Daniella Perruzza
CHE 101
6 June 2004
Just as a child would ask why is the sky blue, why is the ocean blue, and why are
the clouds white, chemical compounds and chemical bonds bring us to the basic
question of what holds things together. Even though atoms or molecules are
electrically neutral, it's their net electrical forces of attraction that account for solids,
liquids and gases, as we know them. We will address why the strong forces of
attraction exist and what is their chemical makeup is. Chemists developed molecular
formulas and strategies to further explain what bonds chemicals in nature. Chemicals
are as unique as fingerprints, no two chemicals have the same molecular structure.
Compounds are substances made of more than one kind of atom or pure element.
There are two kinds of compounds, ionic compounds and molecular compounds. A
molecule is composed of two or more nuclei that contain enough electrons to make it
completely neutral. If the nuclei are composed of the same element such as two
elements of Carbon or two elements of Potassium it is considered a molecular element.
In a molecular compound however, the nuclei are from different elements that form
molecules. '
Ionic compounds are made of two charged elements, either positively charged or
negatively charged elements. Positively charged ions are called cations. Sodium (Na)
ions for example are positively charged. When cations form, electrons are lost from the
largest orbitals first. Negatively charged ions are called anions. Ionic compounds can
form between two elements, one in which loses an electron and the other element that
will gain that electron and became negatively charged. Ions form from elements of
opposite electrical charges that are extremely attracted to each other to form a strong
compound. Many atoms gain or lose electrons to achieve the same number of electrons
as the nearest noble gas. Ionic compounds are often called salts. In a solid form, salts
are held together because opposite charges attract each other. An ionic compound is
the formation of oppositely charged ions. Atoms of certain elements can transfer
electrons between them when they form a compound. It usually forms between a metal
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and a nonmetal. Electronegativity is also another important factor to consider when
atoms bond. Electronegativity is the ability of an atom to attract electrons to itself.
Electronegativity increases from left to right and bottom to top. Fluorine is the most
electronegative. If the electronegativity difference between two atoms is greater than
2.5, one atom will donate an electron to the other, to obtain stability. The atom that
gained an electron obtains a negative charge becoming an anion, while the atom that
donated becomes positively charged, becoming a cation. These particular atoms are
extremely attracted and form an ionic bond. For example, in ordinary table salt (sodium
chloride - NaCl), the sodium atom (a metal) gives up an electron to the chlorine atom (a
nonmetal). Since Na has one less electron, it has a charge of +1. The chlorine atom
becomes a chloride ion and gains one electron, making it have a charge of -1. Since
unlike charges tend to attract each other, the sodium and the chloride ions attract each
other and form an ionic bond. In ionic bonding, electrons are transferred from one atom
or to another, usually from a metal onto a non-metal. In the process of losing or gaining
electrons, the reacting atoms form ions. The oppositely charged ions are attracted to
each other, which form a strong ionic bond. The formula unit of NaCl is therefore Na+
and Cl- ions. Ionic compounds usually have very high melting points and are solid at
room temperature. When they are solid, they do not conduct electricity, but when
melted can, because their electrical ions move freely in a liquid state. Ionic compounds
dissolve easily in water and other polar solvents. Ionic compounds easily conduct
electricity in solutions. With high melting temperatures, ionic compounds tend to form
crystalline solids.
According to L. Ladon, the empirical formula is the simplest formula for a
compound. When NaCl is formed, one can only imagine that only one element of Na
and only one element of Chlorine come together to form this union. However, NaCl is
considered the formula unit, not a molecular formula.
Another type of ion that forms is a monoatomic ion. Monatomic ions have only
one atomic nucleus and are either negative or positively charged. There are two types
of monoatomic ions, cations and anions. Metals form monatomic cations. Once again
cations are positively charged ions. Nickel for example forms two ions, Ni +2 and Ni
+3. There is a special process for naming these ions. Cations are named using the
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element name followed by a Roman numeral and the word ion. For example one could
name Ni +2 and Ni +3 , nickel(II) ion and nickel(III) ion. Anions, which are monatomic
ions derived from nonmetals end in the suffix -ide. The nonmetal Sulfur, for example,
would be called a sulfide ion. Names, symbols and electrical charges are given to ions
to distinguish between cations and anions.
A binary ionic compound to definition is a compound composed of ions of two
different elements, one being a metal and the other a nonmetal. When there is a binary
ionic compound, chemists use certain rules to organize elements and their names. The
cation is always written first in the name and the anion second. For example, iron(III)
iodide, Fe is a metal and is put before the element iodine which is a nonmetal. The
cation is indicated using a Roman numeral immediately following the name of the
cation. The anion is named by adding the suffix -ide to the element name and put after
the parenthesis of the Roman numeral.
When forming ionic compounds, correct formulas must reflect the electrical
neutrality. Metals and nonmetals form neutral charged compounds, having canceled
each other's previous positive or negative charges. If Na +1 decides to form a
compound with Sulfur -2, if would take two Na+1 ions to give enough positive charge to
cancel the charge of -2 of a sulfur ion. To show this ratio, the formula would be written
as Na2S. Subscripts are used to express the smallest whole numbers in a ratio. Sulfur
does not have a subscript because it is given that if there is no subscript there is only
one element in the compound.
Nonmetals tend to gain electrons while metals tend to lose electrons when
forming a bond. Atoms or ions whose outside orbitals contain eight electrons are
extremely stable then those whose outside level contain less. A total of eight electrons
in the outer shell causes the element to become stable. When stating that an element
is "stable", stable simply means that change or sudden reactions would not easily affect
these specific elements. This principle of filling up the energy levels to eight electrons
or to the maximum capacity is called the octet rule. Metals and nonmetals strive to
become stable by reacting with each other to have eight valence electrons. For
example the element Chlorine has seven valence electrons and would like to gain one
more electron to have eight electrons in its outer shell.
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Noble gases, are the least reactive, and most stable of all the elements. The six noble
gases are Helium, Neon, Argon, Krypton, Xenon, and Radon, which are in group VIII in
the periodic table. All noble gases have the maximum number of electrons in their outer
shell. Only Helium has two electrons in its outer shell, which is considered stable, while
the rest have eight valence electrons. These noble gases form neither positive or
negative ions.
When elements combine to form stable bonds, the valency of the reactants
change. Metals like to lose electrons while non-metals will gain them. This reaction that
involves the transfer of electrons between two chemical elements is called a Redox
Reaction. In a redox reaction, two elements will change the existing oxidation number.
One of the elements oxidation number will become more positive or less negative. The
oxidation number of the other element decreases becoming less positive or more
negative. The element that loses electrons is said to be oxidized, while the element that
gains the electron is said to be reduced. When there is an oxidation there must be a
reduction in any reaction. The substance, which loses electrons, is oxidized and the
substance, which gains electrons, is reduced. However, this can get confusing, the
element that gains an electron is reduced and called the oxidizing agent, while the
element that loses the electron is oxidized called the reducing agent.
A covalent bond occurs from sharing electrons between nuclei. Covalent bonds
occur between two atoms in order to produce a mutual attraction in order to share
electrons. The bond is formed when the electron clouds of two atoms overlap. The
nuclei of both atoms feel attracted toward each other and they become bonded. In
covalent bonding, the two electrons are shared and are attracted to the nucleus of both
atoms. Covalent bonds usually are formed between two atoms with similar
electronegativities. Neither atom loses or gains electrons as in ionic bonding. Also, by
the sharing of electrons, each atom is able to achieve stability by having eight valence
electrons. Unlike ionic bonds, covalent bonds require specific atoms. There are two
types of covalent bonding, non-polar bonding and polar bonding. Non-polar bonding
involves the equal sharing of electrons between identical non-metal atoms. Polar
bonding is the unequal sharing of electrons between two different non-metals.
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After a covalent bond has formed, the orbitals of each atom partially merge to
overlap orbitals. The space created by the individual overlapping atomic orbital is called
a molecular orbital. This overlap may cause a decrease or an increase in the energy of
the molecule. Commonly a covalent bond implies the sharing of just a single pair of
electrons. A line connecting each atom represents two shared electrons. The sharing
of two pairs is called a double bond and the sharing of three pairs is called a triple
bond. The triple bond is rare in nature, and atoms have not yet been seen to bond
more than triply. If two electrons, represented by a pair of dots are not shared, they are
known as a lone pair.
A new kind of symbol, the electron- dot structure also known as the Lewis
structures are used to represent an atom's number of electrons. The Lewis structures
and Lewis octet rule was named after Gilbert Newton Lewis who proposed that
chemical bonds be formed between atoms because electrons from the atoms interact
with each other. He suggested that atoms with less than eight valence electrons bond
together to share electrons and become stable with eight electrons in their valence
shell. Lewis revolutionized concepts of bonding and contributed a great deal to a world
of science. The electron-dot symbols for atoms show the symbol for the element and
the number of electrons. The number of electrons is represented as dots around the
symbol for the element. It is important that the number of valence electrons in the
structure be exactly correct, since this shows where all the electrons are. The number
of valence electrons is the number of s and p electrons in the outermost shell. The
number of valence electrons can also be determined by counting across the period, until
the element is found. If there is more than one atom, the number of valence electrons is
the sum of all the atoms in the molecule or polyatomic ion. When writing Lewis
structures for ions, total the value of all negative charges and subtract the value of all
the positive charges. For example, the element Nitrogen is in group 5A, anhasfive
valence eletrons. Carbon in group 4, has four valence electrons.
A dot is used to represent each electron and each dot go on one of the four sides
of the element symbol. The first two electrons are paired up on either the right or left
side and the remaining dots are distributed on each side and double up if multiple. N
has five valence electrons, so the Lewis structure is. Mg has two valence electrons.
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This representation allows one to predict which element will gain or lose electrons
based on their number of dots or electrons to become stable. For example Sodium
(Na), has the electron configuration 1s2 2s2 2p6 3s1, and decides to lose 3s electron to
form the noble gas, Neon. Metals, such as Sodium, tend to lose their valence electrons
to form the octet of the previous period gas. Non-metals, on the other hand, tend to
gain electrons in reactions to become stable. Nitrogen has five valence electrons, and
needs three more electrons to complete the octet subshell, by having eight dots instead
of five. Electron- dot symbols allows a chemist to keep track of the electrons during a
bond. Atoms gain, lose or share electrons to achieve the same amount of electrons as
the nearest noble gas. Atoms once again, want to have enough electrons to fill its
outermost shell. Noble gases are stable because their outmost shells are filled with the
maximum number of electrons in the outmost shell.
The formation of ions can be shown by electron- dot symbols. If Potassium
decides to react with Chlorine, Potassium loses its electron and no dot remains. The
valence shell of Chlorine accepts this electron and now has eight valence electrons and
obtains a negative charge. Brackets are added around the symbol of the ion Chlorine to
show that has no more room for electrons. Atoms that form covalent bonds have the
intention of forming noble gas configurations by gaining as many electrons as they need
to fill their outermost shell. Covalent bonding occurs between atoms with similar
electronegativities, where there is insufficient energy to completely remove an electron
from another atom. Electron negativity is the tendency of an atom in a molecule to
attract electrons toward itself. In non-polar covalent bonding, the difference in
electronegativity is zero. In the case where Chlorine bonds with Chlorine each shares
that one electron with each other. S even dots surround this element and they each
share that one electron to form a pair, so that each element could achieve stability. A
bond between two Chlorine elements would have this structure Cl-Cl. The unshared
pairs of electrons surrounding them would be omitted from the structure. Cl-Cl is also
called a structural formula.
Ions that form very tight bonds and retain their identity within ionic compounds
are known as polyatomic ions. Polyatomic ions are the basic building blocks of many
ionic compounds and theses atoms that form bonds have a net electrical charge.
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Polyatomic cations are usually ammonium, hydronium and mercury. Polyatomic anions
occur in "families." The electron dot structure of ammonia, NH3 has one unshared pair
of electrons left. Another hydrogen ion H+, after losing its 1s electron, comes along and
decides to bond to NH3, making it an ammonium ion NH+4. The cluster of atoms that
shared electrons is an example of a polyatomic ion. But one may ask where the
additional hydrogen ion came from. A large "family" of acids supplied this hydrogen
ion. An acid can be defined as a proton donor. Acids have a sour taste. Acids are
known to be corrosive and change the color of certain vegetable dyes, such as litmus,
from blue to red. Acids tend to react with most metals, such as Sodium. In an
operational sense, an acid is a substance that increases the concentration of the H+ ion
when it dissolves in water. Bases are compounds that increase the concentration of the
OH- ion, after dissolved in water. Bases can be defined as a proton acceptor. Bases
combine with hydrogen ions, unlike acids. To distinguish whether a compound is an
acid or base, one can dissolve it in water and test the solution for an increase in H+ or
OH- concentration. An acid- base reaction can be called a proton transfer. Salt is an
ionic compound and formed from any cation except a hydrogen ion and any anion
except OH- and O2-. It is extremely important to learn the names, charges, and
formulas of the most common polyatomic ions. There is difference between Lewis
structures of polyatomic ions and molecules. It is necessary that one consider the
charge, whether negative or positive when counting valence electrons. Ions with a
negative charge have extra electrons and ions with a positive charge have an excess of
protons and fewer electrons. Some important polyatomic ions Ammonium ion NH+4,
Acetate ion C2H3O2-, Sulfate ion SO4-2, and Phosphate ion PO4-3. In the molecule of
SO32- , there would be 26 valence electrons. The number of valence electrons is 6 +
3(6) + 2 = 26.
When learning how to write Lewis Structures it is important to first write a skeletal
structure, without any valence electrons. One must first be able to express which
element would be considered the central atom in the structure. The less electronegative
element will usually be the central atom, the atom which the others are grouped.
Normally the first atom of the formula will be the central atom. The only exception to this
rule if the element Hydrogen. Hydrogen can never be a central atom because it can only
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bond once and accommodate two electrons. Hydrogen houses only two electrons in its
first shell .In H20, Oxygen must be the central atom, H O H. In CO2, the Lewis structure
would be written O C O. Oxygen is usually not the central atom, but because it was
bonded with hydrogen. More examples are CCL4, H2SO4, NCL3, with the central atom
shown in bold print. The second step in writing a Lewis structure is to add up the
valence electrons and the total equals the number of dots that must be used around the
element. The total number of valence electrons in H2CO3 is 24. Hydrogen has one
valence electron, Carbon has four electrons, and Oxygen has six. Each number is then
multiplied by its own subscript and then these are totaled to give the total amount of
dots in the Lewis structure.
Lewis structures show which atoms are bonded to which, but does not determine
how these atoms are arranged.The shape of any molecule truly depends on the
repulsion's by the electron clouds. The Valence Shell Electron Pair Repulsion Theory or
also known as the VSEPR theory can predict the three-dimensional shape of the
molecules based on their Lewis Dot structure. After a covalent bond has been made,
electron clouds of one pair repel an electron cloud of another, which forms a certain
shape. The shape is determined by each of these electron clouds to stay out of each
other's way. The repulsion are smallest when the electron pairs or groups of electron
pairs are as far apart as possible. When electron pairs are as far apart as possible, they
become stable or in a stable form. All bonds radiate out from the central atom of the
particle. When two electron pairs bond, they form a linear shape with a 180 degree
angle from each other. If three pairs of electrons bond they form a planar triangular with
a 120 degree angle. If four pairs form a bond they form a tetrahedral geometry because
each electron pair faces out in the shape of a pyramid and has a bond angle of 109.5.
When five pairs of electrons form they form a trigonal bipyramidal. A trigonal bipyramid
has two positions. Axial positions are 90 degrees from the equatorial positions, which
are 120° from each other. Six pairs form a octahedral, where each group is 90 degrees
away from the rest. Lone pairs however influence molecular geometry by that fact that
they are one of the repelling groups. The repulsion between lone pairs is greater than
that of a bond, causing the angles between them to decrease.
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Polar molecules are the arrangement of the atoms where in some molecules one
end of that molecule has a positive charge and the other side has a negative charge. A
positive and a negative electrical charge simply mean that the molecule has electrical
poles. Normally, an atom has an even distribution of electrons, but if the electron end up
one side more than the other, a resulting electrical field may occur. Water, H20, for
example is considered a polar molecule where there are excess electrons on the
oxygen side and a lack of or excess positive charges on the hydrogen side. Positive
charges could be imagined to be on the very top of the molecule, while negative
charges at the bottom. A non-polar molecule has electrons that are distributed
symmetrically and the negative and positive charges cancel each other out. There is not
an abundance of negative and positive charges at the opposite sides of a non-polar
molecule. Polar molecules will mix together with other polar molecules to form solutions,
while a polar and non-polar combination will not form a solution when mixed. A solution
is a homogenous mixture of two or more substances. Usually the solution, is a liquid. A
mixture has a solvent and a solute. The solvent has a greater number of moles, while
the solute has less. The most common solvents if water, and a solution may be
described as aqueous. Water and oil, which is a non-polar molecule, form a solution.
However, since alcohol is a polar molecule, and it will form a solution with water.
Interaction between molecules is the basis for solubility. Electrons in a polar covalent
bond are unequally shared between the two bonded atoms, which results in partial
positive and negative charges. The separation of the partial charges creates a dipole.
The word dipole means two poles: the separated partial positive and negative charges.
A polar molecule results when a molecule contains polar bonds in an unsymmetrical
arrangement.
Nonpolar molecules are of two types. Molecules whose atoms have equal or
nearly equal electronegativities have zero or very small dipole moments. A second type
of nonpolar molecule has polar bonds, but the molecular geometry is symmetrical
allowing the bond dipoles to cancel each other.
It is amazing to learn that everything in nature is so unique and ordered. Time
after time, many devoted scientists discovered chemical molecular formulas to help the
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interested amateur scientist understand how structured life really is. Once the formula is
understood, the key to understanding life is unlocked.