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Chemical Foundations:
Elements, Atoms, and
Ions
Elements
115 different elements
88 of which occur naturally, the rest have
been made in laboratories
vary in abundance
9 elements account for most of the
compounds found in earth’s crust (see table
3.1 page 51)
Element Names and SymbolsNames have many sources: descriptions of
their properties, place the element was
discovered, honoring a famous scientist
Element symbols usually consist of the first
letter or first two letters of the element’s
name
The first letter is always capitalized, the
second is not
Examples: Oxygen (O) or Neon (Ne)
The Law of Constant Composition
a compound always contains elements in
exactly the same proportion by mass
this occurs because the molecules that make
up a compound are all identical (ex. H₂O)
John Dalton (early 1800’s) offered an
explanation for this known as Dalton’s
atomic theory
Dalton’s Atomic Theoryelements are made of tiny particles called atoms
all atoms of a given element are identical
the atoms of a given element are different from those of
any other element
atoms of one element can combine with atoms of other
elements to form compounds...a given compound
always has the same relative numbers and types of
atoms
atoms are indivisible in chemical processes...which means
atoms are not created or destroyed in chemical
reactions...a chemical reaction only changes the way
atoms are grouped together
Dalton’s Atomic Theory
Dalton predicted that atoms of two different
elements could combine in different ways to
form different compounds (examples: NO,
NO₂, N₂O)
When the existence of these substances was
verified, Dalton’s atomic theory was widely
accepted
Compounds
always contain the same relative numbers of atoms of
each element
the types of atoms and the number of each type in each
unit (molecule) of a compound are expressed by the
chemical formula
in a chemical formula the atoms are indicated by the
element symbol and the number of each type of atom is
indicated by a subscript
when only one atom of a given type is present, the
subscript 1 is NOT written
Chemical Formula Example
N₂ON: nitrogen
2: two atoms of nitrogen
O: oxygen
no subscript: one atom of oxygen
The Structure Of The Atom
for most of the 1800’s scientists wondered how an atom
was structured
in the late 1890’s J.J. Thomson conducted experiments
using a cathode ray tube to prove that atoms can emit
tiny negative particles which are now called electrons
He predicted that the atom must also contain positive
particles that balance exactly the negative charges
carried by the electrons, giving the atom a charge of
zero overall
Plum Pudding Model
J.J. Thomson and William Thomson (Lord Kelvin, not
related) are credited with this atomic model
The atom is structured like plum pudding (a pudding with
raisins randomly distributed throughout)....think of a
chocolate chip cookie
They believed the atom could be thought of as a uniform
“pudding” of positive charge with enough negative
electrons scattered within to counterbalance the positive
charge
Ernest Rutherford
learned physics in J.J. Thompson’s laboratory
interested in alpha particles, positively charged particles
(made of two protons and two neutrons with a mass
about 7500 times that of an electron but this was not
known then)
1911 he performed an experiment that involved shooting
alpha particles at a thin metal foil surrounded by a
detector screen that flashed every time an alpha particle
hit it
Ernest Rutherford
although most of the alpha particles passed
straight through the foil, some of them were
deflected at large angles, and some were
reflected backwards
he described the results as comparable to
shooting a gun at a piece of paper and
having the bullet bounce back
Rutherford’s Conclusions
the large deflections of the alpha particles was caused by
a center of concentrated positive charge within the atom
that would repel the positively charged alpha particles
Most of the alpha particles passed directly through the foil
because the atom is mostly open space
the deflected alpha particles were those that had a “close
encounter” with the positive center of an atom
those alpha particles that scored a “direct hit” on the
positive center were reflected backwards
The Nuclear Atom
Due to the results of his experiment, Rutherford described
the structure of the atom as having a dense center of
positive charge (the nucleus) around which tiny
electrons moved in a space that was otherwise empty
1919 Rutherford concluded that the nucleus contained
protons (particles located in the nucleus)
a proton has the same magnitude of charge as an
electron, but its charge is positive
Protons = +1 charge and Electrons = -1 charge
Expected ResultsPlum Pudding Model
Actual ResultsNuclear Model
Further Work By Rutherford
Reasoned the hydrogen atom has one proton in the central
nucleus and one electron moving around it
Reasoned that other atoms must have nuclei composed of
many protons bound together in some way
1932 he and James Chadwick showed that most nuclei
contain a neutral particle called a neutron that is slightly
more massive than a proton but has no charge
Modern Concept of Atomic Structure
a tiny nucleus (about 10⁻¹³cm in diameter)
electrons (-1 charge) that move around the nucleus at an
average distance of about 10⁻⁸cm from it
if the nucleus of an atom were the size of a grape, the
electrons would be about one mile away on average
the nucleus contains protons (+1 charge) and neutral
neutrons
the neutrons’ function is not obvious, they may help hold
the protons (which repel each other) together to form
the nucleus (see table 3.4 page 63 for relative masses)
Electrons
the space in which the electrons move accounts for most
of the atomic volume
the electrons are the parts of the atom that intermingle
when atoms combine to form molecules
therefore it is the number of electrons an atom possess
that affects the ways in which it can interact with other
atoms
the number of electrons an atom has determines its
chemical behavior
Isotopes
after the discovery of the neutron, Dalton’s statement that
all atoms of the same element were identical, needed to
be changed
all atoms of the same element contain the same number of
protons and electrons (because an atom is neutral), but
they can have differing amounts of neutrons
atoms with the same number of protons but differing
amounts of neutrons are called isotopes
Isotopes
Atomic Number vs. Mass Number
the number of protons in a nucleus is called the atom’s
atomic number
the sum of the number of protons and neutrons in the
nucleus is called the atom’s mass number
this information can be noted as follows
example written in chemistry text The atomic mass given in the
periodic table is an average of the
atomic mass of each of the isotopes
of that element (atomic weight)
You can always determine the number of neutrons present in a given atom by subtracting the
atomic number from the mass number.
Periodic Table of Elements
a chart that shows all of the known elements and gives a
good deal of information about each one
elements are listed in order of increasing atomic number (#
of protons) as well as in specific horizontal and vertical
columns
the elements were first arranged in this way in 1896 by
Dmitri Mendeleev because of the similarities in the
chemical properties of some elements
it is called a periodic table because as we increase the
atomic numbers, every so often (periodically) an
element occurs with properties similar to those of an
earlier (lower-atomic-number) element
Periodic Table of Elements
Elements with similar chemical properties are
within the same family
Families of elements lie in the same vertical
column on the periodic table, each column is
called a group
Groups are referred to by the number over the
column (see pages 68-69 in text and back
inside cover)
Note: Groups 1A-8A do not include the transition metals. See page 68 for group numbers that
include the transition metals.
Special Group Names
Some groups are given special names in
addition to their group number
Column 1 = alkali metals
Column 2 = alkaline earth metals
Column 7 = halogens
Column 8 = noble gases
a large collection of elements that spans
many vertical columns consists of the
transition metals
Most Elements Are Metals
Chemical Properties of Metals:
efficient conduction of heat and electricity
malleability (can be hammered into thin
sheets)
ductility (can be pulled into wires)
a lustrous (shiny) appearance
Nonmetals
elements that appear in the upper righthand
corner (to the right of the “stair-step”heavy
black line) as well as hydrogen
lack those properties that characterize metals
show much more variation in their individual
properties than metals do
Metalloids (Semimetals)
elements that lie close to the “stair-step”
heavy black line
have a mixture of metallic and nonmetallic
properties
silicon, germanium, arsenic, antimony,
tellurium
Note: See table in text page 70
Natural States of the Elements
most elements are quite reactive: their atoms
tend to combine with those of other elements
to form compounds
therefore, we do NOT often find elements in
nature in pure form (uncombined with other
elements)
Exceptions include: gold, platinum, and silver
Noble Metals
gold, platinum, and silver are members of a class of metals
called noble metals because they are relatively
unreactive
the term noble implies a class set apart
Group 8 is also called the noble gases because these
elements are gases that do not react with other
elements and are found in nature in their pure form
(helium, neon, argon, krypton, xenon, and radon)
Diatomic Molecules
some elements can be found in pure form but not as single
atoms but as two atoms of the same element bonded
together to create a molecule
these molecules are called diatomic molecules
examples: N₂ or O₂
Diatomic Molecules
all of the elemental forms of the group 7
(halogens) contain diatomic molecules
(fluorine, chlorine, bromine, and iodine)
hydrogen, nitrogen, and oxygen are also
diatomic in their elemental forms
Natural States of the Elements
At normal temperatures (about 25°C or 77°F) the elements
found in gaseous form are the noble gases of group 8
(as individual atoms) as well as hydrogen, nitrogen,
oxygen, fluorine, and chlorine (as diatomic molecules)
Two elements are liquids in their elemental forms at the
same temperature: bromine (which is diatomic) and
mercury
All other elements are solids in their elemental forms at
25°C
Allotropes
different forms of the same element
for example, solid carbon is found in three forms:
diamond, graphite, and buckminsterfullerene
Ions
an atom is a neutral entity because it has an
equal number of protons with positive
charges and electrons with negative charges
ions (atoms with charges) are produced by
taking a neutral atom and adding or
removing one or more electrons
Cations
a positively charged ion is called a cation
it is produced when one or more electrons are LOST from
a neutral atom
if an atom loses two electrons the ion it creates has a +2
charge (because now there are two more protons with a
positive charge than there are electrons with a negative
charge)
a cation is named using the name of the parent atom
example: an aluminum atom that has lost 3 electrons is
called an aluminum ion or aluminum cation (Al³⁺)
Anions
a negatively charged ion is called an anion
it is produced when a neutral atom GAINS electrons
if a neutral atom gains one electron it now has an overall -
1 charge because it has one more negatively charged
electron than it does positively charged protons
an anion is named by taking the root name of the atom and
changing the ending to -ide
for example, an atom of fluorine can gain one electron and
will be called a fluoride ion or fluoride anion
Ion Charges and the Periodic Table
metals always form positive ions by losing
electrons
nonmetals form negative ions by gaining
electrons
most of the transition metals form cations with
various positive charges
Note: see table on page 79 in text
Compounds That Contain Ions
substances that contain ions can conduct an electric
current only if the ions can move
the current travels by the movement of the charged ions
in solid NaCl (table salt) the ions are tightly held and
cannot move
when the solid melts and becomes a liquid the structure of
the atoms is disrupted and the ions can move
therefore, an electric current can travel through melted salt
when salt is dissolved in water the ions can move around
and are dispersed throughout the water, allowing it to
conduct a current
Ionic Compounds
a compound that results when a metal reacts with a
nonmetal to form cations and anions
metals lose electrons (become cations) and nonmetals
gain electrons (become anions)
the numbers of cations (+) and anions (-)in an ionic
compound must be equal so that they net charge is
zero
ionic compounds have very high melting points and
conduct an electric current when melted or dissolved in
water
ionic and molecular compounds in solution
salt dissolving in water animation
