CHEMICAL CHANGES

What is a Chemical Change?Molecules and CompoundsChemical Equations and Chemical ReactionsKinds of ReactionsKinds of BondsRustFire

CHEMICAL CHANGES

When Physical Changes happen, there is no change in the atoms or molecules. Changes occur in how molecules are connected and in their separation distance.

Chemical changes occur when atoms enter into different combinations with each other, electrons are exchanged, but the atoms themselves do not change.

Nuclear changes occur when the nucleii of atoms change; neutrons and protons are added, lost or changed.

MOLECULES

Molecules are compounds made up of specific combinations of atoms. Familiar substances may theoretically be divided into single molecules, as modeled here, but no further. Like a strict recipe in which atoms are the ingredients, each molecule has a chemical formula. If any ingredients are subtracted or changed, the molecule becomes something completely different.

CHEMICAL REACTIONS

•A chemical reaction preserves the number of atoms and the total mass involved but redistributes the materials into new arrangements.

•For example, a yellow solid precipitate, lead iodide (PbI2) forms from the mixture of two clear liquids, potassium iodide (KI) and lead nitrate (Pb(NO3)2).

CHEMICAL COMBINATIONS

Elements that do not have a noble-gas configuration (a stable configuration) try to attain such a configuration by entering into chemical reactions.

Stable molecules are formed when atoms combine so as to have outer shells holding eight electrons.

If atoms are no more than a few electrons away from a stable configuration, they generally attain it by losing or gaining electrons to form electrically charged particles called ions.

Positively charged ions (formed by a loss of electrons) are called cations, and negatively charged ions (formed by an electron gain) are called anions.

Ions seldom have a charge greater than three, which means that atoms seldom gain or lose more than three electrons.

FAMILIES and the PERIODIC TABLE

The electrons in the outer shell of an element are called valence electrons. Valence electrons are those electrons that are available to form bonds with other atoms.

Groups of elements with similar electron configurations (arrangements of electrons in their orbitals) behave in a similar way in chemical reactions, so these groups have similar chemical and physical properties. These groups of elements are called families.

The periodic table shows how elements can be grouped into families. Elements with atoms that have one valence electron are in Group I; elements with two valence electrons are in Group II; elements with six are in Group VI; and elements with seven are in Group VII.

Inert GasesAlkali Metals

Alkaline Earth Metals Nonmetals Halogens

Transition Elements (Heavy Metals)

Lanthanide Series

Actinide Series

COMPOUNDS Salt, water, iron rust, and rubber are examples of compounds. A compound is made up of elements, but it looks and behaves quite

differently, as a rule, from any of its component elements. Iron rust, for example, does not look and feel like its components: oxygen gas and iron metal. Some synthetic fabrics, with fibers made from coal, air, and water, do not feel at all like any of the components that make them up. This individuality of properties, as well as other qualities, distinguishes a compound from a simple mixture of the elements it contains.

Another important characteristic of a compound is that the weight of each element in the compound always has a fixed, definite ratio to the weight of the other elements in the compound. For example, water always breaks down into 2.016 parts of hydrogen by weight to 16.000 parts of oxygen by weight, which is a ratio of about 1 to 8, regardless of whether the water came from the Mississippi River or the ice of Antarctica. In other words, a compound has a definite, invariable composition, always containing the same elements in the same proportions by weight; this is the law of definite proportions.

WATER IS A COMPOUND

•Water is an example of a compound.

•A water molecule consists of an oxygen atom and two hydrogen atoms.

KINDS of REACTIONS

Reactions can be classified as:ionic reactions (combining ions to form

insoluble products, or products that won’t dissolve);

oxidation-reduction reactions (involving the transfer of electrons); and

electron-sharing reactions (involving the rearrangement of covalent bonds).

CHEMICAL EQUATIONS

Chemical reactions can be expressed through equations that resemble mathematical equations.

The reactants (the substances that are combined to react with one another) appear on the left side of the equation, and the products (substances produced by the reaction) are written on the right side of the equation.

The reactants and products are typically connected by an arrow

BALANCING EQUATIONS The reaction between sulfuric acid (H2SO4) and sodium hydroxide

(NaOH) to form sodium sulfate (Na2SO4) and water can be written: NaOH + H2SO4 -->H2O + Na2SO4.

This is an incomplete equation, since the same number of atoms does not appear on both sides of the reaction (for example, 1 sodium atom appears in the reactants, but 2 sodium atoms appear in the products). Such a reaction could not actually occur.

Correcting this fault is called balancing the equation. Placing a 2 in front of the NaOH balances the number of sodium atoms, as well as the hydrogen and oxygen atoms. The balanced equation is: 2NaOH(aq) + H2SO4 -->2H2O(l) + Na2SO4(aq).

This equation implies that two molecules of NaOH are necessary to react with each molecule of H2SO4. An equation must be balanced before a chemist can make calculations based on it.

METALS The structure of the atom, in particular the configuration of the electron cloud,

is responsible for the obvious physical differences between metals and nonmetals.

Metals have a characteristic luster, are opaque, can be hammered and drawn into various shapes, and conduct electricity. Nonmetal elements, on the other hand, are often gases, and, if solid, nonmetals are generally brittle, sometimes transparent, and do not conduct electricity.

The atoms of metals have outer shells that contain few electrons and are nowhere near filled (and therefore lack the stability of a noble gas). As a result, all metals tend to easily lose some of these outer electrons.

This means, chemically, that metals tend to form positively charged ions, or positively charged atoms or molecules, when they enter into chemical combination.

Physically, the fact that the outer shells of metal atoms are unfilled means that these “loose” electrons can flow and enable metals to conduct electricity; this fact also accounts for the mechanical properties of metals.

METALLIC BONDS

•Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negative electrons distribute themselves throughout the entire piece of metal and form non-directional bonds between the positive silver ions.

•This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: They are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by non-directional forces.

NON-METALSNonmetals, have outer shells that are nearly filled up

to the stable grouping of eight electrons. In their chemical reactions they tend to add electrons

to achieve the state of a stable noble gas. By adding electrons, nonmetals form negatively charged ions.

They can also add electrons by sharing them with another atom and forming a covalent bond.

The noble gases, with exactly eight electrons in their outer shells (two in the case of helium), are nonmetals.

NONMETALS

NON-METALS• Nonmetals, have outer shells that are nearly

filled up to the stable grouping of eightelectrons.

• In their chemical reactions they tend to addelectrons to achieve the state of a stable noblegas. By adding electrons, nonmetals formnegatively charged ions.

• They can also add electrons by sharing themwith another atom and forming a covalentbond.

• The noble gases, with exactly eight electrons intheir outer shells (two in the case of helium),are nonmetals.

IONIC BONDS

•The bond between the atoms in ordinary table salt (sodium chloride) is a typical ionic bond. In forming the bond, sodium becomes a cation (a positively charged ion) by ‘giving up’ its valence electron to chlorine, which then becomes an anion (a negatively charged ion).

•This electron exchange is reflected in the size difference between the atoms before and after bonding. Attracted by electrostatic forces, the ions arrange themselves in a crystalline structure in which each is strongly attracted to a set of oppositely charged “nearest neighbors” and, to a lesser extent, all the other oppositely charged ions throughout the entire crystal.

COVALENT BONDS Another common type of bond, the covalent bond, results when

two atoms share one or more pairs of electrons in an attempt to fill their outer shells and become more energetically stable. The atoms are held together by the mutual electrostatic attraction between the protons in their nuclei and these electrons. The bonded atoms form a stable unit called a molecule.

For example, because a chlorine atom is one electron short of completing its outer shell (and attaining a noble-gas configuration), two chlorine atoms combine to form a chlorine molecule by sharing two electrons. The atoms thereby complete each other’s outer shell: Cl + Cl →Cl2. Electron sharing distinguishes a covalent bond from an ionic bond in which the electrostatic attraction of the ions results in bonding.

IONIC vs COVALENT BONDS

Covalent bonds tend to form when the bonded atoms have nearly the same attraction for electrons.

Ionic bonds form when the electron-attracting power of the atoms differs markedly.

ACIDS, BASES, and SALTS Acids are compounds (or ions) that react with water

to produce hydrogen ions (H+). Hydrogen ions account for the characteristic

properties of strong acids, such as a sour taste and the ability to react with bases.

Bases are compounds that yield the hydroxide ion (OH-) in water solutions.

Salts are ionic compounds that are generally formed by the reaction of an acid and a base

COMBINING with OXYGEN:RUST

• Oxidation, in its original sense, refers to the combination of oxygen with another substance to produce a compound called an oxide.

• Iron, in the presence of water, combines with atmospheric oxygen to form a hydrated iron oxide, commonly called rust.

Combining with Oxygen

FIRE:• Fire is heat and light resulting from the

rapid combination of oxygen with other materials.

• The light is in the form of flame, which is composed of glowing particles of the burning material and certain gaseous products that are luminous at the temperature of the burning material.

• The conditions necessary for the existence of fire are:

– the presence of a combustible substance, a temperature high enough to cause combustion (called the ignition temperature) and the

– presence of enough oxygen (usually provided by the air)to enable rapid combustion to continue.