CHEMISTRY BASICS - PowerPoint by 2K152b



• Electrons are fundamental subatomic
  particles (elementary) with negative
  electric charge.
  – They have mass although it is so much
    less than the protons mass they are
    considered negligible (≈1/2000)
     • Yet, they can also be described
       as energy
  – Each electron in an atom has it’s own
    energy and exist in a particular volume
    of space that is visualized as a cloud

The Electron
  – An electron has a negative charge
    exactly equal to the charge of a proton
     • Therefore, in a neutral atom there
       are the same number of electrons
       as there are protons
     • In an atom with a charge (an ion)
       there are either more electrons than
       protons (negative ion) or fewer
       electrons than protons (positive ion)

The Electron
The Electron
• Protons are subatomic particles
  with a positive electric charge.
  – They are 2000 times larger than the
    electron with a mass of 1.673×10−27 kg
     • Protons are not elementary particles,
       but are composed of the elementary
       particles, quarks (2 ups and 1 down)
  – Protons are found in the dense
    nucleus in the center of each atom
     • The number of protons in the
       nucleus defines the type
       of atom.

The Proton
• The atomic number on the PT is due
  to the count of protons in the nucleus
  – Oxygen has 8 protons & is atom # 8
  – Potassium has 19 protons & is #19
• The number of protons of a given atom
  never changes in chemical reactions
  – If an ion is formed, it is the electrons that
    reduce or increase, not the protons
• Protons have a charge equal to but
  opposite the charge of the electrons

The Proton
• Neutrons are subatomic particles
  with a neutral electric charge.
  – They are equal in mass with the proton
    with a mass of 1.675×10−27 kg
     • Neutrons are not elementary
       particles, but are composed
       of quarks (1 up and 2 downs)
  – Neutrons are found in the dense
    nucleus in the center of each atom
     • They are primarily responsible
       for the stability of the

The Neutron
The Neutron
• The nucleus is an extremely small
  dense core found in every atom.
  – It is made up of any protons and
    neutrons in the atom (nucleons)
  – It is where you find the mass of any
    given atom
     • Atomic mass is the sum of the masses
        of the protons and the neutrons
  – It is held together by 2 of the universes
    fundamental forces, the strong and
    weak nuclear forces

The Nucleus
• The nucleus is an extremely small
  dense core found in every atom.
  – The size of the nucleus is many times
    smaller than the atom itself
     • Hydrogen atom is 145,000 times
       larger than its nucleus
     • Uranium atom is 23,000 times
       larger than its nucleus
  – Given a collection of atoms of a given
    element, the number of protons never
    change, but the number of neutrons
    may (isotopes)
The Nucleus
• The nucleus is composed of a densely
  packed cluster of protons which are all
  electrically positive
  – If like charges repel why
    doesn’t the nucleus fly apart?
• When 2 protons are in very close
  proximity, there is a strong force
  of attraction between them.
  – similar attraction exists
    when neutrons are close

Nuclear Structure
• These short-range p+-n0, p+-p+, &
  n0-n0 forces hold the nuclear particles
  together, A.K.A strong nuclear forces.
  – When these nuclear forces are strong
    enough the atom is stable
  – If the forces are not strong enough
    the atom (heavier atoms) the atom
    is unstable and becomes

Nuclear Structure
               ELECTRON        PROTON         NEUTRON
 SYMBOL           e-              p+              n0
CHARGE            -1              +1              0
CHARGE (C)   -1.602 x 10−19 +1.602 x 10−19        0
 MASS              0              1               1
MASS (kg)    9.109 x 10-31   1.673 x 10-27   1.675 x 10-27
LOCATION                       nucleus         nucleus
 Summary of the Parts of the Atom
• Basic Truth: All atoms contain the
  same basic parts, but atoms of
  different elements have different
  numbers of protons.
  – The PT lists atoms in consecutive
    order by their Atomic Number (Z)
  – The atomic number is directly
    related to the number of protons
    in the nucleus of each atom
    of that element

Atomic Structure
• The atomic number is found above
  the elemental symbol on the PT and
  it defines the type of element
  – Atomic #47 can only be Ag and it also
    can only have 47 protons in each
  – Because atoms are neutral,
    we know from the atomic number
    the atom must also contain
    47 electrons.

Atomic Structure
• The total number of protons &
  neutrons determines the mass of the
  – Called the Mass Number
  – A Carbon atom, has 6 protons and 6
    neutrons, so its mass number is 12
• If you know the atomic number &
  mass number of an atom of any
  element, you can determine the
  atom’s composition

Atomic Structure
• Every Cl atom has 17 protons, w/o
  exception, but not every Cl atom has
  18 neutrons.
  – Atoms with the same # of protons but
    contain different #s of neutrons are
    called isotopes.
     • Since isotopes of an element have
       different #s of neutrons they have
       different masses

Atomic Structure
• Isotopes are chemically alike because
  they have identical numbers of protons
  and electrons
  – It’s the electrons and protons that are
    responsible for chemical behavior
• Isotopes can be noted using hyphen
  notation (Cl-35 vs Cl-37)
  – elemental symbol hyphen mass

Atomic Structure
        or Na-23                or Na-24

Isotope: one of two or more atoms having
         the same number of protons but
         different numbers of neutrons
• The e- cloud of an atom is held in place
  by attraction with the atom’s nucleus
  – The nucleus is positively charged due
    to the protons.
  – The electron cloud is negatively
    charged due to the electrons
• However, sometimes an atom can
  gain or lose 1 or more of its electrons
  – This creates a charged
    atom called an ion

Atomic Structure
The Charge = protons - electrons

 IONS                                      17 p+
                11 p+                     -18 e-
               -10 e-
If an atom loses an electron it becomes
electrically positive (cation)
If an atom gains an electron it becomes
electrically neutral (anion)
Atomic Structure
Isotopic Symbols
  NAME      SYMBOL   ATOMIC #   p+   n0   MASS #   e-
Hydrogen      H         1        1    0     1      1
 Helium       He        2        2    2     4      2
Lithium       Li        3        3    4     7      3
Beryllium     Be        4        4    5     9      4
 Boron        B         5        5    6    11      5
Carbon        C         6        6    6    12      6
Nitrogen      N         7        7    7    14      7
Oxygen        O         8        8    8    16      8
Fluorine      F         9        9   10    19      9
  Neon        Ne       10       10   10    20      10
                    Atomic   Mass                        Isotopic
  Name       Symb                   # p+   # e-   # n0
                       #      #                          Symbol

 Strontium                                 36


                     82                    78

                                                         33   As
                                                         30       3
                                                         15   P

• If you cut a piece of paper in half and
  then half again and then half again…
  – How small can you get? What is the
    ultimate of smallness?
• These questions have been asked by
  scientists for generations, and is still
  being asked today.
  – We’ve come a long way in under-
    standing what the atom looks like,
    but we are still only using best guess
From Philosophy to Science
John Dalton gave us our first true model of an
atom, and even expanded that to an atomic
theory.                           Wooden beads
                                   to represent
                                 different atoms

From Philosophy to Science
• In 1808, Dalton published a paper that
  for the first time began to explain the
  behavior of chemicals
  – He reasoned that chemicals were
    composed of atoms & that only whole
    #’s of atoms can combine to form
  – His ideas are now called the
    Atomic Theory of Matter

Atomic Theory
 1.All matter is composed of
   extremely small particles called

 2. Atoms of a given element are
    identical in size, mass, and            2
    other properties; atoms of
    different elements differ in size,
    mass, & other properties

Atomic Theory of Matter
  3. Atoms cannot be
     subdivided, created,
     or destroyed

 4. atoms of different
    elements combine in
    simple whole # ratios to
    form chem compds

5. in chemical rxns, atoms
   are combined,               +   +
   separated, or rearranged

Atomic Theory
• Once the Atomic Theory of Matter was
  published, it helped make chemical
  reactions much more predictable
  – Not all of Dalton’s claims are still
    accepted today, but it was a start
    • Atoms CAN be divided into even
      smaller particles
    • Not every atom of an element has
      an identical mass

Atomic Theory
• Scientists soon began to discover that
  atoms are actually composed of
  several basic types of smaller particles
  – it’s the number and arrangement of
    these particles that determine the
    atom’s chemical properties.
• The def. of an atom that emerged
  was, the smallest particle of an
  element that retains the chemical
  properties of that original
Atomic Theory
• J.J. Thomson was the 1st scientist that
  decided that Dalton’s atom needed to
  be updated, so he proposed a new
  model of the atom
  – His model became known as the plum
    pudding model
     • He envisioned the atom being mostly
       positive matter (the pudding)
       – to offset the negative pieces
    • With the negative pieces sprinkled
      in (the plums)

Thomson Model of the Atom
 Negative particles
  embedded in a
 sphere of positive
plasma-like matter.

A ball of chocolate chip
     cookie dough
• Next, Rutherford’s results led him to
  propose a new atomic model.
  – He proposed that all the positive charge
    is concentrated in a small core in the
    center of the atom, AKA nucleus
  – All of the mass of the atom was
    concentrated in the nucleus as well
  – And that the atom is mostly empty
    space with the electrons surrounding
    the positively charged nucleus like
    planets around the sun.

Rutherford Model of the Atom
• The nucleus is in
  the center of the
  atom (the sun)
• The electrons are
  orbiting around
  the nucleus
• Most of the atom,
  therefore, is empty
  space (galaxy)

Nuclear Atom
• Rutherford’s planet system model was
  an improvement over earlier models,
  but it was still not complete.
  – Physics says that charged particles
    (electrons) in motion can’t be in motion
    (orbit) without losing energy
     • Losing energy would cause the
       electron to spiral into the nucleus.
     • The attraction of the electron to the
       nucleus would cause it to spiral into
       the nucleus as well
Bohr Model of the Atom
• Niels Bohr proposed a new model
  that would allow the electrons to be
  outside the nucleus and in orbit
  around the nucleus.
  – His model coupled Rutherford’s model
    with a newer concept regarding energy
    called quantum mechanics
     • Bohr proposed that the electrons
       aren’t on any random orbit around
       the nucleus, they are on
       “special” orbits

Bohr Model
• Bohr’s Model restricts the orbits
  on which an electron can be
  – The bases for what orbit an electron is
    allowed is entirely based on how much
    energy the electron has
     • If it has any more energy or any less
       energy it would be forced to be on a
       different path of different energy
  – The energy of the electron
    is quantized, which means it
    is of a very specific quantity

Bohr Model
  – Each path or level of energy that the
    electron is on is given a label of “n”
     • Such that n=1 is the closest energy
       level to the nucleus
     • n=2 is higher in energy and outside of,
       but adjacent to n=1, and so on…
  –Each energy level can only hold
   a certain number of electrons (2n2)
    • n=1 can hold 2 electrons
    • n=2 can only hold 8 electrons
    • n=3 can hold 18 electrons
    • Etc.
Bohr Model
            Bohr Model of the Atom
Model describes the paths of electrons as energy levels.
The electrons are only allowed to have a certain amount
of energy which restricts their path around the nucleus.
1. Determine the number of electrons in the atom or ion
2. Divide the electrons up among energy levels using
   dots to represent electrons, never putting too many
   on an energy level, and never moving to the next
   level until the previous ones are full

 Be                           Ca

For the nucleus just put the number of protons
and the number of neutrons in the center of the
atom. (e.g. He: 2p, 2n)
• Draw a Bohr model for the
  Nitrogen-15 isotope.

• Draw a Bohr model for the
  Halfnium-179 isotope.

Your Turn:
• Masses of atoms expressed in grams
  are extremely small.
  – An atom of O-16 literally weighs
    0.0000000000000000000000266 g
• For most calcs in chemistry it is easier
  to use a relative atomic mass.
  – for relative masses, an atom was
    arbitrarily chosen as the standard,
    by which all other masses are

Atomic Mass
• The mass of all the other atoms are
  then expressed in relation to the
  – C-12 atom acts as the standard
• A single C-12 atom was assigned
  a mass of 12 atomic mass units (amu)
  – 1 amu is exactly 1/12 of the mass
    of a C-12 atom
  – Carbon has 12 nucleons therefore,
    the mass of a proton = 1 amu

Atomic Mass
• The masses of all other atoms are based
  on the mass of 1 amu
  – Hydrogen has 1 p+ therefore, its most
    common isotope weighs 1 amu
  – Helium has 2 p+ and 2 n0 therefore, its
    most common isotope weighs 4 amus
• Most elements occur naturally as a
  mixture of isotopes.
  – For instance in the two isotopes      of
    Chlorine (Cl-35 & Cl-37)

Atomic Mass
• Each isotope isn’t found 50/50 in nature
  – 75% of all of the Cl atoms
    are the isotope Cl-35
  – 25% of the all of the Cl atoms
    are the isotope Cl-37
• Cl-35 atoms are naturally more
  abundant than Cl-37
  – So, the average mass of Cl (taking
    into account abundance) should be
    closer to 35 than to the
    median of 36.
Atomic Mass
10 Students took a test their grades were:
100   What’s the class average?

Atomic Mass
• Masses on the PT work the same way,
  they are weighted averages of all of the
  naturally occurring isotopes of an
  – The average atomic mass on the PT
    is reflective of both the mass numbers
    of each isotope & the abundance of
    each isotope
       Here’s how the masses were
     calculated on the Periodic Table:

Atomic Mass
Calculating average atomic mass:
     Naturally occurring copper consists
    of 69.17% Cu-63, which has a mass
  of 62.9296011 amu, and 30.83% Cu-65,
      with a mass of 64.9277937 amu.
• The average atomic mass of Cu is
  calculated by multiplying the atomic
  mass of each isotope by its fractional
  abundance and adding the results.

Atomic Mass
 Cu-63   (0.6917)(62.9296011 amu)
 Cu-65 + (0.3083)(64.9277937 amu)
           63.55 amu w/sig amu

• The reported average atomic mass
  of naturally occurring Cu is
  63.55 amu.

Atomic Mass
• Naturally occurring element X exists in three
  isotopic forms: X-28 (27.977 amu, 92.21%
  abundance), X-29 (28.976 amu, 4.70%
  abundance), and X-30 (29.974 amu, 3.09%
  abundance). Calculate the weight of X.
• Lithium has 2 common isotopes, Li-6 with
  a mass of 6.015122 amu & Li-7 with a
  mass of 7.016003 amu. The average
  atomic mass for Li is 6.9409 amu.
  Calculate the percent abundance
  of each?

Your Turn:

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