Basic Chemistry Review

 

Element: a substance that cannot be broken down into simpler chemical substances.

 

Atom: smallest particle of an element that still has the properties of that element.

 

 

 

Nucleus: center of an atom. Atomic nuclei are composed of 2 subatomic particles:

Protons: Positively charged

Large in size

The number of protons in the nucleus determines the identity of an

element. This is the atomic number of the element.

 

Neutrons: Neutral charge

Same size as protons

Neutrons add mass to the nucleus. The number of protons + the

number of neutrons in the nucleus is an elements atomic mass or

mass number.

 

Electrons are negatively charged particles that orbit the nucleus in an electron cloud. The electron cloud is divided energy levels and within each energy level there are sublevels. Each energy level can hold a specific number of electrons:

 

1st energy level holds up to 2 electrons (s sublevel holds both electrons)

2nd energy level holds up to 8 electrons (s sublevel holds 2 and the p sublevel holds 6)

3rd energy level holds up to 18 electrons (s holds 2, p holds 6 and the d sublevel holds 10)

4th energy level holds up to 32 electrons (s holds 2, p holds 6, d holds 10 and the f sublevel holds 14)

 

Electrons are very small. They are so small the mass of an individual electron cannot be measured.

In an atom, the number of protons=the number of electrons, atoms have no electric charge

 


Isotopes: atoms of the same element that have a differing number of neutrons in their nuclei. The

Different number of neutrons gives isotopes different atomic masses.

Ex. Carbon-12 6 protons Carbon-14 6 protons

6 neutrons 8 neutrons

 

 

Pictures of 3 hydrogen isotopes and 3 carbon isotopes. The first hydrogen atom has 1 proton, 1 electron and 0 neutron; the second hydrogen atom has 1 proton, 1 electron and 1 neutron; the third hydrogen atom has 1 proton, 1 electron and 2 neutrons. The first carbon atom has 6 protons, 6 electrons and 6 neutrons; the second carbon atom has 6 protons, 6 electrons and 7 neutrons; the third carbon atom has 6 protons, 6 electrons and 8 neutrons.

 

 

 

Chemical bonding and Chemical compounds

 

Compound: substance composed of two or more elements that have been combined chemically.

The properties of a compound are different than the properties of the elements that make it up.

Ex. 2H2 + O2 2H2O

 

H2 and O2 are both highly flammable, but water is used to put fires out.

 

 

Chemical bonds hold compounds together. There are two main types of chemical bonds:

 


Covalent bonds: form when atoms share electrons to form a compound. Covalent bonds form molecules.

 

Ionic bonds: form when electrons are transferred from one atom to another. Ions are atoms have gained or lost electrons, so they have an electrical charge. Atoms that gain electrons have a negative charge, atoms that lose electrons have a positive charge.Ionic bonds do not form molecules.

 


Chemical equations

 

Chemical equations must obey the Law of Conservation of Matter, so the number of atoms available before a reaction must equal the number of atoms available after the reaction. This is called a balanced equation.

Ex. 2H2 + O2 2H2O

 

4 atoms H2 4 atoms H2

2 atoms O2 2 atoms O2

6 atoms 6 atoms

 

 

Every chemical reaction has 2 parts: the reactants (that join together chemically) and the products

(what is formed in the reaction.)

 

Ex. 2H2 + O2 2H2O

reactants product

 

 

Solutions and Mixtures

 

Mixture: combination of substances where the individual components retain their own properties.

Ex. Sugar mixed with sand.

Mixtures can be separated by physical processes.

 

Solution: a mixture where one substance (the solute) is dissolved in another (the solvent).

Water is the universal solvent.

 

 


Acids and Bases

 

Acid: a substance that released H+ ions when mixed with water.

Acids have a pH of 0 6.

 

Ex. HCl H+ + Cl-

 

Base: a substance that releases OH- ions when mixed with water.

Bases have a pH of 8 14.

 

Ex. NaOH Na+ + OH-

 

A pH of 7 is neutral. It is neither acid nor base.

 

The pH scale measures the concentration of H+ ions in solution. The greater the concentration of hydrogen in the solution, the lower the pH. As the concentration of hydrogen in the solution decreases,

The pH gets higher.

 

The pH scale shows an exponential relationship. Each number on the pH scale represents a factor of 10. What does this mean?

An acid with a pH of 0 is 10 times stronger than an acid with a pH of 1 and it is 1 000 000

times stronger than an acid with a pH of 6.

 

Acids and bases have the ability to neutralize each other. When an acid and a base of equal strength are mixed, the result is a neutral solution.

 

Ex. HCl + NaOH NaCl + H2O

 

Every neutralization reaction will result in an ionic compound that is dissolved in water and the pH will always be 7.

 

Indicators

 

Indicators are chemicals that change color in the presence of other chemicals. There are many indicators that tell whether a solution is acid or base, but these are some of the most common:

 

Litmus paper: blue litmus turns red in an acid

red litmus turns blue in a base

 

Bromthymol blue: blue in a base and yellow in an acid

 

Phenolthelein: colorless in an acid and pink in a base

 


Properties of Water

 

Water is a polar molecule. This means that it bonds covalently, but electrons are not shared equally between the hydrogen atoms and the oxygen atom.

 

 

Since more electrons can be found orbiting the oxygen portion of the molecule, that end has a negative charge. Since fewer electrons can be found around the hydrogen atoms that end has a positive charge.

Because of the difference in charge the molecule acts like a magnet. The positive hydrogen end of the molecule attracts negative ions and compounds and the negative end attracts positive ions and compounds.

Because of this attraction, ionic compounds (ex. Salt) and polar compounds (ex. Sugar) dissolve in water.

 

Water molecules also attract other water molecules. This attraction forms a weak bond called a hydrogen bond. Hydrogen bonds hold many important molecules together (ex. Proteins).

The polarity of water is responsible for many of the unique properties of water.

Adhesion is the tendency of water to stick to the walls of its container.

Responsible for capillary action

 

Cohesion is the tendency for water molecules to stick to each other.

Responsible for surface tension

 

Because of cohesive forces, it is difficult to move water molecules apart. For this reason, water is very slow to gain heat. Once the molecules have been moved apart, though they are difficult to move back together. For this reason, water is slow to lose heat, once it has been warmed.

 

Cohesive forces are also the reason that water expands when it freezes. The hydrogen bonds between water molecules are frozen in place, which makes the molecules less mobile, causing them to remain farther apart.

 


Movement of molecules

 

Molecules of all substances are constantly in motion and are constantly trying to distribute themselves evenly. They do this spontaneously without the use of energy.

 

Diffusion is the movement of molecules from areas of higher concentration to areas of lower concentration. (Follows the concentration gradient.)

Factors affecting diffusion:

Concentration

Most important factor

Steep concentration gradient = fast diffusion

Temperature

Higher temperature = fast diffusion

Pressure

Higher pressure = fast diffusion

 

Osmosis is the diffusion of water across a cell membrane

 

Facilitated diffusion is the diffusion of large molecules using carrier molecules.

 

 

Active transport IS NOT a type of diffusion.

It moves molecules from areas of low concentration to higher concentration (against the

concentration gradient.)

 

It uses energy.