Chemistry
Atomic Structure
Location of Particles:
Protons - In the nucleus
Neutrons - In the nucleus
Electrons - In the electron cloud outside the nucleus in the energy levels
Neutral Atom - An atom is considered neutral when it has the same number of electrons and protons. (p+= e-)
Basic Information On Protons, Neutrons, & Electrons
Atoms are indeed divisible and can be broken down into their subatomic particles, protons, neutrons, and electrons. These too can be broken down even further!
Ions
An ion is an atom or a group of atoms that either has a positive or negative charge. Ions form when the atom gains or loses electrons.
Cations: Positive charge (more protons than electrons)
Anions: Negative charge (more electrons than protons)
Isotopes
Atoms with the SAME number of protons, but have a DIFFERENT amount of neutrons.
(Chemically alike, different mass number)
Atomic Notation
Atomic Number: The number of protons in an element (Always a whole number)
Mass number: The mass of an atom in atomic mass units (total number of protons + neutrons)
Charge:
Protons have a positive charge
Electrons have a negative charge
Charge = number of protons - number of neutrons
Atomic Mass Calculation
What you need:
Number of stable isotopes
Mass of each isotope
Natural % abundance of each isotope
How to Calculate:
Take the AMU (atomic mass units) of each of the isotopes
Multiply each by it's corresponding abundance (the percentage)
Add them up, put it in decimal form
Atomic mass of element =
A.M. isotope1*(%1) + A.M. of isotope2 *(%2)
100 100
EXAMPLE PROBLEM:
Calculate the atomic mass of Carbon if the two common isotopes of carbon have masses of 12.000 AMU (98.89% abundance) and 13.003 AMU (1.11% abundance).
a.m.1= 12.000 AMU a.m.2= 13.003 AMU
%1= 98.89% %2= 1.11%
(.9889)(12.000 AMU) + (0.0111)(13.003 AMU) = 11.8668 AMU + 0.1443333 AMU =
12.0111333 AMU
Electron Configuration
Principal Energy Levels
n = 1, 2, 3, 4, etc.
It describes the average distance of an electron from the nucleus
Energy increases as you move further from the nucleus
Sublevels
Within each energy level, electrons occupy sublevels!
Correspond to the different areas an electron can be located in an atom.
Represented by a letter:
s, p, d, f, g, h, i, etc.
Atomic Orbitals
Each sub level is oriented differently in 3-D space, and each orientation is called an atomic orbital.
Atomic orbitals can hold only two electrons.
Rules for Electron Configuration:
Aufbau Principle: (Bottom-Up)
Electrons will fill up lower orbitals first completely before going to the higher up ones
Hund's Rule: (Singles First)
Each orbital needs to be filled singly occupied first (with all electrons having the same spin) before they start to pair up
Pauli Exclusion Principle: (Opposite Pairs)
Orbitals can only hold up to two electrons, and the electrons must have opposite spins
Types of Sublevels in Energy Levels
S < P < D < F in energy within an energy level
Number of Atomic Orbitals in Sublevels
There are four orbitals, s, p, d, and f which are found in different energy levels called principal quantum numbers. We use the energy level numbers and orbitals in a “code” called electron configuration which helps us express the location of an electron with high probability.
"S" Orbital
The "s" orbital is a spherical shape and can only have 2 electrons.
It is the first orbital in an energy level to be filled when writing electron configurations.
The s orbital only has one orientation, seen in the picture to the right.
This orbital is found in all energy levels.
"P" Orbital
This orbital has 3 orientations.
Can hold 6 total electrons.
The "p" orbitals orientations are shown here.
This orbital is found in energy levels 2-7.
"D" Orbital
The "d" orbital looks like a double peanut.
Can hold 10 electrons total.
This has 5 orientations, one of which is shown to the right.
This orbital is found in energy levels 3-6.
"F" Orbital
The "f" orbital looks like a flower.
Can hold 14 electrons.
This has 7 orientations, one of which is shown to the right.
This orbital is found in energy levels 4 and 5.
Writing Electron Configuration
Notation for Electron Configuration
When writing electron configuration in long form, you need to write out every single sublevel.
The number for the term represents the Principal Energy Level
The variable/letter shows the Sublevel
And the superscript shows the # of electrons in the sublevel
Using the Periodic Table
Find the element on the table.
The block will determine the sublevel (SPDF) and the row will determine the principal energy level (1-7)
Then count over how many blocks it goes over from left to right, that will determine the super script (little exponent thingy).
(This will help you find the last term in your config, after this just fill in all the levels before it)
Example: Phosphorus (P)
For example, lets use this strategy to predict the electron configuration for a neutral atom of Phosphorus
(Find the element on the table)
Phosphorus has an atomic number of 15 and a chemical symbol of P, putting it in that slot on the table. As we can see, it is on the 3rd row in the P block of the table, giving us the symbol: 3p.
(Count over how many blocks it goes, that will determine the super script)
Now if we look on the 3p row, phosphorus is 3rd from the left, which gives us the exponent: 2. So now our final term in the configuration is 3p³.
(After this just fill in all the levels before it)
Finally, (due to the Aufbau principle), we know that all the levels below 3p² were filled out below it. So to get the full configuration we need to add them in. Making our final configuration: 1s²2s²2p⁶3s²3p³.
Excited State
Excited state is any state of the system that has a higher energy than the ground state.
Ground State
Ground state is a state where electrons in a system are in the lowest possible energy levels
The Periodic Table of Elements
History of Periodic Table:
Triads
Sets of three chemically similar elements
Law of Octaves (1865)
Intervals of seven
Mendeleev (1869)
Organized by increasing atomic mass
Impressive because of his predictions for the future of the table
Moseley (1913)
Modern periodic table, rearranged with increasing atomic number
How the Modern Periodic Table is Arranged
Periodic Law:
When elements are arranged in order of increasing atomic number, with patterns in their properties.
Periods (1-7)
Horizontal, do not have the same chemical properties
A new period begins when a new principal energy level begins filling with electrons
Groups (1-18)
Vertical, the same chemical properties
Based on the organization of the outer shell electrons
Metals, Nonmetals, and Metalloids
Metals
Good conductors of heat and electricity
Malleable and ductile
Luster (When a fresh surface of any metal is exposed, it will be very shiny because it reflects light well)
All metals are solid at room temperature
(With the exception of mercury (Hg), which is a liquid)
About 80% of the elements are metals
Nonmetals
Generally a poor conductor of heat and electricity
Not lustrous and malleable
Most properties of nonmetals are the opposite of metals
Nonmetals can be solid, liquid, or gas at room temperature depending upon the element
Their melting points are generally much lower than those of metals
Metalloids
They don’t fit neatly into the categories of metal or non-metal because of their characteristics
EXAMPLE: Silicon is a metalloid because it has luster, but is brittle
Metalloids are elements with properties intermediate between those of metals and non-metals
Metalloids can also be called "semimetals"
Periodic Trends
Atomic Size
Measure of the size of an atom (distance from the nucleus to the outer shells of electrons)
Increasing going down because the addition of shells with each energy level
A higher effective nuclear charge causes greater attractions to the electrons, pulling the electron cloud closer to the nucleus which results in a smaller atomic radius
Decreasing from left to right due to "shielding effect"
Ionic Radius
Ionic radius is determined by measuring the atom in a crystal lattice.
The same trend of atomic radius applies once you divide the table into metal and
nonmetal sections
Removal of electrons results in an ion that is smaller than the parent element (cation)
Addition of electrons results in an ion that is larger than the parent atom (anion)
Ionization Energy
Ionization energy is the required energy that is needed to remove an electron from an atom
Increasing going up because increases as electron shielding remains constant.
This pulls the electron cloud closer to the nucleus, strengthening the nuclear attraction to the outer-most electron, and is more difficult to remove (requires more energy)
Increasing from left to right because the increased distance of the nucleus and highest-energy electron.
This weakens the nuclear attraction to the outer-most electron, and is easier to remove (requiring less energy)
(Same pattern as electronegativity)
Electronegativity
An atom's ability to pull electrons toward itself
Decreasing going down the number because of energy levels increases, and so does the distance between the nucleus and the outermost orbital
Increasing from left to right
Right of the table: Valence shells are more than half full, so these atoms (nonmetals) tend to gain electrons and have high electronegativity
Left of the table: Valence shells are less than half full, so these atoms (metals) tend to lose electrons and have low electronegativity
(Excluding noble gases)
Chemical Equations and Reactions
A chemical equation is a representation of a chemical reaction that displays the reactants and products with chemical formulas. The different components in chemical equation are the formulas and the amount of each reactant and product, the physical states of the reactants and products taking part, and the symbols used in chemical equations.
Unlike in a math equation, a chemical equation does not use an equal sign.
Instead the arrow is called a yield sign and so the equation is described as “reactants yield products."
Chemists keep track of chemical reactions by writing equations as well.
In any chemical reaction one or more substances, called reactants, are converted into one or more new substances, called products
Chemical Equations Example
This equation is called a skeleton equation, that shows only the formulas of the reactants and products with nothing to indicate the relative amounts.
The first step in writing an accurate chemical equation is to write the skeleton equation, making sure that the formulas of all substances involved are written correctly.
All reactants are written to the left of the yield arrow, separated from one another by a plus sign.
Likewise, products are written to the right of the yield arrow, also separated with a plus sign.
Symbols Used in Chemical Equations
How to Balance Chemical Equations
Rules to Keep in Mind:
Count the atoms on each side of the equation (Subscripts)
Only Change the Coefficient
Count polyatomic ions (Ex. H2 O) as one item
Types of Chemical Reactions:
Composition Reaction
General Equation: A 十 B ⟶ AB
Description:
A combination reaction (also known as synthesis reaction) is a chemical reaction in which two or more substances combine to form a single new substance, with the two reactants forming a compound.
IRL Examples: Respiration, Repairing Damaged Tissues, Salt
Importance: Combination reactions are important because they are an umbrella term for more reactions, such as combustion reactions. Also without this we wouldn't be able to combine and form bonds between substances.
Decomposition Reaction
General Equation: AB ⟶ A 十 B
Description:
A decomposition reaction is a chemical reaction in which a compound breaks down into two or more simpler substances. Most require some sort of energy input (heat, light, electricity), and release energy as you break down bonds/create new bonds.
IRL Examples: Digestion
Importance: Decomposition reactions are important because they get compounds and break them down into simpler forms
Combustion Reaction
General Equation: CxHy 十 O2 ⟶ CO2 十 H2O
Description:
A combustion reaction is a chemical reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat.It also qualifies as a combination reaction. This reaction must involve O2 as one reactant.
IRL Examples: Propane used for Gas Grills
Importance: A combustion reaction takes place when a fuel and oxygen react, producing heat or heat and light. This is needed in everyday. Humans have been making practical use of combustion for thousands of years. Cooking food and heating homes have long been two major applications of the combustion reaction.
Single-Replacement Reactions
General Equation: A 十 BC ⟶ AC 十 B
Description:
A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. Metals can only replace metals, and non metals can only replace other non metals (Y+XZ→XY+Z Y is a nonmetal and replaces the nonmetal Z in the compound with X.) Starting materials are always pure elements. The activity series ranks metals/halogens based on their ability to replace what is below it.
IRL Examples: Potassium's Reaction with Water, Iron Rusting
Importance: Metalworking depends on single replacement whether it be to weld with thermite, or to extract metals from their ores.
Double-Replacement Reactions
General Equation: A 十 BC ⟶ AC 十 B
(In this reaction, A and C are positively-charged cation, while B and D are negatively charged ions)
Description:
A double-replacement reaction is a reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water.
IRL Examples: Precipitation Reactions, Acid-Base Reaction, Oxidation-Reduction Reaction
Importance: There are double-replacement reactions in everyday used objects, such as cough syrup and toothpaste.
Acid-Base Reactions
General Equation: Acid + Base = Salt + Water
Description:
This is a reaction between an acid and a base, in which they neutralize with each other and create salts. Most of these reactions are just double replacements. (Always pair positive ions with negative ions)
Acid: sharp taste, corrosive to metals, loses
Base: soapy to the touch, bitter taste, gains
Salt: salty, no similarities with acids and bases
IRL Examples: Baking Soda and Vinegar Experiment
NaHCO₃ + CH₃COOH→ CO₂ + H₂O + NaCH₂COO
Importance: Acids and bases are important in living things because most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to work.
Precipitation Reactions
General Equation: A 十 BC ⟶ AC 十 B
Description:
A precipitate forms in a double-replacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. In other words, it is the formation of an insoluble salt when two solutions containing soluble salts are combined.
IRL Examples: Potassium nitrate (a common active ingredient in toothpaste)
Importance: These reactions are important because they are good at identifying elements in products, they are used for getting magnesium out of seawater and in the human body with antibodies.
Oxidation-Reduction Reaction
General Equation: A 十 B ⟶ A⁺ 十 B⁻
Description:
This is a reaction in which the oxidation number of a molecule, atom, or ion varies by receiving or losing an electron is known as an oxidation-reduction reaction. Redox reactions have two parts: a reduced half and an oxidized half that often happen at the same time. The oxidation number of the reduced half reduces as electrons are gained, while the oxidation number of the oxidized half increases.
IRL Examples: Photosynthesis
Importance: Oxidation-reduction (redox) reactions are important because they are the principal sources of energy on this planet, both natural or biological and artificial.
Exothermic vs Endothermic Reactions:
Exothermic
Exothermic reactions are processes in which energy is released, normally in the form of heat and light.
Endothermic
Endothermic reactions are those in which reactants absorb energy in order to form products.
Writing Out Word Equations:
Example: Solid carbon disulfide burns in oxygen to yield carbon dioxide and sulfur dioxide gases
Identify the type of reaction: Combustion (keywords: "burns in oxygen")
Insert the chemical symbols and their states
Solid carbon disulfide → CS₂
Balance the Equation
Avogadro's Number/The Mole:
Atoms and molecules are extremely small. Counting atoms or molecules is not only unwise, it is absolutely impossible.
EXAMPLE: One drop of water contains about 1022 molecules of water. If you counted 10 molecules every second for 50 years without stopping you would have counted only 1.6 × 1010 molecules. Put another way, at that counting rate, it would take you over 30 trillion years to count the water molecules in one tiny drop.
Chemists needed a name that can stand for a very large number of items. The Italian scientist Amedeo Avogadro provided this new counting unit of measure called the mole.
A mole is the amount of a substance that contains 6.02 × 1023 representative particles of that substance.
The mole is the SI unit for amount of a substance. Just like the dozen and the gross, it is a name that stands for a number.
This number are representative particles in a mole. It is an experimentally determined number.
A representative particle is the smallest unit in which a substance naturally exists. For the majority of elements, the representative particle is the atom.
EXAMPLE: There are therefore 6.02 × 1023 water molecules in a mole of water molecules. There also would be 6.02 × 1023 bananas in a mole of bananas, if such a huge number of bananas ever existed.
Dimensional Analysis:
The Train-Track Method
Identify knowns and unknowns
Place conversion factors
Cancel out units
Calculate
(Ex: # of moles in 1.2 x 10²⁴ atoms of sulfer )
Stoichiometry
Using the relationship/ratios between the coefficients of a chemical equation to calculate the quantity of substances (whether it be particle number, volume, or mass)
The process is just dimensional analysis with more steps.
You convert units to moles, then back from moles into units in the products.