Science Study Guide for the NLN NEX

Chemistry

Chemistry is the study of the atoms and molecules that make up matter, the bonds they form, and the chemical reactions that transform substances.

Atomic Structure

Matter is anything that has mass and volume. Most of the matter we interact with on a daily basis is made of several different types of materials. A substance is a type of matter that is uniform in composition and properties. An element is a substance that cannot be broken down into simpler components. A compound is a substance made of one or more elements chemically bonded together.

The Atom

Atoms are the smallest units of a substance that have all of the properties of that substance. Each element is made of a unique type of atom. Atoms are composed of three particles:

• electrons—small particles that orbit the center of the atom and are negatively charged

• neutrons—uncharged particles found in the nucleus, or center, of the atom

• protons—positively charged particles found in the nucleus of the atom

The Periodic Table

An element is a substance that is made of only one type of atom. It cannot be broken down further or separated into various components. The periodic table contains a list of all possible elements. They are represented with a symbol, such as H for hydrogen or O for oxygen.

An element’s position in the periodic table gives insight into some of its properties:

• period—This is a row of the table. The atoms in each period have the same number of electron shells, the levels at which electrons orbit the nucleus.

• group—This is a column of the table. The atoms in a group have the same number of valence electrons, the electrons in the outermost electron shell.

• metals—These are found on the left side and in the middle of the table.

• nonmetals—These are found on the right side of the table. The last group of nonmetals are the noble gases, which do not form bonds with other atoms.

• metalloids—These are a small group of elements that have transitional properties between metals and nonmetals. They are found along the stairstep between those two groups in the table.

Element Squares

Each square in the periodic table holds information about a particular element. The symbol contains one or two letters to identify the element. The atomic number tell us the number of protons (and electrons, unless it is an ion) an atom contains. The atomic mass is the average mass of an atom, mainly consisting of the number of protons and neutrons it contains, but it also takes into account the mass numbers of various isotopes of that atom that may exist.

Image source: https://openstax.org/books/chemistry-2e/pages/2-5-the-periodic-table

Compounds

A compound is a substance that is made of two or more elements chemically combined. The smallest components of a compound are molecules. Compounds are represented by a chemical formula that includes the elements involved and the ratios of those elements. For example, water’s chemical formula is H2O because it contains a 2:1 ratio of hydrogen and oxygen.

Isotopes and Ions

The number of protons determines the type of atom. The number of electrons or neutrons, however, may vary. When atoms gain electrons they become negatively charged anions, and when they lose electrons they become positively charged cations. The general term for a charged atom is an ion. Atoms that have a different number of neutrons than average are known as isotopes. Isotopes change the mass number, which is the total number of protons and neutrons in an atom. Radioactive isotopes are unstable and break down over time, emitting energy in the form of radiation.

Bonding

Atoms can react with other atoms to form chemical bonds. Two or more atoms chemically bonded together make up a molecule. Molecules are the smallest units of a compound. The electron is the particle responsible for creating chemical bonds, and that can occur in a variety of ways.

Atoms are most stable when their outer (valence) electron shell has eight electrons. This is called the octet rule. They will bond with other atoms to achieve this stability. A Lewis electron dot diagram shows an atom’s symbol, along with dots that represent the number of valence electrons in that atom. It can help us determine how atoms may react with other atoms. The image below shows individual atoms of chlorine, which have seven valence electrons each, combining to form a molecule of chlorine in which they each share an electron to make eight.

Image source: https://openstax.org/books/chemistry-2e/pages/7-3-lewis-symbols-and-structures

Ionic Bonds

Ionic bonds form when one atom gains an electron from another, creating two ions whose opposite charges cause them to bond together. The atom that gains electrons becomes negatively charged, and the atom that loses electrons becomes positively charged. Atoms with six to seven valence electrons are likely to gain electrons from atoms that have only one to two valence electrons.

Covalent and Polar Covalent Bonds

A covalent bond occurs when atoms share electrons. The shared electrons spend time around each of the atoms in the bond, filling their valence shells and making them stable. If the electrons are shared equally between the atoms, it will be a nonpolar covalent bond. Larger atoms in a bond tend to pull electrons more strongly toward themselves, resulting in an unequal sharing of electrons. This creates a polar covalent bond in which the molecule has positively and negatively charged regions. A dipole forms when the opposite charges are equal in strength on opposite sides of the molecule.

States of Matter

Substances can undergo changes in state, known as a phase change. The phases of matter and the processes that change them are shown in the diagram below:

Phase changes occur with a change in temperature and/or pressure. Each substance has a unique set of points at which it will change phase. Vapor pressure measures the tendency of a substance to change to the gas phase from a solid or liquid. Vapor pressure increases with temperature.

Mixtures

A mixture is made of several substances physically combined (rather than chemically bonded, as with a compound). The components of a homogeneous mixture are evenly distributed, while those in a heterogeneous mixture are not.

Solutions

One type of homogeneous mixture is a solution. Solutions form when a solute dissolves in a solvent. Solubility measures a solute’s tendency to dissolve in a particular solvent. Solutions can be separated through filtration (passing the solution through a filter that traps the solute and allows the solvent through) or through distillation (evaporating and recollecting the solvent, leaving the solute behind). Chromatography is a technique that is used to separate several solutes in a solution.

Tinctures

A tincture is a concentrated solution whose solvent is a type of alcohol, typically ethanol. Tinctures often have solutes extracted from plants or animals and are used in medicine and homeopathy.

Emulsions

An emulsion is a mixture of two liquids that are not soluble in each other, where small droplets of one liquid are evenly dispersed throughout the other. Emulsions may be unstable and separate over time (such as oil and vinegar), or they may be stable and remain mixed (such as the fat droplets in milk).

Changes in Matter

Different types of matter can combine or separate to form new types of matter. Depending on what is happening at the molecular level, these changes can be classified as chemical or physical changes.

Chemical Changes

A chemical change is one in which chemical bonds between atoms are broken and/or reformed with other atoms in a chemical reaction. The reactants are the chemicals that are present before the reaction, and the products are the chemicals formed after the reaction. The products will have different properties than the reactants.

Physical Changes

A physical change occurs when mixtures are created or separated, but the atoms involved do not break or form new bonds. Physical changes occur when, for example, you make a solution of salt water, melt a cube of ice, or shred paper. In each example, the components retain their chemical compositions.

Chemical Reactions

A chemical reaction occurs when bonds between atoms are broken and reformed. When energy is absorbed during a chemical reaction, that is an endothermic reaction, whereas when energy is released, that is an exothermic reaction. Reactions will continue until a balance between products and reactants is reached, a state known as equilibrium. One important principle in chemical reactions is that the number and types of chemicals must be equal before and after the reaction. This is known as the law of conservation of mass.

Combination or Synthesis Reaction

In a synthesis reaction, two reactants combine to form one new product. This can also be called a combination reaction, since two chemicals are being combined into one:

A + B → AB

Single Replacement Reaction

In a single replacement reaction, a compound replaces one element with another. This involves a compound and an element as reactants and a different compound and element as the products:

AB + C → AC + B

Double Displacement

A double displacement (or double replacement) reaction involves two compounds that break bonds and exchange elements:

AB + CD → AC + BD

Decomposition Reaction

A decomposition reaction occurs when one compound breaks down into two or more compounds or elements:

AB → A + B

Reaction Rate

The speed at which a reaction takes place is its reaction rate. This is influenced by the types of chemicals involved, the temperature, the pressure, and other factors, such as the presence of a catalyst, which is explained in the next section.

Catalysts

A catalyst is a chemical that increases the rate of a reaction, but is not used up in the reaction itself. In order for a reaction to proceed, energy is required. This is called the activation energy. Catalysts speed up a reaction by lowering the activation energy needed for the reaction to proceed.

Acid/Base Chemistry

Acids and bases are solutions with particular properties. Acids are chemicals that form hydrogen ions (H+) in solution. Bases are chemicals that form hydroxide ions (OH-) in solution. The pH scale measures the strength of acids and bases. It ranges from 0 to 14. A pH of 7 is neutral, with anything less than 7 indicating an acid, and anything greater than 7 indicating a base. The further the number is from 7, the stronger the acid or base. When acids and bases react, a neutralization reaction occurs in which they cancel each other out to form water and a type of salt. Certain substances can serve as indicators because they change color when they come into contact with an acid or a base.

Chemical Equilibrium

Reactions will continue until a balance between products and reactants is reached, a state known as equilibrium. One important principle in chemical reactions is that the number and types of chemicals must be equal before and after the reaction. This is known as the law of conservation of mass.

Organic Chemistry Basics

Organic chemistry is the study of organic compounds, which are chemicals containing carbon. Organic compounds constitute the building blocks of life. Because carbon is able to form four bonds with many types of atoms, it is the foundation for the complex molecules of which living things are made.

Organic Compounds

There are four main biological molecules that make up all life. These complex molecules are made of monomers (subunits) that bond together to make polymers (large chains of monomers). Each type of molecule has a distinct structural formula, a diagram that shows its atoms, bonds, and configurations.

Nucleic Acids

Nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules contain the code for the structure and function of living things. They are made of monomers called nucleotides.

Carbohydrates

Carbohydrates are our main source of energy and form some cellular structures. They are made of monomers called monosaccharides, such as glucose. Two monosaccharides combine to form a disaccharide, such as sucrose. Long chains of monosaccharides form polysaccharides, such as starch. In animals, carbohydrates that are not needed right away are stored as glycogen. Lugol’s solution, which is used as a cell stain and disinfectant, also reacts with starches to change color from orange/brown to purple/black. Benedict’s solution can detect simple sugars in a solution and changes from blue to orange/red when heated in the presence of simple sugars. This indicator can be used to detect the presence of glucose in urine when testing for diabetes.

Lipids

Lipids, also called fats, are used for energy storage, cell membranes, insulation, and waterproofing. They are made of two types of monomers called fatty acids and glycerol.

Proteins

Proteins are among the most diverse types of biomolecules. Proteins can function as hormones, structural molecules, enzymes, and antibodies that fight infection. They are composed of amino acids. Biuret solution detects proteins in food, urine, and other body fluids by changing the color from blue to pink/purple.

Reactions

Certain types of reactions are key in building up and breaking down the components of the biomolecules described here.

Dehydration Synthesis

A dehydration synthesis reaction, also known as a condensation reaction, is used to combine two molecules, resulting in the creation of a water molecule. This type of reaction joins monomers together to form polymers.

Hydrolysis

A hydrolysis reaction is the opposite of dehydration synthesis. With hydrolysis, two subunits of a large molecule break apart with the help of water and a catalyst. This type of reaction occurs during digestion as the body breaks food into smaller pieces.

Oxidation/Reduction

Hydrocarbons are chemicals composed of only hydrogen and carbon. If a hydroxyl group (-OH) replaces one of the hydrogen atoms in a hydrocarbon, an alcohol will form. Alcohols can transform into aldehydes or ketones through the process of oxidation (a reaction in which a molecule gains an oxygen atom), and back into alcohols through reduction, the opposite process.