Science and Technology and Engineering: Chemistry (Review)

Past Atomic Models and Theories
There have been many revisions to theories regarding the structure of atoms and their particles.
Part of the challenge in developing an understanding of matter is that atoms and their particles are too small to be seen. It is believed that the first conceptualization of the atom was developed by Democritus in 400 B.C.
Some of the more notable models are the solid sphere or billiard ball model postulated by John Dalton, the plum pudding or raisin bun model by J.J. Thomson, the planetary or nuclear model by Ernest Rutherford, the Bohr or orbit model by Niels Bohr, and the electron cloud or quantum mechanical model by
Louis de Broglie and Erwin Schrodinger. Rutherford directed the alpha scattering experiment that discounted the plum pudding model. The shortcoming of the Bohr model was the belief that electrons orbited in fixed rather than changing ecliptic orbits.

Models of Atoms
Atoms are extremely small. A hydrogen atom is about  in diameter.
According to some estimates, five trillion hydrogen atoms could fit on the head of a pin. Atomic radius refers to the average distance between the nucleus and the outermost electron. Models of atoms that include the proton, nucleus, and electrons typically show the electrons very close to the nucleus and revolving around it, similar to how the Earth orbits the sun. However, another model relates the Earth as the nucleus and its atmosphere as electrons, which is the basis of the term 'electron cloud.' Another description is that electrons swarm around the nucleus. It should be noted that these atomic models are not to scale. A more accurate representation would be a nucleus with a diameter of about 2 cm in a stadium. The electrons would be in the bleachers.
This model is similar to the not-to-scale solar system model.

Structure of Atoms
All matter consists of atoms. Atoms consist of a nucleus and electrons. The nucleus consists of protons and neutrons. The properties of these are measurable; they have mass and an electrical charge.
The nucleus is positively charged due to the presence of protons. Electrons are negatively charged and orbit the nucleus. The nucleus has considerably more mass than the surrounding electrons. Atoms can bond together to make molecules.
Atoms that have an equal number of protons and electrons are electrically neutral. If the number of protons and electrons in an atom is not equal, the atom has a positive or negative charge and is an ion.

Atomic Number, Neutrons, Nucleon, and Element
Atomic number (proton number) - The atomic number of an element refers to the number of protons in the nucleus of an atom. It is a unique identifier. It can be represented as Z. Atoms with a neutral charge have an atomic number that is equal to the number of electrons.
Neutrons - Neutrons are the uncharged atomic particles contained within the nucleus. The number of neutrons in a nucleus can be represented as 'N.'
Nucleon - This refers collectively to both neutrons and protons.
Element - An element is matter with one particular type of atom. It can be identified by its atomic number or the number of protons in its nucleus. There are approximately 117 elements currently known, 94 of which occur naturally on Earth. Elements from the periodic table include hydrogen, carbon, iron, helium, mercury, and oxygen.

Molecules
Electrons in an atom can orbit different levels around the nucleus. They can absorb or release energy, which can change the location of their orbit or even allow them to break free from the atom. The outermost layer is the valence layer, which contains the valence electrons. The valence layer tends to have or share eight electrons. Molecules are formed by a chemical bond between atoms, a bond that occurs at the valence level. Two basic types of bonds are covalent and ionic. A covalent bond is formed when atoms share electrons. An ionic bond is formed when an atom transfers an electron to another atom. A cation or positive ion is formed when an atom loses one or more electrons. An anion or negative ion is formed when an atom gains one or more electrons. A hydrogen bond is a weak bond between a hydrogen atom of one molecule and an electronegative atom (such as nitrogen, oxygen, or fluorine) of another molecule. The Van der Waals force is a weak force between molecules.
This type of force is much weaker than actual chemical bonds between atoms.

Interaction of Atoms to Form Compounds
Atoms interact by transferring or sharing the electrons furthest from the nucleus. Known as the outer or valence electrons, they are responsible for the chemical properties of an element. Bonds between atoms are created when electrons are paired up by being transferred or shared.
If electrons are transferred from one atom to another, the bond is ionic. If electrons are shared, the bond is covalent. Atoms of the same element may bond together to form molecules or crystalline solids. When two or more different types of atoms bind together chemically, a compound is made. The physical properties of compounds reflect the nature of the interactions among their molecules. These interactions are determined by the structure of the molecule, including the atoms they consist of and the distances and angles between them.

Matter
Matter refers to substances that have mass and occupy space (or volume). The traditional definition of matter describes it as having three states: solid, liquid, and gas. These different states are caused by differences in the distances and angles between molecules or atoms, which result in differences in the energy that binds them. Solid structures are rigid or nearly rigid and have strong bonds. Molecules or atoms of liquids move around and have weak bonds, although they are not weak enough to readily break. Molecules or atoms of gases move almost independently of each other, are typically far apart, and do not form bonds. The current definition of matter describes it as having four states. The fourth is plasma, which is an ionized gas that has some electrons that are described as free because they are not bound to an atom or molecule.

Most Abundant Elements in the Universe and on Earth
Aside from dark energy and dark matter, which are thought to account for all but four percent of the universe, the two most abundant elements in the universe are hydrogen (H) and helium (He). After hydrogen and helium, the most abundant elements are oxygen, neon, nitrogen, carbon, silicon, and magnesium. The most abundant isotopes in the solar system are hydrogen-1 and helium-4. Measurements of the masses of elements in the Earth's crust indicate that oxygen (O), silicon (Si), and aluminum (Al) are the most abundant on Earth. Hydrogen in its plasma state is the most abundant chemical element in stars in their main sequences but is relatively rare on planet Earth.

Energy Transformations
The following are some examples of energy transformations:

Electric to mechanical: Ceiling fan
Chemical to heat: A familiar example of a chemical to heat energy transformation is the internal combustion engine, which transforms the chemical energy (a type of potential energy) of gas and oxygen into heat. This heat is transformed into propulsive energy, which is kinetic. Lighting a match and burning coal are also examples of chemical to heat energy transformations.
Chemical to light: Phosphorescence and luminescence (which allow objects to glow in the dark) occur because energy is absorbed by a substance (charged) and light is re-emitted comparatively slowly. This process is different from the one involved with glow sticks. They glow due to chemiluminescence, in which an excited state is created by a chemical reaction and transferred to another molecule.
Heat to electricity: Examples include thermoelectric, geothermal, and ocean thermal.
Nuclear to heat: Examples include nuclear reactors and power plants.
Mechanical to sound: Playing a violin or almost any instrument
Sound to electric: Microphone
Light to electric: Solar panels
Electric to light: Light bulbs

Relationship Between Conservation of Matter and Atomic Theory
Atomic theory is concerned with the characteristics and properties of atoms that make up matter.
It deals with matter on a microscopic level as opposed to a macroscopic level. Atomic theory, for instance, discusses the kinetic motion of atoms in order to explain the properties of macroscopic quantities of matter. John Dalton (1766-1844) is credited with making many contributions to the field of atomic theory that are still considered valid. This includes the notion that all matter consists of atoms and that atoms are indestructible. In other words, atoms can be neither created nor destroyed. This is also the theory behind the conservation of matter, which explains why chemical reactions do not result in any detectable gains or losses in matter. This holds true for chemical reactions and smaller-scale processes. When dealing with large amounts of energy, however, atoms can be destroyed by nuclear reactions. This can happen in particle colliders or atom smashers.

Difference Between Atoms and Molecules
Elements from the periodic table such as hydrogen, carbon, iron, helium, mercury, and oxygen are atoms. Atoms combine to form molecules. For example, two atoms of hydrogen (H) and one atom of oxygen (O) combine to form one molecule of water ().

Chemical and Physical Properties
Matter has both physical and chemical properties. Physical properties can be seen or observed without changing the identity or composition of matter. For example, the mass, volume, and density of a substance can be determined without permanently changing the sample. Other physical properties include color, boiling point, freezing point, solubility, odor, hardness, electrical conductivity, thermal conductivity, ductility, and malleability. Chemical properties cannot be measured without changing the identity or composition of matter. Chemical properties describe how a substance reacts or changes to form a new substance. Examples of chemical properties include flammability, corrosivity, oxidation states, enthalpy of formation, and reactivity with other chemicals.

Chemical and Physical Changes
Physical changes do not produce new substances. The atoms or molecules may be rearranged, but no new substances are formed. Phase changes or changes of state such as melting, freezing, and sublimation are physical changes. For example, physical changes include the melting of ice, the boiling of water, sugar dissolving into water, and the crushing of a piece of chalk into a fine powder. Chemical changes involve a chemical reaction and do produce new substances. When iron rusts, iron oxide is formed, indicating a chemical change. Other examples of chemical changes include baking a cake, burning wood, digesting a cracker, and mixing an acid and a base.

Physical and Chemical Properties and Changes
Both physical changes and chemical reactions are everyday occurrences. Physical changes do not result in different substances. For example, when water becomes ice it has undergone a physical change, but not a chemical change. It has changed its form, but not its composition. It is still

 

Elements, Compounds, Solutions, and Mixtures
Elements - These are substances that consist of only one type of atom.
Compounds - These are substances containing two or more elements. Compounds are formed by chemical reactions and frequently have different properties than the original elements. Compounds are decomposed by a chemical reaction rather than separated by a physical one.
Solutions - These are homogeneous mixtures composed of two or more substances that have become one.
Mixtures - Mixtures contain two or more substances that are combined but have not reacted chemically with each other. Mixtures can be separated using physical methods, while compounds cannot.

Heat, Energy, Work, and Thermal Energy
Heat - Heat is the transfer of energy from a body or system as a result of thermal contact. Heat consists of random motion and the vibration of atoms, molecules, and ions. The higher the temperature is, the greater the atomic or molecular motion will be.
Energy - Energy is the capacity to do work.
Work - Work is the quantity of energy transferred by one system to another due to changes in a system that is the result of external forces, or macroscopic variables. Another way to put this is that work is the amount of energy that must be transferred to overcome a force. Lifting an object in the air is an example of work. The opposing force that must be overcome is gravity. Work is measured in joules (J). The rate at which work is performed is known as power.
Thermal energy - Thermal energy is the energy present in a system due to temperature.

Types of Energy
Some discussions of energy consider only two types of energy: kinetic energy (the energy of motion) and potential energy (which depends on relative position or orientation). There are, however, other types of energy. Electromagnetic waves, for example, are a type of energy contained by a field. Another type of potential energy is electrical energy, which is the energy it takes to pull apart positive and negative electrical charges. Chemical energy refers to the manner in which atoms form into molecules, and this energy can be released or absorbed when molecules regroup. Solar energy comes in the form of visible light and non-visible light, such as infrared and ultraviolet rays. Sound energy refers to the energy in sound waves.

Chemical Reactions
Chemical reactions measured in human time can take place quickly or slowly. They can take fractions of a second or billions of years. The rates of chemical reactions are determined by how frequently reacting atoms and molecules interact. Rates are also influenced by the temperature and various properties (such as shape) of the reacting materials. Catalysts accelerate chemical reactions, while inhibitors decrease reaction rates. Some types of reactions release energy in the form of heat and light. Some types of reactions involve the transfer of either electrons or hydrogen ions between reacting ions, molecules, or atoms.
In other reactions, chemical bonds are broken down by heat or light to form reactive radicals with electrons that will readily form new bonds. Processes such as the formation of ozone and greenhouse gases in the atmosphere and the burning and processing of fossil fuels are controlled by radical reactions.

Reading Chemical Equations
Chemical equations describe chemical reactions. The reactants are on the left side before the arrow and the products are on the right side after the arrow. The arrow indicates the reaction or change. The coefficient, or stoichiometric coefficient, is the number before the element and indicates the ratio of reactants to products in terms of moles. The equation for the formation of water from hydrogen and oxygen, for example, is
. The 2 preceding hydrogen and water is the coefficient, which means there are 2 moles of hydrogen and 2 of water. There is 1 mole of oxygen, which does not have to be indicated with the number 1. In parentheses, g stands for gas, l stands for liquid, s stands for solid, and aq stands for aqueous solution (a substance dissolved in water). Charges are shown in superscript for individual ions, but not for ionic compounds. Polyatomic ions are separated by parentheses so the ion will not be confused with the number of ions.

Balancing Equations: An unbalanced equation is one that does not follow the law of conservation of mass, which states that matter can only be changed, not created or destroyed. If an equation is unbalanced, the numbers of atoms indicated by the stoichiometric coefficients on each side of the arrow will not be equal. Start by writing the formulas for each species in the reaction. Count the atoms on each side and determine if the number is equal. Coefficients must be whole numbers.
Fractional amounts, such as half a molecule, are not possible. Equations can be balanced by multiplying the coefficients by a constant that will produce the smallest possible whole number coefficient.

is an example of an unbalanced equation. The balanced equation is
, which indicates that it takes two moles of hydrogen and one of oxygen to produce two moles of water.

Periodic Table
The periodic table groups elements with similar chemical properties together. The grouping of elements is based on atomic structure. It shows periodic trends of physical and chemical properties and identifies families of elements with similar properties. It is a common model for organizing and understanding elements. In the periodic table, each element has its own cell that includes varying amounts of information presented in symbol form about the properties of the element. Cells in the table are arranged in rows (periods) and columns
(groups or families). At minimum, a cell includes the symbol for the element and its atomic number. The cell for hydrogen, for example, which appears first in the upper left corner, includes an 'H' and a '1' above the letter. Elements are ordered by atomic number, left to right, top to bottom.


Solutions
A solution is a homogeneous mixture. A mixture is two or more different substances that are mixed together, but not combined chemically. Homogeneous mixtures are those that are uniform in their composition. Solutions consist of a solute (the substance that is dissolved) and a solvent (the substance that does the dissolving). A. example is sugar water. The solvent is the water and the solute is the sugar. The intermolecular attraction between the solvent and the solute is called solvation. Hydration refers to solutions in which water is the solvent. Solutions are formed when the forces between the molecules of the solute and the solvent are as strong as the forces holding the solute together. A. example is that salt (NaCl) dissolves in water to create a solution. The  and the 
ions in salt interact with the molecules of water and vice versa to overcome the intramolecular forces of the solute.

Mixtures, Suspensions, Colloids, Emulsions, and Foams
A mixture is a combination of two or more substances that are not bonded. Suspensions are mixtures of heterogeneous materials. Particles in suspensions are usually larger than those found in true solutions. Dirt mixed vigorously with water is an example of a suspension. The dirt is temporarily suspended in water, but the two separate once the mixing is ceased. A mixture of large (1 nm to 500 nm) particles is called a colloidal suspension. The particles are termed dispersants and the dispersing medium is similar to the solvent in a solution. Sol refers to a liquid or a solid that also has solids dispersed through it, such as milk or gelatin. An aerosol spray is a colloid suspension of gas and the solid or liquid being dispersed. An emulsion refers to a liquid or a solid that has small droplets of another liquid dispersed through it. A foam is a liquid that has gas dispersed through it.

pH
The potential of hydrogen
(pH) is a measurement of the concentration of hydrogen ions in a substance in terms of the number of moles of  per liter of solution. A lower pH indicates a higher concentration, while a higher pH indicates a lower  concentration.
Pure water has a neutral pH, which is 7. Anything with a pH lower than water (less than 7) is considered acidic. Anything with a pH higher than water (greater than 7) is a base. Drain cleaner, soap, baking soda, ammonia, egg whites, and sea water are common bases. Urine, stomach acid, citric acid, vinegar, hydrochloric acid, and battery acid are acids. A pH indicator is a substance that acts as a detector of hydrogen or hydronium ions.
It is halochromic, meaning it changes color to indicate that hydrogen or hydronium ions have been detected.

Properties of Acids
When they are dissolved in aqueous solutions, some properties of acids are that they conduct electricity, change blue litmus paper to red, have a sour taste, react with bases to neutralize them, and react with active metals to free hydrogen. A weak acid is one that does not donate all of its protons or disassociate completely. Strong acids include hydrochloric, hydriodic, hydrobromic, perchloric, nitric, and sulfuric. They ionize completely. Superacids are those that are stronger than 100 percent sulfuric acid. They include fluoroantimonic, magic, and perchloric acids. Acids can be used in pickling, a process used to remove rust and corrosion from metals. They are also used as catalysts in the processing of minerals and the production of salts and fertilizers. Phosphoric acid () is added to sodas and other acids are added to foods as preservatives or to add taste.

Properties of Bases
When they are dissolved in aqueous solutions, some properties of bases are that they conduct electricity, change red litmus paper to blue, feel slippery, and react with acids to neutralize their properties. A weak base is one that does not completely ionize in an aqueous solution, and usually has a low pH. Strong bases can free protons in very weak acids.
Examples of strong bases are hydroxide compounds such as potassium, barium, and lithium hydroxides. Most are in the first and second groups of the periodic table. A superbase is extremely strong compared to sodium hydroxide and cannot be kept in an aqueous solution. Superbases are organized into organic, organometallic, and inorganic classes. Bases are used as insoluble catalysts in heterogeneous reactions and as catalysts in hydrogenation.

Properties of Salts
Some properties of salts are that they are formed from acid base reactions, are ionic compounds consisting of metallic and nonmetallic ions, dissociate in water, and are comprised of tightly bonded ions. Some common salts are sodium chloride (NaCl), sodium bisulfate, potassium dichromate (), and calcium chloride (). Calcium chloride is used as a drying agent, and may be used to absorb moisture when freezing mixtures. Potassium nitrate () is used to make fertilizer and in the manufacture of explosives. Sodium nitrate () is also used in the making of fertilizer. Baking soda [sodium bicarbonate ()] is a salt, as are Epsom salts [magnesium sulfate ()]. Salt and water can react to form a base and an acid. This is called a hydrolysis reaction.

Unique Properties of Water
The important properties of water () are high polarity, hydrogen bonding, cohesiveness, adhesiveness, high specific heat, high latent heat, and high heat of vaporization. It is essential to life as we know it, as water is one of the main if not the main constituent of many living things. Water is a liquid at room temperature. The high specific heat of water means it resists the breaking of its hydrogen bonds and resists heat and motion, which is why it has a relatively high boiling point and high vaporization point. It also resists temperature change. Water is peculiar in that its solid state floats in its liquid state. Most substances are denser in their solid forms. Water is cohesive, which means it is attracted to itself. It is also adhesive, which means it readily attracts other molecules. If water tends to adhere to another substance, the substance is said to be hydrophilic. Because of its cohesive and adhesive properties, water makes a good solvent. Substances, particularly those with polar ions and molecules, readily dissolve in water.

Kinetic Theory of Gases
The kinetic theory of gases assumes that gas molecules are small compared to the distances between them and that they are in constant random motion. The attractive and repulsive forces between gas molecules are negligible. Their kinetic energy does not change with time as long as the temperature remains the same. The higher the temperature is, the greater the motion will be. As the temperature of a gas increases, so does the kinetic energy of the molecules. In other words, gas will occupy a greater volume as the temperature is increased and a lesser volume as the temperature is decreased. In addition, the same amount of gas will occupy a greater volume as the temperature increases, but pressure remains constant. At any given temperature, gas molecules have the same average kinetic energy. The ideal gas law is derived from the kinetic theory of gases.

Organic Compounds
Two of the main characteristics of organic compounds are that they include carbon and are formed by covalent bonds. Carbon can form long chains, double and triple bonds, and rings. While inorganic compounds tend to have high melting points, organic compounds tend to melt at temperatures below 300° C.
They also tend to boil, sublimate, and decompose below this temperature. Unlike inorganic compounds, they are not very water-soluble. Organic molecules are organized into functional groups based on their specific atoms, which helps determine how they will react chemically. A few groups are alkanes, nitro, alkenes, sulfides, amines, and carbolic acids. The hydroxyl group () consists of alcohols. These molecules are polar, which increases their solubility. By some estimates, there are more than 16 million organic compounds.

Hydrogen Bonds
Hydrogen bonds are weaker than covalent and ionic bonds and refer to the type of attraction in an electronegative atom such as oxygen, fluorine, or nitrogen. Hydrogen bonds can form within a single molecule or between molecules. A water molecule is polar, meaning it is partially positively charged on one end (the hydrogen end) and partially negatively charged on the other (the oxygen end). This is because the hydrogen atoms are arranged around the oxygen atom in a close tetrahedron. Hydrogen is oxidized (its number of electrons is reduced) when it bonds with oxygen to form water.
Hydrogen bonds tend not only to be weak but also short-lived. They also tend to be numerous. Hydrogen bonds give water many of its important properties, including its high specific heat and high heat of vaporization, its solvent qualities, its adhesiveness and cohesiveness, its hydrophobic qualities, and its ability to float in its solid form. Hydrogen bonds are also an important component of proteins, nucleic acids, and DNA.

Inorganic Compounds
The main trait of inorganic compounds is that they lack carbon. Inorganic compounds include mineral salts, metals and alloys, non-metallic compounds such as phosphorus, and metal complexes. A metal complex has a central atom (or ion) bonded to surrounding ligands (molecules or anions). The ligands sacrifice the donor atoms (in the form of at least one pair of electrons) to the central atom. Many inorganic compounds are ionic, meaning they form ionic bonds rather than share electrons. They may have high melting points because of this. They may also be colorful, but this is not an absolute identifier of an inorganic compound. Salts, which are inorganic compounds, are an example of inorganic bonding of cations and anions. Some examples of salts are magnesium chloride () and sodium oxide (). Oxides, carbonates, sulfates, and halides are classes of inorganic compounds. They are typically poor conductors, are very water soluble, and crystallize easily.
Minerals and silicates are also inorganic compounds.