ELEMENTS, COMPOUNDS AND MIXTURES AND HOW THEY ARE REPRESENTED

Transcription

1 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

2 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!

3 WHAT ARE COMPOUNDS? Made up of 2 or more different elements that are chemically combined. They are represented by formulas Ex: H 2 O, NaCl, C 6 H 12 O 6, CO 2 Compounds have different properties than their original elements They cannot be separated by physical means Unlike elements, compounds can only be broken down to simpler substances through a chemical reaction

4 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)

5 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

6 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!

7 ELEMENT, COMPOUND, OR MIXTURE???

8 QUICK CHECK Element, Compound, or Mixture? Element Compound Mixture Mixture Compound 1. Platinum Pt 2. Carbon Dioxide 3. Air O 2, N 2, and Ar CO 2 4. Brass Alloy of Cu and Zn 5. Glucose C 6 H 12 O 6

9 MOLECULES A molecule is 2 or more atoms chemically bonded Water-2 atoms of hydrogen and one atom of oxygen-together they form one molecule of H 2 O All compounds are molecules but not all molecules are compounds H 2 is a molecule, but not a compound H 2 O is both a molecule and a compound (notice the 2 different elements)

10 QUICK CHECK Which substances are molecules and which substances are both molecules and compounds? Molecule Both Both Molecule Both 1. O 2 2. CO 2 3. C 6 H 12 O 6 4. Cl 2 5. NH 3

11 CHEMICAL FORMULAS A shorthand way of representing compounds If chemical symbols are the letters, combined, these are the words. Ex: NH 3 = ammonia, C 3 H 7 OH = 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. O 2 -Oxygen H 2 -Hydrogen Cl 2 -Chlorine

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

13 COEFFICIENTS 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: 2H 2 SO 4 2 molecules of Sulfuric Acid Coefficient

14 COEFFICIENTS 2H 2 SO 4 = 2 molecules of Sulfuric Acid A coefficient is distributed to ALL elements in a compound To find the number of atoms: 2 H 2 (for a total of 4 H atoms) 2S (for a total of 2 S atoms) 2O 4 (for a total of 8 O atoms)

15 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. 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.

16 COVALENT BONDS The attraction of each atom s nucleus for the valence electrons of the other atom pulls the atoms together. 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. A covalent bond happens if the attractions are strong enough in both atoms and if each atom has room for an electron in its outer energy level. Atoms covalently bonded as a molecule are more stable than they were as separate atoms Examples of covalent bonds: H 2 (hydrogen), H 2 O (water), O 2 (oxygen), CH 4 (methane), and CO 2 (carbon dioxide). Covalent Bonds - animation

17 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? A sodium and chlorine atom are near each other. An electron from each atom feel the attraction from the other atom's nucleus. The attraction by chlorine is stronger than the attraction by sodium. 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.

18 METALLIC BONDS When two atoms share electrons, they form a covalent bond. When one atom takes an electron away from another and the resulting positive and negative ions are attracted to each other, those atoms have formed an ionic bond. 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)

19 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.

20 LET S MAKE MODELS OF BONDING!

21 STUDENT ACTIVITY INSTRUCTIONS: Materials: Molecular Model Kits 1 kit per group of 4 students Student Handout Formula cards 1 set per group of 4 students colored pencils periodic table Procedure: Students will practice reading formulas, counting atoms, building molecules, and identifying bond types. Quick tips: If a compound is made from a metal and a non-metal, its bonding will be ionic. If a compound is made from two non-metals, its bonding will be covalent.

22 WHEN DONE Place the materials back where instructed and clean up your table!