ELEMENTS, COMPOUNDS AND MIXTURES AND HOW THEY ARE REPRESENTED 8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances

WHAT ARE ELEMENTS? – RECALL… Elements are pure substances Made of only one kind of material Has definite properties, and Is the same all throughout They cannot be broken down into simpler substances without losing their identity Represented by a symbol (Mg, Na) They’re on the periodic table!

WHAT ARE COMPOUNDS? Made up of 2 or more different elements that are chemically combined. They are represented by formulas Ex: H2O, NaCl, C6H12O6, CO2 Compounds have different properties than their original elements (* more on next slide) They cannot be separated by physical means Unlike elements, compounds can be broken down to simpler substances through a chemical reaction

COMPOUNDS The properties of the elements that make up a compound are often quite different from the properties of the compound itself Sodium – Na = highly reactive metal Chlorine – Cl = poisonous gas Sodium Chloride = NaCl (table salt)

WHAT ARE MIXTURES? Mixtures - two or more substances that are physically combined and retain the properties of their substances  Mixture of elements – brass (mixture of copper and zinc) Mixture of elements and compounds – air Mixture of compounds – sand, saltwater Solution – particles are evenly distributed

TYPES OF MIXTURES Homogeneous - Entire mixture looks the same throughout Ex. Milk, Bronze Heterogeneous – Parts of the mixture look different Ex. Fruit Salad, Trail Mix Both types of mixtures can be separated by a physical change!

ELEMENT, COMPOUND, OR MIXTURE ? ? ?

QUICK CHECK Element, Compound, or Mixture? Mixture Mixture Compound Platinum Pt Carbon Dioxide CO2 Air O2, N2, and Ar Brass Alloy of Cu and Zn 5. Glucose C6H12O6

MOLECULES A molecule is 2 or more atoms chemically bonded that act like one unit Water-2 atoms of hydrogen and one atom of oxygen-together they form one molecule of H2O All compounds are molecules but not all molecules are compounds H2 is a molecule, but not a compound H2O is both a molecule and a compound (notice the 2 different elements)

QUICK CHECK Which substances are molecules and which substances are both molecules and compounds? Molecule Both Both Both O2 CO2 3. C6H12O6 Cl2 NH3

CHEMICAL FORMULAS A shorthand way of representing compounds If chemical symbols are the “letters,” combined, these are the “words.” Ex: NH3 = ammonia, C3H7OH = rubbing alcohol Sometimes, the formula represents a molecule of a single element. These are called diatomic molecules. This is how that element is naturally found. O2-Oxygen H2-Hydrogen Cl2-Chlorine

CHEMICAL FORMULAS - SUBSCRIPTS Subscripts are small numbers used in chemical formulas Shows the elements & number of atoms of each element in a molecule H2SO4 Subscript Elements Hydrogen: 2 atoms Sulfur: 1 atom Oxygen: 4 atoms  7 atoms total

COEFFICIENTS (SLIDE 1) A formula may begin with a number If there is no number, then “1” is understood to be in front of the formula. This number is called the coefficient The coefficient represents the number of molecules of that compound or atom needed in the reaction For example: 2H2SO4 – 2 molecules of Sulfuric Acid Coefficient

COEFFICIENTS (SLIDE 2) 2H2SO4 = 2 molecules of Sulfuric Acid Note - A coefficient is distributed to ALL elements in a compound To find the number of atoms: 2 H2 (for a total of 4 H atoms) 2S (for a total of 2 S atoms) 2O4 (for a total of 8 O atoms)

BONDING - HOW DO ATOMS JOIN TOGETHER? A chemical bond is an interaction that holds atoms together – this is known as a molecule (this was mentioned earlier) When we talked about valence electrons, we learned that when the outer energy level is not full, that atom seeks to either gain or lose electrons to be stable. Atoms form bonds with other elements (or sometimes itself) in order to lose, or gain electrons to be “happy” or stable. Bonds are formed to get the atoms to a lower energy state. There are several types of bonds, let’s go over some main ones.

COVALENT BONDS As the attractions bring the atoms together, electrons from each atom are attracted to the nucleus of both atoms, which “share” the electrons. The sharing of electrons between atoms is called a covalent bond, which holds the atoms together as a molecule. Atoms covalently bonded as a molecule are more stable than they were as separate atoms Examples of covalent bonds: H2 (hydrogen), H2O (water), O2 (oxygen), CH4 (methane), and CO2 (carbon dioxide). Covalent Bonds - animation

IONIC BONDS - VIDEO In ionic bonding, atoms transfer electrons to each other. Ionic bonds require at least one electron donor and one electron acceptor. Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion. Common examples: NaCl – table salt, NaBr - sodium bromide

IONIC BONDS - VIDEO Chlorine is a greenish poisonous gas and sodium is a shiny, soft, and very reactive metal. But when they react, they form sodium chloride (table salt). In the video, the drop of water helps expose the atoms at the surface of the sodium so that they can react with the chlorine. The formation of the salt crystals releases a lot of energy. What is happening? An electron is transferred from sodium to chlorine. Sodium becomes a positive ion and chlorine becomes a negative ion. Positive and negative ions attract each other and form an ionic bond and the compound sodium chloride.

IONIC BOND IMAGE: The Na gives up the electron to the Cl. The Na is more stable with 8 valence electrons. The Cl has one more than stable so it has an over all negative charge.

METALLIC BONDS A metallic bond is pretty different from covalent and ionic bonds, but the goal is the same: to achieve a lower energy state. Instead of a bond between just two atoms, a metallic bond is a sharing of electrons between many atoms of a metal element. The electrons sort of “hover” in between the atoms! Watch this! Bonds (18 min), And/or: Bonds (9 min)

WAYS THAT BONDS ARE REPRESENTED We can’t “see” bonds, but scientists need to communicate about them, systems have been created to discuss bonding. Two are typical – model (the dots shown) and “displayed” which shows the bonds. A displayed formula shows all the bonds in the molecule as individual lines. You need to remember that each line represents a pair of shared electrons.

EVERYDAY COMPOUND OR POISON? HW: due next class