•Compounds and Chemical Change

• This is a scanning tunneling electron microscope image of DNA, showing the actual grooves of the double helix. The scale is in Angstroms (1 Angstrom = 10-10m), and each twist of the helix is about 35 Angstroms. A molecule like this one determined the characteristics that other people recognize as you.

• An atom is the smallest unit of an element that can exist alone or in combination with other elements.

• A molecule is the smallest particle of a compound, or a gaseous element, that can exist and still retain the characteristic chemical properties of a substance.

• (A)The noble gases are monatomic, occurring as single atoms. (B) Many gases, such as hydrogen and oxygen, are diatomic, with two atoms per molecule. (C) Ozone is a form of oxygen that is triatomic, occurring with three atoms per molecule

• A chemical reaction is a change in matter in which different chemical substances are created by forming or breaking chemical bonds.– Chemical bonds are formed when atoms of elements bind

together to form compounds.– Chemical bonds are broken when a compound is

decomposed into simpler substances.

• (A)New chemical bonds are formed as a green plant makes new materials and stores solar energy through the photosynthesis process. (B)The chemical bonds are later broken, and the same amount of energy and the same original materials are released. The same energy and the same materials are released rapidly when the plant materials burn, and they are released slowly when the plant decomposes.

• Magnesium is an alkaline earth metal that burns brightly in air, releasing heat \and light. As chemical energy is released, a new chemical substance is formed. The new chemical material is magnesium oxide, a soft powdery material that forms an alkaline solution in water (called milk of magnesia).

• A chemical equation is a shorthand representation of a chemical reaction.– The substances that are changed in to reaction are called

the reactants.– The substances that are formed in the reaction are called

the products.– chemical reactions and energy flow can be explained by

the making or breaking of chemical bonds.

-- chemical bonds can be explained in terms of changes in the electron structure of atoms

• Valence Electrons and Ions

• Valence electrons are those electrons that occur on the outer shell of atoms.– These are the electrons that are responsible for chemical

reactions– Inner electrons are in lower energy levels and the orbitals

are filled.• These electrons are therefore not available for interactions with

other electrons.

• The Octet Rule– Atoms attempt to acquire an outer orbital with eight

electrons through chemical reactions (octet rule)– This gives them an outer shell configuration like their

nearest noble gas and therefore they become stable.– From the family number of the representative elements,

you can determine the number of valence electrons, and therefore the number of electrons necessary to gain the stable configuration

– Sodium is in Group IA and therefore has 1 electron in its outer shell.• If sodium loses one electron if becomes Na+ and has 8

electrons in its outer shell.• It then has the electron configuration of Ne (neon) and

has a filled outer shell configuration.

• (A)A sodium atom has two electrons in the first energy level, eight in the second energy level, and one in the third level. (B) When it loses its one outer, or valence, electron, it becomes a sodium ion with the same electron structure as an atom of neon (C).

• Chemical Bonds

• Introduction– Atoms gain or lose electrons through chemical reactions

to gain a filled outer shell and a state of lower energy.– A chemical reaction forms a chemical bond that is an

attractive force that holds atoms together in a compound.– Ionic bonds are formed when an atom transfers an

electron to another atom during a chemical reaction• The opposite charges resulting forms an electrostatic attraction

between the ions that are formed.

• formed between a metal on the left of the table and a

nonmetal on the right side of the table.

– Covalent bonds form when atoms share electrons in a chemical bond. (generally between nonmetals)

– Metallic bonds form in metals. (accounts for high

electrical conductivity)

These new bonds form compounds which can be described in two ways:

– Molecular orbital theory describes the electrons as belonging to the whole molecule which gives the orbital its own shape, orientation, and energy levels.

– Isolated atom description considers the electrons around the atoms as being isolated from the rest of the molecule.

• Ionic Bonds– Ionic bonding occurs when one atom transfers an

electron to another atom• The difference in electrical charge results in an electrostatic

attraction between unlike electrical charges

• This occurs when a metal reacts with a nonmetal

• Sodium chloride crystals are composed of sodium and chlorine ions held together by electrostatic attraction. Each sodium ion is surrounded by six chlorine ions, and each chlorine ion is surrounded by six sodium ions. A crystal builds up like this, giving the sodium chloride crystal a cubic structure.

• You can clearly see the cubic structure of these ordinary table salt crystals because they have been magnified about ten times.

– Energy and Electrons in Ionic Bonding• Example:

– energy + Na Na+ + e-

– Cl + e- Cl- + energy– Na+ + Cl- Na+Cl- + energy

• The energy that is released in steps 2 and 3 is greater that that absorbed in step one and an ionic bond is formed.

– This energy is called the heat of formation.– It is also the amount of energy required to

decompose the compound (sodium chloride)

• Two rules for keeping track of electrons in ionic bonding reactions.

– Ions are formed when atoms gain or lose electrons to achieve a noble gas configuration

– The number of electrons that are lost must equal the number of electrons that are gained.

– Electrons are not created or destroyed in a chemical reaction

– Ionic Compounds and Formulas• The formula of a compound describes what elements are in the

compound and in what proportions.• Compounds that are held together by ionic bonds are called

ionic compounds.• The elements in Group IA and IIA tend to lose electrons for

form positive ions, the ion charge is equal to the family number• The elements in Group VIA and VIIA tend to gain electrons to

form negative ions, ion charge equals 8 minus family number.• The elements in families IIIA and VA have less tendency to form ionic compounds, except for those in higher periodsPolyatomic ions are charged groups of atoms that actlike a singe unit in ionic compound. These are held together bycovalent bonds.

• Covalent Bonds– A covalent bond is a chemical bond that is formed when

two atoms share a pair of electrons.– H. + H. H:H

– Covalent Compounds and Formulas• Since a pair of electrons is shared in a covalent bond, the

electrons move throughout the entire molecular orbital.

• In the above example, since both hydrogen share the electron pair, each hydrogen has a filled valence shell, since it has the electron configuration of helium.

• Compounds that are held together by covalent bonds are called covalent compounds.

• Covalent compounds form from atoms on the right side of the periodic table

• Families IVA through VIIA, the number of unpaired electrons

(number of covalent bonds) is eight minus family number.

• (A)Two hydrogen atoms, each with its own probability distribution of electrons about the nucleus. (B) When the hydrogen atoms bond, a new electron distribution pattern forms around the entire molecule, and both electrons occupy the molecular orbital.

– Multiple Bonds.

• In electron dot notations, a pair of electrons can be represented by a pair of dots : .

–This can be a bonding pair or a lone pair (non-bonding pair).

• Bonding pairs can also be represented by lines connecting atoms.

H:H = H—H

• When one pair of electrons is shared, it is called a single bond.

H-H

• When two pairs of electrons are shared it is called a double bond.

• H H \ / C=C / \ H H

• When three pairs of electrons are shared it is called a triple bond.

–H-C=C-H

• Acetylene is a hydrocarbon consisting of two carbon atoms and two hydrogen atoms held together by a triple covalent bond between the two carbon atoms. When mixed with oxygen gas (the tank to the right), the resulting flame is hot enough to cut through most metals.

– Coordinate Covalent Bonds• A coordinate covalent bond is one in which the

electron pair comes from one atom.

H+ + NH3 NH4+

H H .. .. H+ + :N:H H:N:H .. .. H H

• In this example both electrons from the new covalent bond come from the lone pair (non-bonding electrons) around the nitrogen.

Bond Polarity

• Electronegativity.– Electronegativity is the ability of an atom to attract

bonding electrons.– Elements with higher values have the greatest attraction

for bonding electrons.– The difference in electronegativity can be used to predict

whether a bond will be ionic or covalent.• If the absolute difference is 0.5 or less, the bond will be

covalent.

• If the absolute difference is 1.7 or more the bond will be ionic.

• Electronegativities of the elements. These values are comparative only, assigned an arbitrary scale to indicate the relative tendency of atoms to attract shared electrons.

– When the absolute difference is between 0.5 and 1.7 a covalent bond is formed, but one in which the electron pair is not shared equally.

– This type of bond is called a polar covalent bond.– A polar covalent bond results in areas of partial positive

charge and areas of partial negative charge since the electrons spend more time around the more electronegative atom.

– The poles are created by an uneven charge distribution,

not an imbalance of electrons and protons.

• The absolute difference in electronegativities determines the kind of bond formed.

• Composition of Compounds

• These substances are made up of sodium and some form of a carbonate ion. All have common names with the term "soda" for this reason. Soda water (or "soda pop") was first made by reacting soda (sodium carbonate) with an acid, so it was called "soda water."

• Ionic Compound Names– Ionic compounds that are formed from metal ions are

named by naming the metal ion (electropositive ion) first, followed by the nonmetal (electronegative ion)

– The ending of the nonmetal is changed to end in -ide– When a metal can have various oxidation states the

oxidation state is give by roman numerals in parenthesis after the name of the metal. Older naming added –ic &

-ous to the end of the name of the metal, for the higher and lower of two possible charges.

-- some ionic compounds have polyatomic ions, thereby

containing 3 or more elements. ( Metal hydroxides)

(Salts: -ate, -ite, per, hypo)

• Ionic Compound Formulas– Rules

• The symbol for the positive element is written first, followed by the symbol of the negative element

• Subscripts are used to indicate the numbers of ions needed to produce an electrically neutral compound.

• Example calcium chloride– calcium is Ca2+ and chlorine is Cl-

– in order to balance charges there needs to be two negative charges to balance the 2+ on the calcium

– the formula is therefore CaCl2

• A battery and bulb will tell you if a solution contains ions.

• Covalent Compound Names

– Names for covalent compounds uses Greek prefixes to indicate numbers of atoms of each element

• The first element in the formula is named first with a prefix indicating the number of atoms if the number is greater than one.

• The stem name of the second element in the formula is named next, with a prefix used with the stem name if two elements can form more than one compound

– The suffix –ide is again used.( either ionic or covalent)

• Example

– CO = carbon monoxide

– CO2 = carbon dioxide

• Covalent Compound Formulas– The systematic name tells you the formula– Formulas indicate how many atoms of one element

combine with atoms of another element.– The number of covalent bonds that an atom can form is

called its valence.– Lone pairs create the possibility of creating coordinate

covalent compounds.

• Once you understand chemical names and formulas, you can figure out what chemical compounds are contained in different household products. For example, (A) washing soda is sodium carbonate (NA2CO3) and (B) oven cleaner is sodium hydroxide (NaOH), which is also known as lye.

• As you can see by studying these two charts, there is a relationship between the number of bonding electron pairs and the number of lone electron pairs and the shape of a molecule.