Physical Science Chapter 20: Chemical Bonds
Compounds • A compound is a combination of atoms from 2 or more different elements, in a definite ratio • The properties of a compound may be completely different from those of the elements which make it up. • Example: water (H2O)
Chemical Formulas • A short-hand way to write the name of a chemical compound is by using a chemical formula, which is like a "recipe" for a compound. • It tells what elements are in the compound, and how many atoms of each element.
The # of atoms of each element is shown using a subscript. • Example: Sulfuric acid, H2SO4 Hydrogen - 2 atoms Sulfur - 1 atom (understood subscript) Oxygen - 4 atoms
Molecule - smallest sample of a compound. • Molecular mass - the mass of a single molecule of a compound. • Ex. H2O (2x1) + (1x16) = 18
C6H12O6 is the chemical formula for a simple sugar. What is the molecular mass of this compound?
Chemical stability • Some elements do not join together with other elements to form compounds. These un-reactive elements are said to already be chemically stable. • All of the elements in G18 are this way.
The way that an element reacts with other elements is determined by it's outer shell electrons. (Outer shell electrons refers to the electrons that are farthest out from the nucleus in the electron cloud) • Atoms which react with each other do so by either gaining, losing, or sharing electrons with other atoms.
Think of it like an egg carton. It can only hold a certain number of eggs. And when the carton is full, then it doesn’t need any more.
Likewise, atoms of different elements can only have a certain # of electrons in their electron clouds, and if they are full then they are stable, and won’t react with other elements. • Elements with 8 o.s.e.’s, or a full first level (2 e-’s) are stable, and will not react.
However, elements with only a partially-filled outer shell may react with certain other elements.
An electron dot diagram can show how many outer shell electrons an atom has, and whether or not it can hold any more, and therefore whether that atom will react or not.
Electron Dot Diagrams • (Note: The only elements we will concern ourselves with now are the G1 & G2 metals, and the non-metals, since these are the most common reactive elements) • An element with 8 electrons in the outer shell is considered full, and stable (un-reactive). This is called the Octet Rule. Ex. He-2 ose's, Ne-8, Ar-8, Kr-8
A quick way to determine the # of outer shell electrons (G1, G2, and the non-metals) is to use the last # of the group #. For example: Mg G2 2 o.s.e.’s N G15 5 o.s.e.’s Cl G17 7 o.s.e.’s Kr G18 8 o.s.e.’s
A dot diagram shows an elements chemical symbol with 1 dot for each outer shell electron up to 8. So.....magnesium has 2 ose's, and would therefore have 2 dots surrounding it. Mg
Examples: H He Al C N O Cl Ne
Noble gases have 8 o.s.e.’s, which is why they are stable and un-reactive.
Chemical Bonds • When 2 atoms do react with each other to form a new substance, this new substance is held together with chemical bonds. A chemical bond is a force holding the atoms in a compound together. • Think of bonds like the connecting sticks in a Tinkertoy set.
Types of Bonds • There are 2 main types of chemical bonds: 1. Ionic - bonds created by the attraction of opposite charges (+, -) like a magnet 2. Covalent - bonds created by the sharing of electrons
Ionic Bonds • When 2 atoms react with each other, they both want to have a stable relationship (i.e. have 8 outer shell electrons) • Sometimes, one atom will "steal" electrons from another atom in order to become stable.
For example, sodium (Na) has 1 outer shell electron and chlorine (Cl) has 7. If chlorine were to gain 1 electron, then it would have 8, and be stable. So it "steals" sodium's electron to become stable. Sodium, however, now drops down an energy level and ends up with 8 outer shell electrons also. So everybody's happy.
When 1 atoms steals (gains) electrons, it will take on a negative charge since that is the charge of the electrons. • Likewise, the atom which gave up electrons will take on a positive charge, since it lost negatively charged electrons. • So………the 2 atoms now have opposite charges which attract each other, just like a magnet.
When the atoms join together, the positive and negative charges will cancel each other out, forming a neutral molecule. • Ions are individual atoms with a positive or negative charge due to gained or lost electrons • Ionic bonds form when the oppositely charged ions attract each other
Covalent Bonds • A covalent bond is formed when atoms combine together by sharing electrons. • Covalent bonds usually occur between 2 non-metals • Example: H has 1 o.s.e. and Cl has 7. By sharing, H will have 2 (stable) and Cl will have 8 (stable)
A diatomic molecule is one with 2 atoms of the same element covalently bonded. • Ex.: Cl2 (chlorine gas) O2 (oxygen gas)
Binary Compounds • A binary compound is one composed of atoms from exactly 2 different elements. • In order to determine the ratio between 2 elements which will produce a compound it is often useful to use oxidation numbers (numbers which show how many electrons are gained, lost, or shared when 1 atom combines with another).
Using oxidation numbers to determine chemical formulas. Oxidation numbers show the number of electrons gained, lost, or shared when an atom combines. Oxidation numbers are the same for elements in the same groups.* Ex.: G17 elements gain 1 electron when bonding, and thus have an ox. # of -1.
+1 +2 +3 +4 -3 -2 -1 0
The criss-cross method Oxidation numbers can be written as a superscript for each of the elements in a binary compound. Those numbers are then criss-crossed down and used as subscripts for the other element. • Example: Mg+2 N-3 Cross the +2 and the -3 down to the subscript of the other element, drop the signs, and you end up with Mg3N2.
Example #2: Calcium has an oxidation # of +2, chlorine is -1, what is the formula for a compound of calcium and chlorine?
Naming Binary Compounds • To name a binary compound, you simply say the name of the first element, and then add the suffix -ide to the root of the second element. • Example: Na + Cl = sodium chloride Al + N = aluminum nitride Ca + P = calcium phosphide
Toxic and Corrosive Materials • Many compounds are completely harmless to humans, while some are very harmful. • Corrosive materials attack and/or destroy metals, living tissue, and other materials. Acids are corrosive, although their relative strength will vary greatly.
Toxic materials are poisonous. • If toxic materials are ingested, vomiting is often induced to remove the materials from the body. This is usually not the case with corrosive materials. Why?
Sample Problems Write formulas, give names, and calculate molecular masses for each of the following compounds: Al + Cl Ca + O Na + N H + S K + F
The following covalently bonded diatomic molecules are listed in order from most stable to least stable. What do you think is the main determining factor of stability in these molecules? • He • N2 • O2 • Cl2
Identify 2 specific materials for each of the following: A. Corrosive B. Toxic C. Corrosive, but not toxic D. Toxic, but not corrosive (assume materials are at everyday common strengths)
Identify the following: -P3 element with 6 outer shell electrons -the lightest inert element -the lightest binary compound (17 a.m.u.) -the lightest diatomic molecule
Provide the following information for a binary compound composed of hydrogen and sulfur: • Compound name • Compound formula • Molecular mass
Give 1 example for each of the following: • A diatomic molecule which is very stable • A compound formula which reduces down to a 1:1 ratio • An element which loses 2 electrons when ionically bonding • An element which does not follow the Octet Rule
Physical Science Ch. 21: Chemical Reactions
A chemical reaction is where 1 or more substances are chemically changed into new substance(s) • Example: Changing hydrogen gas and oxygen gas into water
In order for a chemical reaction to take place, you need both reactants and products. • Reactants are the substances used to produce the reaction. • Products are the new substances formed as a result of the reaction.
Chemical Equations A short-hand way to show a chemical reaction by using formulas and symbols is called a chemical equation. Example: H2 + O2g H20 In a chemical equation, the reactant(s) are always on the left hand side, and the product(s) are always on the right.