases

These are inert gases. This group is known to not readily react with other elements.

We will continue to study the periodic table through out this course. This is one of the most important tools a chemist uses on a daily basis!!

The Mole

One of the most fascinating and sometimes frustrating aspects of chemistry and chemical reactions is the fact you can’t usually see what is happening. You can’t physically see an atom, or a molecule, or a compound. We see a grouping of an extremely large number of these atoms in, for example, a glass of water. What if you needed to dissolve vinegar in water in exactly 1 molecule vinegar to 1 molecule water? You can’t see the molecules so you can’t separate them or count them, you can’t rely on weighing water and vinegar, because we have seen all of the elements have a different weights and different densities, so H₂O isn’t the same as CH₃CO₂H.

We need a way to compare the elements, compounds, and molecules.

That is the basis for the mole.

The mole, is essentially a way to count atoms, no matter what their size is.

The word “mole” was introduced around 1896, from the Latin word moles meaning “heap” or “pile”.

The official definition:

A mole is the amount of substance that contains as many elementary particles (atoms or molecules) as there are atoms in 12.0 grams of carbon-12 isotope.

First, the mole always contains the same amount of particles, no matter what the substance.

How many? 1 mole = 6.022045 X 10²³ particles

This is Avogadro’s number, named after Amadeo Avogadro, an Italian lawyer and physist.

Moles translate to mass, using the atomic mass number for each element.

Thus, the molar mass is equal to the atomic weight of each element.

This is essential!!

1 mole = 6.023 X 10²³particles = atomic weight (g/mol)

So, Let’s look at Oxygen

Oxygen’s molar mass = 16.0 grams/mole or g/mol = 6.023 X 10²³ atoms of O

Let’s work a problem

Compounds, Molecules, and Molecular Formulas

The combination of elements in precise, well defined ratios into a pure substance called a compound.

Any compound can by decomposed into its individual elements!!

Compounds, a pure substance made up of two or more elements in a fixed ratio.

The Law of Constant Composition, For example, water is 89% oxygen and 11% hydrogen, not matter where it comes from.

The smallest unit of a compound which retains the characteristics of that compound is a molecule.

Molecules are groups of two or more atoms held together by the forces of chemical bonds. The composition of a molecule is represented by a molecular formula.

Let’s look at ordinary table salt.

NaCl

sodium chloride

Why do we have salt? Why form? Why is NaCl a stable molecule? We know elemental sodium is a metal, and elemental chlorine is a gas, yet NaCl is a white solid, why?

This molecule is comprised of two different ions.

Remember: Ions are atoms or groups of atoms that carry an electrical charge. The two charges, positive and negative attract, so a positive ion and a negative ion attract each other.

So if Na existed as a positive ion and Cl existed as a negative ion, they would attract. This is exactly what happens and that attraction is what we called our chemical bond.

Na⁺Cl^-

We will get into a much deeper discussion of chemical bonding, ions, and orbitals later in the course.

But notice the positions of both elements on the periodic table. There are general trends within the periodic table that we will build on through out the semester, including this one.

What is your positive ion was Ca⁺², ions aren’t limited to + or -1.

You would need a (-2) element or 2 (-1), thus CaCl₂.

Molecules are always in strict ratios to form neutral species!!

Molecular formulas tell you the number of ions and the positive ions are always listed first, like NaCl.

What does that mean for our mole?

1 mole = 6.023 X 10²³ particles and particles can be molecules.

The molecular mass is equal to the masses of each element in the molecule.

NaCl = 22.9898 g/mol(Na) + 35.453g/mol(Cl) = 58 g/mol NaCl

Let’s work another problem

What if we are a chemist and we were given a sample of colorless liquid, could we find the molecular formula? Yes. Elemental Analysis is an established qualitative analysis that gives us the % composition of the elements in that powder.

Let’s say we had the elemental analysis done, and the results told us…..

H – 1.6%

N – 22.2%

O – 76.2%

Can we get to the molecular formula?

Yes. HNO₃

Let’s clarify here.

The empirical formula is the simplest formula, meaning the lowest possible ratio equivalents, like BH₃

The molecular formula is what the molecule actually exists as., B₂H₆.

So if we have 100g of our sample:

We convert each to moles.

Then we find the ratio

Notice we used moles here.

Let’s do another problem here.

Let’s consider what information the molecular formula gives us,

If we use moles,

1 mol of a molecular formula = 1 mol of each element in the formula (in ratio)

Let’s do a problem.