Transcript Chapter 2 Sections 2.5-2.7 Powerpoint

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•Tiny nucleus: diameter of about 10-13 cm.
•Electrons: move about the nucleus at an average distance
of about 10-8 from it.
If the nucleus were the size of the ball
bearing, a typical atom would be the size
of the stadium.
•Nucleus contains protons (positive charge) and neutrons
(neutral).
•Electrons (negative charge) surround the nucleus.
•Nucleus
•small size with extremely high density
•accounts for most of atom’s mass
•piece of nuclear material about the size of a pea would
have a mass of 250 million tons
•Electrons
•account for most of atom’s volume
•their number and arrangement account for an atom’s
chemical behavior
•involved when different atoms bond
•number of protons in the nucleus of an atom
•the atomic number identifies the element
•has the symbol, Z
•total number of protons and neutrons in the
nucleus of an atom
•has the symbol, A
•mass number = number of protons + number of neutrons
•atoms with the same number of protons but different
number of neutrons
•Isotopes are defined by two numbers, its atomic number
and its mass number.
•Isotopes are represented by nuclear symbols or by using
hyphen notation.
Nuclear Symbol
Mass number
(protons + neutrons)
Atomic number
(number of protons)
A
X
Z
Element symbol
Hyphen notation
Element name-mass number
Example: symbol for one particular isotope of sodium
Mass number
Atomic number
23
11 Na
Element symbol
or Sodium-23
Two isotopes of
sodium.
Both have 11
protons and 11
electrons, but
differ in the
number of
neutrons.
•Chemical bonds: forces that hold atoms together in
compounds.
•Covalent bond: results from sharing of electrons between
different atoms; the resulting collection of atoms is called a
molecule.
•Simplest method to represent a molecule is a chemical
formula.
•Chemical formula: the symbols for the elements are used
to indicate the types of atoms present and subscripts are
used to indicate the relative numbers of atoms.
Chemical formula for methane: CH4
Different representations of molecules.
Structural formula for methane. Individual bonds
are shown. May or may not indicate the actual
structure of the compound.
Ball-and-stick model of
methane.
Space-filling model of
methane. This type of
model shows the relative
sizes of atoms as well as
their relative orientation in
the molecule.
•A second type of bond results from attractions between
ions.
•An ion is an atom or groups of atoms that has a net
positive or negative charge resulting from a loss or gain of
electrons.
•The best known ionic compound is common table salt, or
sodium chloride, which forms when neutral sodium and
chlorine react.
•Consider the formation of the sodium ion below.
•When an ion forms, electrons are transferred from one
atom to another (the number lost must equal the number
gained).
•With one electron transferred, sodium has 11 protons and
only 10 electrons so has a net 1+ charge – it has become a
positive ion.
•A positive ion is called a cation.
•Represented in shorthand form: Na → Na+ + e-
•Consider the formation of the chlorine ion below.
•If an electron is added to chlorine, it has 17 protons and 18
electrons which produce a net 1- charge – it has become a
negative ion.
•A negative ion is called an anion.
•Represented in shorthand form: Cl + e- → Cl-
•Anions and cations = opposite charges = attraction
•Ionic bonding: force of attraction between oppositely
charged ions.
•Solid composed of oppositely charged ions is called an ionic
solid or a salt.
•Polyatomic ion: an ion composed of two or more atoms.
•Can form ionic bonds with other
simple ions or with other polyatomic
ions.
Ammonium
nitrate
•Periodic Table: a chart showing all the elements arranged
in columns with similar chemical properties.
•Letters in the boxes are the symbols for the elements.
•The number above the symbol is the atomic number
(number of protons).
•Most of the elements are metals (found to the left of the
staircase in the periodic table).
•Characteristic physical properties:
•efficient conduction of heat and electricity
•malleability (they can be hammered into thin sheets)
•ductility (they can be pulled into wires)
•(often) a lustrous appearance
•all are solids at room temperature except mercury
•Tend to lose electrons to form positive ions.
•Found to the right of the staircase on the periodic table.
•Encountered mostly in the form of compounds or mixtures
of compounds.
•Poor conductors of heat and electricity.
•Neither malleable nor ductile.
•Many are gases at room temperature and many are solids
(Bromine is the only liquid).
•Usually exist as molecules in their elemental form.
•Tend to gain electrons to form negative ions.
•Grouped along the staircase.
•Properties lie between those of metals and nonmetals.
•Tend to be semiconductors; they conduct electricity, but
not nearly as well as metals.
•Generally, metalloids behave as nonmetals, both chemically
and physically.
•Periodic Table
•Horizontal rows are called periods.
•Vertical columns are called groups or families.
•Elements in same column have similar chemical
properties.
•Group 1 elements are called the alkali metals.
•very active elements
•readily form ions with a 1+ charge
•Group 2 elements are called the alkaline earth metals.
•reactive and form ions with a 2+ charge
•Groups 3 through 12 are called the transition metals.
•form colored ions
•variable oxidation states
•Group 17 are called the halogens.
•very reactive nonmetals
•form ions with 1- charge
•exist as diatomic molecules
•Group 18 are called the noble gases.
•exist as monatomic (single-atom) gases
•little chemical reactivity
•The elements in the two long rows below the main body of
the periodic table are called the inner transition elements.
•Elements 58-71 are called the lanthanide elements
because they follow lanthanum (Z = 57).
•Elements 90-103 are called the actinide elements because
they follow actinium (Z = 89).