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Ch. 2 Atoms, Molecules, and
Ions
2.1 The Atomic Theory of Matter
• Early Greek philosophers thought that the material world must be
made up of tiny indivisible particles they called atomos, meaning
"indivisible or uncuttable"
• Later, Plato and Aristotle formulated the notion that there can be no
ultimately indivisible particles
• As chemists learned to measure the amts of elements that reacted
with one another to form new substances, the ground was laid for NA
atomic theory that linked the idea of elements with the idea of atoms
Dalton's Atomic Theory
• 1803-1807 John Dalton formulated the following postulates:
• Each element is composed of extremely small particles called atoms
• All atoms of a given element are identical to one another in mass and other
properties, but the atoms of one element are different from the atoms of all
other elements
• Atoms of an element are not changed into atoms of a different by chemical
rxns; atoms are neither created nor destroyed in chemical rxns
• Cmpds are formed when atoms of more than one element combine; a given
cmpd always has the same relative # and kind of atoms
• According to Dalton's atomic theory atoms are the smallest particles
of an element that retain the chemical identity of the element
2.1 cont
• An element is composed of only one kind of atom, whereas a cmpd
contains atoms of two or more elements
• The law of constant composition: In a given cmpd, the relative #s and
kinds of atoms are constant
• Based on Dalton's Law Postulate #4
• Law of conservation of mass (Law of conservation of matter) –
Postulate #3
• The total mass of materials present after a chemical rxn is the same as the
total mass present before the rxn
• Atoms always retain their identities and that atoms taking part in a
chemical rxn rearrange to give new chemical combinations
2.1 cont
• A good theory should not only explain the facts known but should
also predict new ones
• Dalton used his theory to deduce the law of multiple proportions
• If two elements A and B combine to form more than one cmpd, the masses of
B that can combine with a given mass of A are in the ratio of small whole #s
2.2 The Discovery of Atomic Structure
• We now know that the atom is composed of still smaller subatomic
particles
• The atom is composed in part of electrically charged particles, some with a
positive (+) charge and some with a negative (-) charge
• Particles with the same charge repel one another, whereas particles
with unlike charges are attracted to one another
• In the mid-1800s scientists began to study electrical discharge
through partially evacuated tubes (tubes that had been pumped
almost empty of air)
• A high voltage produces radiation within the tube
Cathode Rays and Electrons
• This radiation became known as cathode rays bc it originated from
the negative electrode, or cathode
• It was not initially clear clear whether the rays were an invisible
stream of particles or a new form of radiation
• Experiments showed that cathode rays are deflected by electric or
magnetic fields in a way consistent with their being a stream of (–)
electrical charge
• JJ Thomson concluded that cathode rays are streams of (-) charged
particles
• Thomson's paper is generally accepted as the "discovery" of what later
became known as the electron
2.2 cont
• Earnest Rutherford revealed three types of radiation: alpha, beta, and
gamma radiation
• Each type differs in its response to an electric field
• The paths of both alpha and beta radiation are bent by the electric field, although in
opposite directions, whereas gamma radiation is unaffected
• Alpha particles have a 2+ charge
• Beta particles have a 1- charge
• Gamma radiation does not consist of particles and carries no charge
The Nuclear Atom
• Thomson proposed that the atom consisted of a uniform (+) sphere of
matter in which the e- were imbedded
• This so-called "plum pudding" model was very short-lived
• In 1910-1911, Rutherford disproved this model with the gold foil
experiment
• Subsequent experimental studies led to the discovery of both (+)
particles (protons) and neutral particles (neutrons) in the nucleus
• Protons discovered by Rutherford in 1919
• Neutrons discovered by James Chadwick in 1932
2.3 The Modern View of Atomic Structure
• Only three subatomic particles (proton, neutron, and electron) have
bearing on chemical behavior
• Proton charge 1+
• Electron charge 1• Neutrons carry no charge and are therefor electrically neutral
• Every atom has an equal # of electrons and protons, and so atoms
have no net electrical charge
• Protons and neutrons reside in the nucleus of the atom, which, as
Rutherford proposed, is extremely small
2.3 cont
• The e- are attracted to the protons in the nucleus by the force that
exists btwn particles of opposite electrical charge
• Bc it would be cumbersome to express such small masses of atoms in
grams, we use instead the atomic mass unit, or amu
• 1 amu = 1.66054 * 10-24g
• The masses of the proton and neutron are very nearly equal, and
both are much greater than that of the e- (5.486 * 10-4)
• It would take 1836 e- to equal the mass of one proton
• The nucleus contains most of the mass of an atom
2.3 cont
• A convenient, although non-SI, unit of length used to express atomic
dimensions is the angstrom (Å)
• One Å = 10-10m
• EX The diameter of a penny is 19mm. The diameter of a silver atom is
2.88Å. How many atoms could arranged side by side in a straight line
across the diameter of a penny?
Ag atoms = (19mm)(10-3m/1mm)(1Å/10-10 m)(1 Ag atom/2.88Å)
= 6.6*107 Ag atoms
Atomic Numbers, Mass # and Isotopes
• The # of protons in the nucleus of an atom of any particular element
is called the element's atomic #
• Bc an atom has no net electrical charge, the # of e- it contains must equal the
# of protons
• i.e. C has 6 protons and 6 e- and O has 8 protons and 8 e-
• Atoms of a given element can differ in the # of neutrons they contain
and consequently in mass
• The atomic # is shown by the subscript,, and the superscript, called
the mass #, is the total # of protons plus neutrons in the atom
2.3 cont
• Bc all atoms of a given element have the same atomic #, the subscript
is redundant and is often omitted. Thus, the symbol for carbon-12 can
be represented simply as 12C
• Atoms w/ identical atomic #s but different mass #s (same # of protons
but different # of neutrons) are called isotopes
2.4 Atomic Weights
• Most elements occur in nature as mixtures of isotopes. We can
determine the average atomic mass of an element by using the
masses of various isotopes and their relative abundance
• i.e. C is composed of 98.93% 12C and 1.07% 13C. The masses of these isotopes
are 12 amu and 13.00355 amu, respectively.
(0.9893)(12 amu) + (0.0107)(12.0035 amu) = 12.01 amu
• The average atomic mass of each element (in amu) is also known as
its atomic weight
2.5 The Periodic Table
• Periodic table was developed in 1869
• If the elements are arranged in order of inc atomic #, their chemical
and physical properties are found to show a repeating, or periodic,
pattern
• i.e. Each of the oft reactive metals comes immediately after one of the
nonreactive gases
• The arrangement of elements in order of inc atomic #, with elements
having similar properties placed in vertical columns, is known as the
periodic table
2.5 cont
• The horizontal rows are called periods
• The vertical columns are called groups
• Elements that belong to the same group often exhibit similarities in physical
and chemical properties
• Except for H, all the elements on the left side and in the middle of the
periodic table are metallic elements, or metals
• The majority of elements are metallic, and they all share many characteristic
properties, such as luster and high electrical and heat conductivity
• All metals, except Hg, are solids at room temp
2.5 cont
• The metals are separated from the nonmetallic elements, or
nonmetals, by a diagonal steplike line that runs from B to At
• H, although on the left side, is a nonmetal
• Many of the elements that lie along the line that separates metals
from nonmetals have properties that fall between those of metals
and nonmetals
• These elements are often referred to as metalloids
2.6 Molecules and Molecular Compounds
• Only the noble gases (Group 18) are normally found in nature as
isolated atoms
• Most matter is composed of mcules or ions, both of which are formed
from atoms
• A mcule is an assembly of two or more atoms tightly bound together
• The resultant "package" of atoms behaves in many ways as a single, distinct
object
• Oxygen normally found in air consists of mcules that contain 2 oxygen
atoms
• We represent this molecular form by the chemical formula 02
2.6 cont
• The subscript in the formula tells us that two oxygen atoms are
present in each mcule
• A mcule that is made up of two atoms is called a diatomic mcule
• A mcule that is made up of more than two atoms is a polyatomic
mcule
• O also exists as 03, ozone
• Even though "normal" oxygen and ozone are composed of only O
atoms, they exhibit very different chemical and physical properties
• 02 is essential to life, but 03 is toxic; 02 is odorless my whereas 03 has a sharp,
pungent smell
2.6 cont
• The elements that normally occur as diatomic mcules are H, O, N, and
the halogens (Group 17/7A)
• Cmpds that are composed of mcules are called molecular compounds
and contain more than one type of atom
• i.e. A mcule of water consists of two H atoms and one O atoms, and is
represented by the chemical formula H20
• Lack of a subscript on the O atom represents that there is one O atom in water
• Chemical formulas that indicate the actual #s and types of atoms in a
mcule are called molecular formulas
• Chemical formulas that give only the relative # of atoms of each type
in a mcule are called empirical formulas
2.6 cont
• The subscripts in an empirical formula are always the smallest
possible whole # ratios
• i.e. The molecular formula for hydrogen peroxide is H2O2, whereas its
empirical formula is HO
• For most substances the molecular formula and the empirical formula are
identical
• Molecular formulas provide more information about mcules than do
empirical formulas
• You can determine the empirical formula from the molecular formula, but not
conversely
Picturing Mcules
• The structural formula of a substance shows which atoms are
attached to which within the mcule
• The atoms are represented by their chemical symbols, and the lines
are used to represent the bonds that hold the atoms together
• A structural formula usually does not depict the actual geometry of
the mcule, that is, the actual angles at which atoms are joined
together
• A structural formula can be written as a perspective drawing to give
some sense of the 3D shape
2.6 cont
• Scientists rely on various models to help visualize mcules
• Ball-and-stick models show atoms as spheres and bonds as sticks
• A space-filling model depicts what the mcule would look like if the atom
• The molecular formula of a substance summarizes the composition of
the substance, but does not show how the atom comes together to
form the mcule
• The structural formula of a substance shows which atoms are
attached to which within the mcule
2.7 Ions and Ionic Compounds
• If e- are removed from or added to a neutral atom, a charged particle
called an ion is formed
• An ion with a + charge is called a cation
• A – charged ion is called an anion
• The net charge on an ion is represented by a superscript
• Ex Na2+ Cl2-
• In general, metal atoms tend to lose e- to form cations, whereas
nonmetal atoms tend to gain an e- to form anions
2.7 cont
• There are polyatomic ions which consist of atoms joined as in a mcule
2.8 Naming Inorganic Compounds
2.9 Some Simple Organic Compounds