Chemistry Review

Chemistry Review A Slam Dunk Crash Course on the Periodic Table

Atoms Atoms are made of protons (+), electrons (-), and neutrons (no charge). Protons and Neutrons are heaviest (1 amu) and make up the nucleus (center of atom). Electrons are light and surround the nucleus in “clouds”. Electrons are responsible for the reactions, bonds, and what make one element different from another.

Atoms cont. Atoms usually have the same number of protons and electrons so that the (+) and (-) charges are equal. When they aren’t equal, the atom is called an ion. Having more electrons than normal gives the atom a (-) charge and less electrons than normal gives the atom a (+) charge. Positive ions are cations (cats are positive) and negative ions are anions (ants are negative).

Atoms cont. The atomic number (top number on periodic table) tells you the number of protons (or electrons) in a neutral atom (not an ion). i.e. How many protons are in an atom of Helium? Calcium? What about electrons?

Sample Chemistry Question Compared to the charge and mass of a proton, an electron has… • The same charge and a smaller mass • The same charge and the same mass • An opposite charge and smaller mass • An opposite charge and the same mass

Sample 2 A strontium atom differs from a strontium ion in that the atom has a different… • Number of electrons • Number of protons • Atomic number • Mass number

Sample 3 A positive ion is known as a… • Cation • Anion • Oxyanion • Valence ion

Elements Elements are made of one type of atom (the element Helium is made only of Helium atoms). Elements are arranged according to atomic number (number of protons) on the periodic table.

Periodic Table The periodic table is arranged by group (family) and period. Groups or families are vertical columns of elements. Periods are horizontal rows of elements. Elements in the same group have the same number of outer (valence) electrons and have similar properties. Those in the same period are not necessarily alike at all.

Periodic Table Cont. Different groups or families have different traits… Alkali metals (group 1) are most reactive (one outer electron) Alkaline earth metals (group 2) are next reactive (two outer electrons) Noble gases (group 8) are non-reactive (8 outer electrons…full)

Sample 4 Potassium is a highly reactive metal. Which element would you expect to also be reactive? • Lithium • Calcium • Neon • Hydrogen

Sample 5 Which element would you expect to be most reactive when placed in water? • Magnesium • Sodium • Argon • Iron

Bonding Atoms bond to fill their outer electon shells and become more stable (have 8 outer electrons like noble gases). Atoms either share or transfer their electrons with other atoms to create a bond between them. ELECTRONS FORM BONDS! Atoms that share electrons form covalent bonds. Atoms that transfer electrons form ionic bonds.

Bonding Cont. • Covalent bonds usually happen between two nonmetal atoms (not groups 1 & 2). • An atom can only have one bond per each outer (valence electron- group number). • Electrons in covalent bonds travel back and forth between the two atoms. • Equal sharing of the electrons in the bond is called nonpolar covalent bonding. • Unequal sharing of the electrons in the bond is called polar covalent bonding.

Bonding Cont. Ionic bonding usually happens between a metal and a nonmetal. Usually one atoms transfers its electrons to the other so that both have a complete (full) outer shell. If a bond forms between two metals, it is called a metallic bond.

Sample 6 Covalent bonds involve the _____of electrons. • Transfer • Combination • Absorbtion • Sharing

Sample 7 What is the maximum number of covalent bonds that an atom of carbon can form? • 1 • 2 • 3 • 4

Sample 8 At STP, which substance is the best conductor of electricity? • Nitrogen • Neon • Sulfer • Silver

pH pH is a measure of how strong acids and bases are. pH is a number on scale of 0 to 14. 0-6 = Acid 7 = Neutral 8-14 = Base

Sample 9 pH is a measure of … • Strengths of acids and bases • The pressure of gas • The heat produced by a chemical reaction • Weight of metals

Sample 10 When measured on the pH scale an acid is… • =7 • >5 • <7 • >7

Reactions Chemical reactions are exciting! They are the breaking and rearranging of bonds to create new substances. Breaking bonds releases energy stored in them. Creating new bonds requires energy. What you start with in chemical reactions are called the reactants. What you end up with are called the products. i.e. 6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O

Reactions Cont. Endothermic reactions require energy (bonds are forming) and the temperature will decrease (energy being used up = less heat). Exothermic reactions release energy (bonds are broken) and the temperature will increase (energy given off = more heat).

Reactions Cont. The signal for whether energy was gained or lost in a reaction is ∆H (heat of reaction). A positive ∆H means the reaction is endothermic (heat is absorbed) and if ∆H is negative, the reaction is exothermic (heat is released). If the temperature goes up during a reaction, the reaction is exothermic and the ∆H is negative.

Reactions Cont. ∆H can also be called enthalpy. Enthalpy is the heat absorbed in a reaction (to make new bonds). Catalysts are chemicals that can speed up a reaction without changing it or being involved in it at all! Catalysts have the ability to change (increase) the potential energy of the PRODUCTS (end result).

Equilibrium Reactions tend to continue until equilibrium (balance is reached). The rate (speed) of reaching equilibrium can be increased by increasing the… • Concentration of the reactants • Temperature • Presence of a catalyst • Physical state of the reactants

Equilibrium We know that equilibrium was reached when the rate of reactants becoming products is equal to the amount of products becoming reactants.

Density Density is mass / volume. Density is a measure of how tightly packed molecules are.

Gas Volume & Pressure • Volume is the size or space molecules occupy. Big volume = more space • As gases heat up, they spread out and have a greater volume. • Gases that are pressurized have less volume. • Volume of a gas is directly related to temperature and inversely related to pressure.

Entropy • Entropy is the change from greater to less potential energy (things tend to go from more to less organization). • Relationship between states of matter and entropy. Gas = Highest entropy Liquid = ``Medium'' entropy Crystal/solids = Lowest entropy • If this is unclear, think about which one takes more energy to create. To get from crystal to liquid or liquid to gas you need to add energy, which makes the system more disordered (more enthalpy).

Chemistry Review