Chapter 4: Elements, Atoms, and Ions

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Chapter 4 Elements, Atoms, and Ions
Chemistry 1020 Interpretive chemistry Andy
Aspaas, Instructor
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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

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Daltons 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

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Conclusions from Daltons 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

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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

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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

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Thomsons plum pudding model

• Atoms are indeed breakable, contrary to Daltons
  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

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Rutherfords 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 Thomsons plum pudding model

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Rutherfords nuclear model

• Most of atoms mass is in a tiny dense center
  called the nucleus
• Positively charged
• Only 1/10 trillionth the volume of the atom
• Nucleuss positive charge balances electrons
  negative charge
• Electrons fly around in the empty space
  surrounding nucleus

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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