Unit+5

= Periodic Table =

**Historytoc**
With so many elements already found and the possibility of more being discovered, chemists needed a way to organize them. Many systems were tried in order to make some sort of pattern in their properties to match the table. The modern periodic table, based on atomic number and electron configuration, was created primarily by a Russian chemist, Dmitri Mendeleev, and a German physicist, Julius Lothar Meyer, both working independently. They both created similar periodic tables only a few months apart in 1869. Mendeleev created the first periodic table based on atomic weight. He observed that many elements had similar properties, and that they occur periodically, hence the name, periodic table. From this, he made the periodic law. His **periodic law** states that the chemical and physical properties of the elements vary in a periodic way with their atomic weights. The modern one states that the properties vary with atomic number, not weight. Elements in Mendeleev's table were arranged in rows called **periods**. The columns were called groups or families. Elements of each group had similar properties. By Mendeleev's theory, they should have been perfectly arranged by increasing atomic weight. There were some inconsistencies in the arrangement of the elements according to his law, however it wasn't until the early 1900's (1914) that a Prof Henry Moseley, a British Physicist, was able to determine the atomic numbers of all the known elements using an experimental technique. Moseley then proceeded to rearrange the elements according to increasing atomic numbers. Moseley's arrangement seemed to clear up the contradictions and inconsistencies of Mendeleev's arrangement. Moseley's periodic law is now considered the current Periodic Law. The Periodic Table of the Elements is constructed and arranged so that similar chemical properties were arranged in vertical columns called **groups**. Because chemical properties are based on electron configurations we can use the table to predict chemical properties for elements.

Periodic Table of the Elements
Group 1 is also called the **alkali metal** family. These are strong metals that are unusually soft, silver in color, and very reactive toward oxygen and water. These elements are so reactive toward oxygen and water vapor that they are stored under an inert liquid to protect them from oxygen and water vapor. Group 2 is called the **alkaline earth metals**. These metals are not as soft as Group 1 metals. They also react more mildly with oxygen and only react with water at temperatures where the water is steam. Groups 3-12 are referred to as the **transition metal** groups. These metals are not as predictable because of the shielding effect of the inner electrons. Group 13 is referred to as the **boron** **family**. Group 14 is referred to as the **carbon family**. Group 15 is referred to as the **nitrogen family.** Group 16 is referred to as the **oxygen family**. Group 17 is referred to as the **halogen** **family**. Group 18 is referred to as the **Noble gas** group previously known as the inert gas group.

=Compounds= With just over 100 elements, how is it possible for there to be so many different forms of matter? Think of the thousands of words that you use. You know that there are only 26 letters in the English alphabet. The letters are used in different combinations to make up all the letters that we use. In a similar way, the elements combine to form matter. When two or more elements chemically combine, it is called a **compound**. The physical and chemical properties of a compound are very different from those of the elements that make it up. Compounds are made up of molecules. A molecule is made up of two or more atoms chemically combined. A **molecule** is the smallest particle of a compound that can exist that has all the properties of that compound. Just as all atoms of an element are alike, all molecules of a compound are alike. A compound has an identity all its own. It may be much different from the elements that form it. C6H12O6 - sugar Sugar, a white solid crystal, is a compound made from carbon, hydrogen, and oxygen. None of these elements are sweet. Carbon is black. Hydrogen and oxygen are gases. The sweetness and white color of sugar are its own properties.
 * Chemical formulas** are often used to represent compounds. In chemical formulas, symbols show which elements are present. Chemical formulas are like a recipe. They recipe lists the ingredients and how much of each is necessary to make the compound. Elements in a given compound are always combined in the same ratio. For example, H2O is the chemical formula for water. The ratio between hydrogen and oxygen in salt is always 2 to 1. This means that there are two hydrogen atoms combined to one oxygen atom. Notice how subscripts are used to indicated how many atoms are present in one molecule of a compound. Subscripts are written as small numbers to the right and slightly lower than the elements symbol. Also notice that one is implied and not written.

Naming Compounds
Compounds are given names that come from the names of their elements. Naming compounds can become very complex with allot of different rules depending upon what type of bond is created. however, there are basic rules that you can use to name most of the more common compounds. All compounds have two names. The ion with the positive charge is always written before the ion with the negative charge. (as they are found on the periodic table from left to right.) The first name of the compound comes from the first element in the formula. A subscript after the first element does not generally affect its name. The second part of the name can become a little more involved. The second name follows these rules: a. The second word tells you the second element. b. The second word ends in IDE, that's the suffix. c. The second word tells you how many atoms there are in the compound. If there is subscript, then you must add a prefix to the section at the beginning of the word. For example using oxygen
 * First name**
 * Second name**
 * Subscript || Prefix || Name ||
 * 1 || mono || monoxide ||
 * 2 || di or bi || dioxide ||
 * 3 || tri || trioxide ||
 * 4 || tetra || tetraoxide ||
 * 5 || penta || pentoxide ||
 * 6 || hexa || hexoxide ||

If the compound has more than two elements, the second name is one of the following: These names are called **p****olyatomic ions**. Polyatomic ions are groups of atoms that act as if they were one element. When naming compounds that contain polyatomic ions, you will sometimes see the polyatomic ion in parentheses and a subscript written outside. To correctly name these compounds, the numbers inside parentheses must match the number given in the rule. The numbers outside the parentheses do not affect the name. Second name- //chlorine// Compound name - //potassium chloride// || Second name- //hydroxide// Compound name - //barium hydroxide// || Second name- //chlorine// Compound name - //aluminum trichloride// || Second name- //bromine// Compound name - //magnesium dibromide// || Second name- //carbonate// Compound name - //calcium carbonate// ||
 * NO3 || Nitrate ||
 * OH || Hydroxide ||
 * PO4 || Phosphate ||
 * CO3 || Carbonate ||
 * SO4 || Sulfate ||
 * KCl || First name - //potassium//
 * Ba(OH)2 || First name - //barium//
 * AlCl3 || First name - //aluminum//
 * MgBr2 || First name - //magnesium//
 * CaCO3 || First name - //calcium//

=Chemical Bonds= Chemical properties of an elements are basically how different elements bond to each other to form compounds. The valence shell or the outer shell (the shell with the highest energy level) is the one mainly responsible for how an element reacts to form compounds. The core electrons are the rest of the electrons, and they are buried deep within the atom and usually do not play a role in chemical reactions. The **octet rule** says that atoms tend to gain, lose or share electrons so as to have eight electrons in their outer electron shell. It is a very useful rule but there are //many// bonding situations where it does //not// apply. The noble gases are the elements that have filled their orbitals completely. All atoms would like to have the same number of valence electrons as a Noble gas because of their great stability. They appear on the column on the far right side of the periodic table, and they include helium, neon, argon, krypton, xenon, and radon.

Covalent bonds are formed as a result of the sharing electrons. These usually occur when non-metal atoms form compounds. The dash is symbolic of the bonding pair.
 * Covalent Bonds**

A single covalent bond would be the sharing of two electrons between the two bonded atoms. Examples are:

H-H H-Cl F-F

A double covalent bond is two pairs of electrons being shared. Examples are:

O=O C=O C=C

An ionic bond is the force of attraction between oppositely charged ions. Ions are charged particles that form during chemical changes when one or more valence electrons transfer from one atom to another. Na Cl Positive and negative charges attract one anther. This attraction is called electrostatic attraction. An ionic bond results from the electrostatic attraction between oppositely charged ions. The resulting compound is called an ionic compound. Even though the ions that bond are charged, the compound formed is neutral because the charges of the ions cancel each other through ionic bonding. The sum of the charges in a compound is always zero! Ca combines with Cl to form an ionic compound. 2 electrons are lost from Ca. This gives Ca a 2+ charge. Since Cl needs only one electron to complete its outer shell, 2 Cl atoms are need for each Ca2+ atom. (Cl -). Thus we get the chemical formula CaCl2 for the compound formed. [ [|Back to the Top] ]
 * Ionic Bonds**

=Chemical Reactions= When a chemical reaction occurs, chemical bonds between atoms or ions break and new bonds form between different atoms or ions. A chemical reaction creates one or more new substances. The new substance has properties that are different from the properties of the original substances. A //chemical equation// is an expression that uses symbols to describe a chemical reaction. An equation is like a sentence in chemical terms. The sentence begins at the left with the formulas for the starting materials. These are called the //reactants//. They are the substances that undergo a chemical change. NaOH + HCl NaCl + H2O An arrow connects the two sides of the equations. The arrow is called a //yield// sign. Sometimes it is pronounced "produces". The sentence ends with the formulas for the new substances formed by the reaction. These substances are called the //products//. AgNO3 + NaCl AgCl + NaNO3 Most chemical reactions can be divided into four main types Fe + S FeS HgO Hg + O  BeF2 + Mg  MF2 + Be  AgNO3 + KCl  AgCl + KNO3 [ [|Back to the Top] ]
 * 1) synthesis
 * 2) decomposition
 * 3) single replacement
 * 4) double replacement
 * Synthesis** - means to put together or combine
 * Decomposition** - to decompose or break down
 * Single Replacement** - the reactants are an element and a compound. The products are a different element and a different compound. One element takes the place of an element in a compound.
 * Double Replacement** - two compounds react to form two new compounds.

=Balancing Equations= During chemical reactions, substances are changed into new and different substances through a rearrangement of their atoms. By using chemical formulas, you can describe a chemical reaction. The description of a chemical reaction using symbols and formulas is called a **chemical equation**. When charcoal burns in a BBQ grill, carbon atoms combine with oxygen molecules in the air to form carbon dioxide. Carbon atoms plus oxygen molecules produce carbon dioxide. The chemical equation for this reaction is C + O2 CO2 The arrow is read **yields**. The substances on the left of the arrow in an equation are called **reactants**. The substances on the right of the arrow in an equation are called **products**. According to the **law of conservation of mass**, the mass of all substances before a reaction equals the mass of all substances after the reaction. Equations are balanced to show that atoms are neither lost nor created during a chemical reaction. Atoms are just rearranged. Na + Cl NaCl When you write a chemical equation, the total number of atoms on each side of the yield symbol must be the same. This is called **a balanced equation**. Chromium dioxide is commonly found as a coating on magnetic recording tape. The reaction is carried out under high temperatures and in the presence of a magnetic field to produce the magnetic coating. The chemical equation for the reaction that forms chromium dioxide is 2 CrO3 2 CrO2 + O2  Notice in this equation that the numeral 2 precede the formula for chromium trioxide. This numeral is the coefficient. A **coefficient** is a numeral in a chemical formula that represents the number of molecules. This in this equation, two molecules of chromium trioxide yield two molecules of chromium dioxide and one molecule of oxygen. To balance a chemical reaction, count the number of atoms of each element in both the reactants and the products. Then determine the numbers that, when multiplied by the number of atoms in the reactants or products, will make the number of atoms on either side of the arrow equal. Next check the equations by counting the number of atoms in the reactants and the products. If the equation is balanced, the number of atoms on each side will be the same. Zn + HBr H2 + ZnBr2 In a balanced chemical equation, there are always an equal number of atoms on each side of the equation. For example, if there are a total of 6 nitrogen atoms on one side of an equation, there must be a total of 6 on the other side. Mg +O2 2MgO To balance this equation, you simply write a 2 in front of Mg on the left side of the equation. The balanced equation is 2Mg + O2 2MgO To balance an equation you may change the numbers in front of any chemical. Keep in mind, however, that you may not change the subscript on any chemical formula. [ [|Back to the Top] ]