Transcript Metathesis Problems (and Some Solutions) Identified Through

Chapter 4: Elements, Atoms, and Ions
Chemistry 1020: Interpretive chemistry
Andy Aspaas, Instructor
Elements
• All substances on earth are made from a combination of 114
or so elements
– 88 found in nature, others are man-made
• Abundance: % found in nature
– Oxygen most abundant on earth & in human body (by
mass)
– Abundances vary in different parts of environment
• Each element has its own symbol
– One or two letters - first is always capitalized
Dalton’s atomic theory
• According to Dalton:
– Elements are composed of tiny unbreakable
particles called atoms
– All atoms of a given element are identical
– Atoms of a given element are different from those
of any other element
– Atoms of one element combine with atoms of
other elements to form compounds
– Atoms are indivisible by chemical processes
Conclusions from Dalton’s atomic theory
• Law of constant composition
– All samples of a pure compound contain the same
proportions of the elements
– Chemical formulas used to show those proportions
• Law of conservation of mass
– Atoms are never created or destroyed, only rearranged
– Atoms of one element cannot change into atoms of
another element
– All atoms present at the beginning of a reaction are
present at the end
Compounds and chemical formulas
• A compound is a pure substance that is composed
of atoms of two or more elements
• Compounds are described by giving the number and
type of each atom in the simples unit of the
compound
• Each element represented by letter symbol
• Quantity of atoms of each element written as
subscripts (subscript 1 never written)
• Polyatomic groups in parentheses if more than one
Structure of the atom
• J.J. Thomson investigated beams known as cathode
rays
– Made of tiny negatively charged particles called
electrons
– Smaller than a hydrogen atom!
– Atoms of different elements produce the same
electrons
Thomson’s plum pudding model
• Atoms are indeed breakable, contrary to Dalton’s
theory
• Electrons are suspended in a positively charged
electric field (to balance electrons’ neg. charge)
• Mass of atom is due mostly to electrons
• Atom is mostly empty space
Rutherford’s gold foil experiment
• Tried to prove Plum Pudding model
• Shot “bullets” of alpha particles through thin sheet of
gold atoms
• Expected alpha particles to fire straight through
– Most indeed did
– But about 2% were deflected by very large
angles
– Disproved Thomson’s plum pudding model
Rutherford’s nuclear model
• Most of atom’s mass is in a tiny dense center called
the nucleus
– Positively charged
– Only 1/10 trillionth the volume of the atom
– Nucleus’s positive charge balances electrons’
negative charge
• Electrons fly around in the empty space surrounding
nucleus
The modern atom
• Nucleus composed of two types of particles
– Protons: +1 charge
– Neutron: 0 charge, mass similar to proton
• Electrons: -1 charge, outside of nucleus
Isotopes
• Number of protons defines which element an atom is
– Called atomic number, found on periodic table
• Atoms of an element with different numbers of neutrons are
called isotopes
• All isotopes of an element behave identically in chemical
reactions
• But have different masses
• Identified by mass number
– Mass number = # protons + # neutrons
• Isotope symbols contain mass number, atomic number, and
element symbol
Elements
• Arranged in pattern called Periodic Table
• Properties can be predicted based on position in table
• Metals
– About 75% of elements
– Lustrous, malleable, ductile, conduct heat and electricity
• Nonmetals
– Dull, brittle, insulators
• Metalloids or semi-metals
– Properties of both metals and nonmetals
Groups and periods
• Elements with generally similar chemical and
physical properties are in the same column
• Columns are called groups or families
– Different numbering schemes
• Rows are called periods
Regions of the periodic table
• Main group = representative elements
– “A” columns in label
• Transition elements
– All metals
• Bottom rows: Inner transition elements or Rare earth
elements
– All metals
– Inserted into periodic table after La and Ac
A few important groups
• Group 8A: Noble gases
– Colorless gases at room temperature
– Non-reactive (inert)
– Found in nature as single atoms uncombined
• Group 7A: Halogens
– Very reactive nonmetals
– Exist as diatomic molecules in nature (Cl2, Br2,
etc)
– React with metals to form ionic compounds
Allotropes
• Some solid nonmetallic elements can exist in
different forms with different physical properties
– Allotropes: these different forms
• Different physical properties are from different
arrangements of atoms in the solid
• Allotropes of carbon:
– Diamond
– Graphite
– Buckminsterfullerene (C60)
Ions
• Ions are atoms or groups of atoms that carry an electrical
charge
• Cations: positive charge (loss of electrons)
• Anions: negative charge (gain of electrons)
• Unlike charges attract, so cations and anions are attracted to
each other
• Electrolytes: ions that dissolve in water and cause it to
conduct electricity
• Ionic compound: combination of cations and anions that form
a pure substance with no total charge
Atomic structures of ions
• Metals always form cations
• Loss of one electron creates a +1 charge
– Na = 11 e-, Na+ = 10 e– Ca = 20 e-, Ca2+ = 18 e• Cations have same name as uncharged metal
• Charge can be determined from group number
Atomic structures of ions
• Nonmetals form anions
• Gain of 1 electron = -1 charge
• Anions named by changing ending of element name
to -ide
– Fluorine: F + 1 e- = F- (fluoride ion)
– Oxygen: O + 2 e- = O2- (oxide ion)
• Charge determined by taking 8 - group number
Writing ionic formulas
• Ionic compound must have no net charge
• So change number of ions to cancel out positive and
negative charges
• Compound made of Mg2+ and Cl– Must have formula MgCl2