## Chapter 4, Section 1 - Matter

Matter - Anything that has mass and takes up space.
Mass - The amount of matter in an object.

Properties of Matter:
Physical Properties can be observed without changing the composition of the substance.
• Density
• Color
• Hardness
• Freezing/Boiling Points
• Electrical Conductivity
Chemical Properties describe how a substance reacts with other substances.

Element - A substance that cannot be separated or broken down into simper substances by chemical means.
• Any element contains only one type of atom.
• Elements are represented by chemical symbols.
• One or two letters - the first is always CAPITALIZED, the second is always lower case.
• Examples...
• Hydrogen - H
• Helium - He
• Atom - The smallest piece of an element that has all of the element's chemical properties.
• Atoms join together to make molecules.
• Molecule - The smallest unit of a substance that behaves like that substance.
• Some elements can exist as single atoms, some make molecules with more than one atom of the same element.
• Example:  O2 = Two atoms of oxygen, bonded together.  (Still one element, because they are the same type of atom.)
Atoms are made of subatomic particles...

 Particle Charge Mass Location in the Atom Proton +1 1 amu In the nucleus. Neutron 0 1 amu In the nucleus. Electron -1 1/2000 amu In the electron cloud (orbiting the          nucleus).

Protons determine the element.
• Each element has a specific number of protons.
• Example - All carbon atoms have six protons.
Atoms have no overall charge.
• The number of protons in an atom equals the number of electrons, so the positives and negatives cancel out.
• If an atom gains or loses electrons, it becomes charged and is now an ion (an atom with a charge).
The electric force holds the atom together.
• Positive and negative charges attract each other - this is called the electric force.
• The charges of protons and electrons hold the atom together.
Atoms of the same element always have the same number of protons, but can have different numbers of neutrons.
• Atomic Number (Z) - The number of protons in the nucleus of an atom.
• Each element has its own specific atomic number.  This number doesn't change!
• Mass Number (A) - The total number of protons and neutrons in the nucleus of an atom.
• The number of neutrons can vary from atom to atom of the same element.
• Isotopes - Atoms of the same element (same number of protons) with different numbers of neutrons.
• Some isotopes are more common than others...
• Protium - 1(hydrogen with one proton) - most common isotope of hydrogen.
• Deuterium - 2(1 proton, 1 neutron) - 1 out of 6,000 hydrogen atoms on Earth.
• Tritium - 3(1 proton, 2 neutrons) - very rare.  Unstable and decays over time.
• Radioisotopes - give off radiation and decay into other isotopes over time.
• To find the number of protons, neutrons or electrons in an atom...
• # Protons = Atomic Number
• # Neutrons = Mass Number - Atomic Number
• # Electrons = Atomic Number
Atoms have mass - but their mass is very small.
• Unified Atomic Mass Unit (u) - A unit of mass that is 1/12 the mass of a carbon-12 atom (with 6 protons and 6 neutrons).
• Also called atomic mass unit (amu).
• The mass of one proton or one neutron is about 1u.  (Electrons are too small to affect atomic mass significantly.)
• Average Atomic Mass - The average mass of the different isotopes of an element, as they are found in nature.
• Chlorine has two isotopes: 35Cl and 37Cl.  The 35Cl isotope is more common, so the average atomic mass is 35.453u.

The Periodic Table
• Elements are arranged in order of their Atomic Numbers.
• Elements in the same column (vertical) have similar chemical properties.
Electrons are arranged in energy levels (shells).
• Valence Electrons - the electrons in an atom's outermost energy level.
• Only valence electrons can interact with other atoms.
• The nucleus and inner shells are protected, and cannot interact.
• So, valence electrons determine the element's chemical properties.
Elements in the same group (column) tend to have the same number of valence electrons.

Elements can be grouped into three main categories...
• Metals
• Tend to lose electrons.
• Good conductors of heat and electricity.
• Ductile - Can be drawn into a wire.
• Malleable - Easily shaped and formed.
• Most are shiny.
• Solid (except mercury).
• Nonmetals
• Tend to gain electrons.
• Insulators - Poor conductors of heat and electricity.
• Not ductile or malleable - solid nonmetals are usually brittle.
• Most are not shiny.
• Can be solid, liquid or gas.
• Semiconductors (Metalloids)
• Share some properties with metals, some with nonmetals.
• Conduct electricity under certain conditions.  (Better conductor than nonmetal, not as good as metal.)
• Also called metalloids.
• Often used in computer chips and electronic devices.

## Chapter 4, Section 2 - Combinations of Atoms

Compound - A substance made up of atoms of different elements.
• Compounds contain more than one type of atom.
• The atoms in a compound always combine in the same proportions.
• Example: Water always has one oxygen atom and two hydrogen atoms.
• The properties of a compound are different than the properties of the elements that make it up.
• Compounds are represented by chemical formulas.
• Chemical formulas show how many of each kind of atom that are in a molecule.
• Example:  Water = H2O
• The small numbers (subscript) in a formula tell how many atoms of the element that comes before it.  In water, the "2" stands for two hydrogen atoms.  If there's no number, than there is one of these atoms.  (One oxygen atom in water.)
Chemical reactions happen when substances change to form new substances.
• Reactant - A substance that participates in a chemical reaction.
• Product - A substance that forms in a chemical reaction.
In a chemical reaction, the atoms are rearranged.
• Atoms are not created or destroyed.
• There are the same number and types of atoms before and after the reaction.

Chemical Equations use symbols to represent chemical reactions and to show the relationship between reactants and products.

2H2  +  O2  =>  2H2O

Chemical equations must be balanced.  The same number and type of atoms on each side of the arrow.  (Law of Conservation of Mass.)

Ion - An atom or molecule that has gained or lost electrons and, therefore, has a net positive or negative charge.
Atoms that do not have a full outer orbital can gain or lose electrons so that their outermost orbital is full.  This is called ionization.
• An atom that loses electrons becomes positively charged.
• An atom that gains electrons becomes negatively charged.
Chemical Bond - The force that holds atoms/ions together in a molecule.
• Ionic Bond - The attraction (force) between oppositely charged ions.  These ions form when electrons are transferred from one atom to another.
• Covalent Bond - A bond formed when atoms share one or more pairs of electrons.
• Atoms do not always share electrons equally.
• When two identical atoms bond, the shared electrons are equally attracted to both nucleus.  This makes anonpolar covalent bond.
• When different atoms bond, the electrons are more attracted to one nucleus than the other - they are not shared equally.  This makes a polar covalent bond.
• Fluorine is the strongest electron attractor - the closer an element is to F on the periodic table, the greater its attraction for electrons.
Mixture - A combination of different substances, which are not chemically combined.
• Mixtures can be physically separated into their components.  Pure substances cannot.
• Mixtures are classified by how well their components mix...
• Homogeneous Mixtures - The components are evenly spread throughout the mixture.  (a.k.a. solutions)
• Example - In Kool-Aid, the Kool Aid particles are evenly spread throughout the water.  It looks like one substance.
• Heterogeneous Mixtures - The components are not evenly  spread throughout the mixture.
• Example - In a chocolate chip cookie, some parts have chocolate chips, others don't.  You can see the different parts.
• Note - You can't always see the different parts in a heterogeneous mixture.

Resources

Chapter Assignment