Chapter 20 Chemical Bonds

Stability In Bonding Roughly 110 elements on the periodic table can form a nearly infinite number of compounds Elements bond with each other, seeking stability Only one group of elements (Noble Gases) are stable without bonding All others must bond with either another atom of the same element or a different element to be stable

Combined Elements Elemental copper is a shiny metal The Statue of Liberty is copper, why is she a pale green color? The copper that the statue is made of reacted with oxygen and sulfur to form a compound called copper sulfate, that has its own properties

Combined Elements Sodium (Na) is a soft, silvery metal that is highly reactive– it actually catches fire in the presence of oxygen Chlorine (Cl) is a green poisonous gas. Together, they make a compound that you can put on your food, table salt, NaCl Table salt’s compound name is Sodium Chloride

Chemical Formulas A chemical formula tells what elements are in a compound, and in what numbers Subscripts (a number written below) show how many atoms are in each compound. If there is no subscript, you can assume it is 1 Example, H2O, water, has two atoms of Hydrogen and one atom of oxygen

Some Familiar Compounds

Atomic Stability An atom is considered stable if its outer shell electrons are full This is considered an “Octet”, because 8 electrons fill the outer shell of some atoms Outer shell electrons are called “valence” electrons and are the electrons that are involved in bonding

Dot Diagrams Uses dots around the chemical symbol to show number of valence electrons

Dot Diagrams, Cont Write the symbol of the element Look at the periodic table and determine the number of outer shell, or valence electrons Start on the top, put one dot on each side as you move clockwise Exceptions: He (helium) has a full shell at two electrons, so put those dots on top Group 1: Alkali metals (including H) put the one dot to the right

Stability is Reached Atoms form chemical bonds with each other to reach chemical stability– to fill up their outer shells They will either give away, take, or share electrons to do this

Types of Bonds When atoms gain or lose an electron, they become an ion A positive ion, such as Ca +2, gives away 2 electrons A negative ion, such as Chlorine -1, gains one electron A bond where electrons are transferred, (given or taken) is called an IONIC bond

Ionic Bonds Ionic bond is the force of attraction between opposite charges in an ionic compound For the most part, ionic bonds are formed by elements that are far apart on the periodic table A metal and a non-metal Metals tend to lose electrons Non-metals tend to gain electrons Once ionic bonds are formed, the resulting compound has a net charge of zero

Sharing Electons Non metals, when bonding with other non metals, are unlikely to gain or lose electrons The attraction that forms between atoms when they share electrons is called a covalent bond A neutral particle that forms due to a covalent bond is called a molecule

Types of Covalent Bonds Single covalent bonds share one pair of electrons, example is water H2O this is called a single bond A multiple covalent bond shares two or three pairs of electrons, called a double or a triple bond, example N2 Covalent bonds form between non-metals Many covalent compounds are liquids or gases at room temperature

Bond Types

Polar vs Non-Polar Polar covalent compounds are molecules that have ends that have partial opposite charges This means that the electrons are shared unevenly One atom has a greater “pull” on the shared electrons than another A non-polar compound mean that the electrons are shared evenly with no partial charges

Polar vs. Non Polar Carbon Tetrachloride is non-polar Water is Polar

Diatomic Atoms Some atoms are so reactant that they can’t exist as only one atom They must bond with themselves in order to reach stability They are called “diatomic” which literally means “two atoms” The diatomic atoms are: Nitrogen, Oxygen, Fluorine, Chlorine, Iodine, Hydrogen, Bromine Magic 7- they make a 7 and then add hydrogen They are all written with a subscript: Cl2

Diatomic Atoms

Binary Ionic Compound A binary ionic compound is one that consists of two elements bonded together where electrons are transferred You need to know what elements are involved and how many electrons are gained or lost The element’s OXIDATION NUMBER tells how many electrons are transferred when ions are formed Oxidation numbers are often referred to as “charge”

Oxidation Numbers

Special Ions- have more than one oxidation number

Compounds are Neutral When compounds are formed, their net charge must be zero Therefore, when you are putting compounds together and writing their formulas, their oxidation numbers must equal zero Sodium (Na +1) and Chlorine (Cl -1) together are NaCl Magnesium (Mg +2) and Chlorine (Cl -1) together are MgCl2 It takes two chlorine atoms to cancel out the +2 charge of Mg to zero Criss-Cross method

Naming Ionic Compounds Step 1: Write the name of the positive ion Step 2: Determine if the positive ion has a special oxidation number (refer to table 2.) If it does, you’ll have to determine its oxidation number and write roman numerals after the name Step 3: Write the root of the negative ion (chlor-, ox-, phosph-) Add –ide to the end of the root of the negative ion

Naming Ionic Compounds NaCl Step one: Sodium Step two: not needed, it only has one possible oxidation number Step three: Sodium Chlor- Step four: Sodium Chloride Now, you do MgI

Naming Compounds with polyatomic ions A polyatomic ion is a special ion that is made of many atoms Look at the chart to the right To write the names of a special compound with polyatomic ion, you write the name of the positive ion and then the name of the polyatomic ion Ex K2SO4 is potassium sulfate

Naming Covalent Compounds Naming covalent compounds uses prefixes that tell how many of the atoms are present in the molecule Often the prefix for one, mono is omitted but is used for emphasis in some cases

Naming Covalent Compounds CO is Carbon Monoxide CCl4 is Carbon Tetrachloride H2O is Dihydrogen Monoxide Your Turn NO2 N2O5