Notes, summaries, assignments, exams, and problems for Chemistry

Chemical Bonds and Substance Types

The enormous variety of substances is possible thanks to the fact that the atoms of most elements have a great ability to join other atoms, either among different elements or atoms of the same element. These forces are known as chemical bonds. The result of chemical bonding ranges from a discrete group of atoms, called a molecule, to a structure of millions of atoms ordered regularly in space, forming a lattice or crystal.

Classifying Chemical Substances

Elements and Compounds

If atoms of the same element join, they form an elemental substance, whereas if they are different elements, they form a chemical compound.

Molecular Substances

Substances which are formed by molecules are called molecular substances. The... Continue reading "Chemical Bonds and Substance Classification" »

Electron Spin and Orbitals

Spin refers to the movement of an electron around the nucleus and on its own axis. There are two possible spin directions: +1/2 and -1/2. When two electrons have opposite directions of rotation, they are represented with small arrows, one pointing up and the other down.

An orbital is occupied when it contains two electrons. An orbital with a single unpaired electron is represented by a single arrow. Orbitals are represented as rectangles called quantum boxes, within which the electrons are indicated.

The electronic configuration of an atom describes how the electrons are arranged within the atom.

Hund's Rule

Hund's rule states that an electron cannot completely fill an orbital until all orbitals within that sublevel contain... Continue reading "Electron Configuration and Chemical Bonding Basics" »

Intermolecular forces (IMFs) are attractive forces that exist between molecules. They are responsible for holding molecules together in condensed phases (liquids and solids) and influence various physical properties such as melting points, boiling points, and solubility.

Dipole-Dipole Interactions

These forces occur between molecules that possess a permanent nonzero dipole moment. In such molecules, electrons accumulate in one part (creating a negative pole), while another part becomes electron-deficient (forming a positive pole). If the dipole is sufficiently large, the electrostatic forces between these dipoles can significantly influence the substance's properties. For instance, substances with strong dipole-dipole interactions often have... Continue reading "Intermolecular Forces: Molecular Interactions and Properties" »

Mol

The unit of amount of substance in the SI. It is the amount of substance that contains as many basic entities as there are atoms in 12 g of the isotope carbon-12.

Avogadro's Number (NA)

One mole contains Avogadro's number of particles: NA = 6.022045 × 10²³.

Molar Mass (M)

Mass of one mole of atoms or molecules. The numerical value of molar mass in grams matches the value of the mass in amu of its formula.

Molar Volume

Volume of 1 mole of a substance at a certain temperature (the temperature must be stated), whether solid, liquid, or gas.

Following Avogadro's principle, 1 mole of gas occupies the same volume when measured under the same conditions of pressure (p) and temperature (T), irrespective of the gas. Under standard conditions (0 °C and... Continue reading "Fundamental Chemistry Concepts & Laws" »

Chemical Reactions

A chemical reaction is a process in which one or more substances react to form other substances with different properties.

Chemical Reactions and Chemical Equations

The chemical equation is the symbolic representation of a chemical reaction.

Activation Energy

For a chemical reaction to be possible, particles must collide with a minimum energy.

Reaction Energy

The energy exchange that occurs in the course of a chemical reaction is called the energy of reaction.

Speed of Reaction

The speed at which a chemical reaction takes place.

Factors Influencing the Reaction Rate

• Nature of Reagents: Generally, covalent substances lead to slow reactions at room temperature, while ionic substances react quickly when dissolved.
• Temperature: The reaction

Fundamental Chemical Substances: Elements and Compounds

Defining Simple Substances: Elements

Simple substances, or elements, are the basic building blocks of matter. They combine in fixed proportions by mass to form compounds.

Classification of Elements

• Metallic Elements

  Located on the left side of the periodic table. All are solids at room temperature, except mercury (Hg), which is a liquid with a melting point of -39 °C.

• Nonmetals

  Found on the right side of the periodic table. Examples include carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulfur (S), selenium (Se), and the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). They exist in various physical states; for instance, sulfur is solid, chlorine is gaseous, and bromine

Bioelements: Chemical Basis of Life

Bioelements are the chemical elements that constitute living matter. They are classified based on their abundance:

Classification of Bioelements

• Primary Bioelements

  These make up the vast majority of living matter. They include Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S) – often abbreviated as CHONPS.

• Secondary Bioelements

  These include Sodium (Na+), Potassium (K+), Calcium (Ca2+), Magnesium (Mg2+), and Chlorine (Cl-). Although present in smaller proportions than primary bioelements, they are essential for life. In aqueous environments within living organisms, they are always found in their ionized forms.

• Trace Elements (Oligoelements)

  These are required in very small amounts.

Metallic Materials

Metallic materials are inorganic substances containing one or more elements. They may contain metals and also some non-metallic elements. The metallic elements include iron, copper, aluminum, nickel, magnesium, and titanium. Non-metallic elements include carbon, nitrogen, and oxygen.

Characteristics of Metal Components

• Have a crystalline structure
• Are good thermal and electrical conductors
• Many have mechanical resistance and are ductile at room temperature
• Can also exhibit high strength at elevated temperatures

A metal alloy is a combination of two or more metals. They are normally divided into two classes:

• Ferrous metals and alloys that contain a high percentage of iron
• Nonferrous metals and alloys containing no iron or relatively

Why Life Is Based on Carbon and Not on Silicon

Although silicon (Si) also has four electrons in its valence shell and is much more abundant in the Earth's crust than carbon (C), life is based on C and not on Si. This is because C-C, C=O, and C-N bonds are weak enough to form stable chains and rings, while also being able to break to make way for other molecules. This allows living organisms to obtain the energy contained in these bonds. In contrast, Si-Si and Si-O-Si-O chains are so stable that they are unalterable.

Descriptive Biology

Descriptive biology is based on the simple observation of the structure and functioning of living things, to make descriptions of what is observed. Until the 17th century, it was based solely on simple observation... Continue reading "Carbon-Based Life and the Scientific Method" »

Types of Materials

Materials are substances whose properties make them useful for the fabrication of structures, machinery, and other products.

Material Classification Groups

Materials are typically classified into groups:

• Metals and Alloys (e.g., iron, steel, aluminum)
• Polymers (e.g., nylon, polyurethane)
• Ceramics and Glass (e.g., alumina, magnesia)
• Composite Materials (e.g., wood, cermets)

Material Properties

Key material properties include:

• Chemical Properties
• Physical Properties
• Mechanical Properties
• Aesthetic and Economic Properties
• Fabrication Properties

Chemical Properties

Chemical properties include behavior related to oxidation and corrosion.

Oxidation

Oxidation occurs when a material combines with oxygen, forming oxides.

Corrosion

Corrosion is a form... Continue reading "Understanding Material Properties and Classes" »