Elements, Atoms

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Elements, Atoms IonsChapter 4
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Elements

• Over 112 known, 88 of which occur naturally.
• others are man-made (made in labs)
• Abundance is the mass percent in the Earths
  crust, oceans and atmosphere
• oxygen most abundant element and accounts for
  about 20 of Earths atmosphere
• the abundance and form of an element varies from
  place to place but given as an overall average
• Each element has a unique symbol
• The symbol of an element may be one letter or two
• if two letters in the symbol, only the first
  letter is capitalized.

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The Periodic Table
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Daltons Atomic Theory

• 1. Elements are composed of atoms
• 2. All atoms of a given element are identical
• all carbon atoms have the same characteristics of
  every other atom of carbon
• 3. Atoms of a given element are different from
  those of any other element
• carbon atoms have different characteristics than
  oxygen atoms

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John Dalton (1766 1844) was an English
scientist who made his living as a teacher in
Manchester.
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Daltons Atomic Theory

• 4. Atoms of one element combine with atoms of
  other elements to form compounds.
• Law of Constant Composition
• all samples of a compound have the same
  proportion of the elements as in any other sample
  of that compound
• Chemical Formulas

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Dalton pictured compounds as collections of
atoms. Here NO, NO2, and N2O are represented.
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Daltons Atomic Theory

• 5. Atoms are indivisible in a chemical process.
• all atoms present at the beginning of a chemical
  process must also be present at the end of the
  process.
• atoms are not created or destroyed, they must be
  conserved.
• atoms of one element cannot be turned into atoms
  of another element
• You cannot turn atoms of lead into atoms of gold

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Formulas Describe Compounds

• Compound - distinct substance that is composed of
  the atoms of two or more elements and always
  contains exactly the same relative masses of
  those elements.
• Compounds are described by the elements in them
  and how many atoms of each element are in that
  compound.
• Chemical formula indicates the type and number
  of each element in a given compound.
• if there is only one atom of an element, the
  number is not written after the symbol, it is an
  implied 1.

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Atomic Structure History

• Discovery of the Electron
• In 1897, J.J. Thomson used a cathode ray tube to
  deduce the presence of a negatively charged
  particle.
• Cathode ray tubes pass electricity through a gas
  that is contained at a very low pressure.

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Schematic of a cathode ray tube.
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Conclusions from the Study of the Electron

• Cathode rays have identical properties regardless
  of the element used to produce them.
• Therefore, all elements must contain identically
  charged electrons.
• Atoms are neutral, so there must be positive
  particles in the atom to balance the negative
  charge of the electrons
• Electrons have so little mass that atoms must
  contain other particles that account for most of
  the mass

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The Electron

• Tiny, negatively charged particle
• Very light compared to the mass of the rest of
  the atom
• 1/1836th the mass of a proton

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Thomsons Atomic Model

• Thomson believed that the electrons were like
  plums embedded in a positively charged pudding,
  thus it was called the plum pudding model
  (easier to think of as chocolate chips" in
  chocolate chip cookie dough.

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Rutherfords Gold Foil Experiment
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Rutherfords Gold Foil Experiment

• bullet alpha particles, target atoms gold
  foil
• ?? particles are positively charged
• gold atoms are about 50 larger than a particles.
• Alpha particles are helium nuclei
• Particles were fired at a thin sheet of gold
  foil
• Particle hits on the detecting screen (film) are
  recorded

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(a) The results that the metal foil experiment
would have yielded if the plum pudding model had
been correct. (b) Actual results.
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Rutherfords Results

• Over 98 of the ? particles went straight through
• About 2 of the ? particles went through but were
  deflected by large angles
• About 0.01 of the ? particles bounced off the
  gold foil
• Most of the volume of the atom is empty space

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Rutherfords Nuclear Model

• The atom contains a tiny dense center called the
  nucleus
• the volume is about 1/10 trillionth the volume of
  the atom
• The nucleus is essentially the entire mass of the
  atom (extremely dense)
• The nucleus is positively charged
• the amount of positive charge of the nucleus
  balances the negative charge of the electrons
• The electrons move around in the empty space of
  the atom surrounding the nucleus

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Structure of the Nucleus

• The nucleus was found to be composed of two kinds
  of particles
• Some of these particles are called protons
• charge 1
• mass is about the same as a hydrogen atom
• Since protons and electrons have the same amount
  of charge, for the atom to be neutral there must
  be equal numbers of protons and electrons
• The other particle is called a neutron
• has no charge
• has a mass slightly more than a proton

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The Modern Atom

• We know atoms are composed of three main pieces -
  protons, neutrons and electrons
• The nucleus contains protons and neutrons
• The nucleus is only about 10-13 cm in diameter
• The electrons move outside the nucleus with an
  average distance of about 10-8 cm
• therefore the radius of the atom is about 100,000
  times larger than the radius of the nucleus

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A nuclear atom viewed in cross section.
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Atomic Particles
Particle Charge Mass Location
Electron -1 0 Electron cloud
Proton 1 1 Nucleus
Neutron 0 1 Nucleus
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Atomic Structure

• The number of protons in an atom of a given
  element is the same as the atomic number (Z).
• found on the Periodic Table, whole for each
  element

Element of protons Atomic (Z)
Carbon 6 6
Phosphorus 15 15
Gold 79 79
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Atomic Structure

• Mass number protons neutrons always a whole
  number.
• of Neutrons mass number - of protons
• Atomic mass larger number in each elements box
  on the periodic table. If you round the atomic
  mass of an element to the closest whole number
  you generally get the mass for that element.

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Atomic Structure

• of Electrons of protons if the atom is
  neutral
• If the chemical symbol is written with a charge,
  representing an ion, the charge indicates the
  number of electrons that have been added or
  removed from the atom.

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Atomic Structure

• If the ion has a positive charge (cation),
  subtract that charge from the of protons to get
  the number of electrons.
• If the ion has a negative charge (anion), add
  that charge number to of protons to get the
  number of electrons.
• of Electrons protons charge
• Charge protons - electrons

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Nuclear Symbols
Mass number (p no)

• 
  Charge (if any)

Element symbol
Atomic number (number of p)
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Finding of particles and charge

• 2713Al3
• 3517Cl-1
• 13756Ba2

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Isotopes

• Isotopes - atoms of an element with the same
  number of protons and electrons, but different
  numbers of neutrons.

Isotope Protons Electrons Neutrons Nucleus
Hydrogen1 (protium) 1 1 0
Hydrogen-2 (deuterium) 1 1 1
Hydrogen-3 (tritium) 1 1 2
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Two isotopes of sodium.
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Isotopes Examples

• 3517Cl 3717Cl
• H-1 H-2 H-3
• Copper 63 Copper 65

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Determining Average Atomic Mass

• To determine the average atomic mass for an
  element, you must
• Multiple the percentage (percent abundance) of
  each isotope of the element by its mass number.
• Add the products of the multiplications together.
• Divide by 100.
• Your answer should be very close to the atomic
  mass of the element for that element

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Average Atomic Mass Examples

• Find the average atomic mass of each of the
  following elements from their percentages and
  mass numbers.
• 69.17 63Cu and 30.83 65Cu
• 5.85 Fe-54, 91.75 Fe-56, 2.12 Fe-57 and 0.28
  Fe-58

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Elements

• Arranged in a pattern called the Periodic Table
• Elements listed by increasing atomic number.
• Originally elements were listed by increasing
  atomic mass (Dmitri Mendeleev), but later it was
  found that by placing the elements in order of
  increasing atomic number (Moseley).
• Position on the table allows us to predict
  properties of the element

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Mendeleevs Original Periodic Table
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The modern periodic table.
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The Modern Periodic Table

• Elements with similar chemical and physical
  properties are in the same column
• Columns are called Groups or Families
• Rows are called Periods
• The properties of the elements in one period are
  very similar to the element directly above or
  below it.

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Elements

• Metals
• about 75 of all the elements
• lustrous, malleable, ductile, conduct heat and
  electricity
• Nonmetals
• dull, brittle, insulators, do not conduct heat
  and electricity
• Metalloids (Si, Ge, As, Sb, Te, Po, At)
• also know as semi-metals
• some properties of both metals nonmetals

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Figure 4.12 The elements classified as metals
and as nonmetals.
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Important Groups

• Group 18 Noble Gases
• He, Ne, Ar, Kr, Xe, Rn
• all colorless gases at room temperature
• very non-reactive, practically inert
• found in nature as a collection of separate atoms
  uncombined with other atoms

• Noble Metals
• Ag, Au, Pt
• all solids at room temperature
• least reactive metals
• found in nature uncombined with other atoms

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Platinum is a noble metal used in jewelry and in
many industrial processes.
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The Modern Periodic Table

• Main Group Representative Elements
• A columns (Groups 1,2 and 13-18)
• Transition Elements
• B columns (Groups 3-12) all metals
• Bottom rows Inner Transition Elements Rare
  Earth Elements (Lanthanides and Actinides)
• metals
• really belong in Period 6 7

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Figure 4.11 The periodic table.
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Important Groups - Halogens

• Group 17 Halogens
• very reactive nonmetals
• react with metals to form ionic compounds

• Fluorine F2
• pale yellow gas
• Chlorine Cl2
• pale green gas
• Bromine Br2
• brown liquid that has lots of brown vapor over it
• Only other liquid element at room temperature is
  the metal Hg
• Iodine I2
• lustrous, purple solid

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Other Important Groups

• Alkali metals (Group 1) extremely reactive
  metals generally react with halogens to form
  salts. Form ions with a 1 charge.
• Alkali earth metals (Group 2) not as reactive
  as alkali metals. Form ions with a 2 charge.
• Transition metals less reactive than metals of
  Groups 1 2. Form ions that usually have more
  than one possible charge.

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Natural State of Elements

• Most elements are solids at room temp. and can
  exist as single atoms.
• All metals except for mercury and gallium are
  solids at room temp.
• Nonmetals can be found in all three common states
  of matter sulfur (solid), bromine (liquid) and
  oxygen (gas).

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Argon gas consists of a collection of separate
argon atoms.
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In solid metals, the spherical atoms are packed
closely together.
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(a) Sodium chloride (common table salt) can be
decomposed to the elements (b) sodium metal (on
the left) and chlorine gas.
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Liquid bromine in a flask with bromine vapor.
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Diatomic Molecules

• Diatomic molecules exist only as pairs of atoms
  of these particular elements. They cannot exist
  as single atoms.
• Remember the 7 on the Periodic Table plus
  Hydrogen to remember all of the diatomic
  molecules.
• H2, N2, O2, F2, Cl2, Br2, I2

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Figure 4.11 The periodic table.
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Gaseous nitrogen and oxygen contain diatomic
(two-atom) molecules.
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The decomposition of two water molecules (H2O) to
form two hydrogen molecules (H2) and an oxygen
molecule (O2).
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Allotropes

• Many solid nonmetallic elements can exist in
  different forms with different physical
  properties, these are called allotropes
• the different physical properties arise from the
  different arrangements of the atoms in the solid
• Allotropes of Carbon include
• diamond
• graphite
• buckminsterfullerene

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Graphite and diamond, two forms of carbon.
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The three solid elemental forms of carbon
(allotropes).