Combinations of Atoms There are 92 naturally-occurring elements in the universe. Like letters of the alphabet, different atoms of different elements combine, or bond, to form different types of matter. Compounds are made up of two or more different elements bonded together. Compounds are different than the atoms they are made of.

Chemical Properties Depending on the elements in a compound, a compound may react with other compounds. Iron will rust when exposed to oxygen and water, but it won’t if it is melted and mixed with chromium and nickel metals to make stainless steel.

Bonds

Chemical Bonds Chemical bonds are forces that hold the atoms together in compounds. Bonds typically form when atoms share or transfer electrons.

Electron Energy Levels a)Atomic number = number of protons in the nucleus. Since protons are positive in charge, the nucleus attracts an equal number of negative electrons. b)Electrons vary in the amount of energy they possess, and they occur at certain energy levels. c)Electron energy levels determine how an atom behaves when it encounters other atoms.

Electrons are placed in energy levels according to rules: 1)The 1st energy level can hold up to two electrons. 2)The second energy level can hold up to eight. 3)After that some more complicated rules apply. You will learn these rules later. 4)The outermost energy level always attempts to stabilize with eight electrons. It does this by sharing or transferring electrons. 5)The electrons in the outermost energy level are called “valence electrons.”

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons in the last level. C would like to N would like to O would like to gain 4 electrons gain 3 electrons gain 2 electrons

Why are electrons important? Elements have different electron configurations. –A configuration is just an arrangement. –Different electron configurations mean different levels of bonding. –A carbon atom looks like this:

An Atom of Oxygen

1.Ionic bonds 2.Covalent bonds 3.Metallic bonds 4.Hydrogen bonds Chemical bonds happen when an atom attempts to fill its outside electron energy level. There are four main types of bonds:

Ionic Bonds An IONIC BOND is formed between two or more atoms by the transfer of electrons. Ionic bonds happen between a metal and a nonmetal. A good example is between sodium (Na) and chlorine (Cl). Sodium is a metal, chlorine is a nonmetal.

When sodium gives its electron to chlorine, it loses a negative and has one more positive proton than electrons. Chlorine gains the electron and has an extra negative charge. This makes sodium a positive ion and chlorine a negative ion. More on Ionic Bonds

Covalent Bond A COVALENT BOND forms by the sharing of electrons in the outermost energy level of two or more atoms. Covalent bonds are between two or more nonmetals.

Metallic Bond A METALLIC BOND is between atoms of metals. A metallic bond holds metal atoms together very strongly. Electrons flow around many nuclei of atoms, acting like sheepdogs keeping sheep together.

Metallic Bond, A Sea of Electrons

Hydrogen Bonds Hydrogen bonds do not involve the sharing or transfer (giving) of electrons. Hydrogen bonds occur because of unequal sharing of electrons. Water molecules have “polar” bonds because of this.

How water attracts other water molecules because of polarity

Hydrogen Bonding in Water gilbert2/tutorials/chapter_10/water_h_bond/

-about-snowflakes-a-flurry-of-ideas-for- science-and-math-integrationhttp://beyondpenguins.ehe.osu.edu/teaching -about-snowflakes-a-flurry-of-ideas-for- science-and-math-integration Hydrogen Bonding in Ice

A mixture is two or more elements or compounds put together, but not bonded. –Heterogeneous –Homogeneous –Solution Mixtures

Hetero means “different.” Heterogeneous mixtures are not mixed evenly and can be fairly easily separated. The different parts can be seen. Examples –Vegetable stew –A bowl of Halloween candy –A group of people Heterogeneous Mixtures

Homo means “same.” Homogeneous mixtures are mixed evenly throughout. The different parts cannot be easily told apart from one another. Examples –Salt water –Air Homogeneous Mixtures

Solutions are homogenous mixtures. They are often liquids. Alloys of metals are also considered solutions, even though they are solids. Alloys are different metals mixed, not bonded, together when melted. Solutions

Mixtures can be separated by sifting, evaporating, or by hand. Mixtures are between elements or compounds. Examples –Sugar and salt (C 6 H 12 O 6 mixed with NaCl) –Salt and pepper These methods are physical. Separating Mixtures

Compounds must be separated by breaking bonds between atoms. This method is chemical. Examples –Fire –Digestion –Breathing/respiration –Photosynthesis Separating Compounds