Atomic Structure
Atom – The smallest particle of an element that retains its chemical identity
• Aristotle – continuous view of matter
• Democritus – discontinuous view of matter
• Democritus – believed that all matter consisted of extremely small particles
that could not be divided. ( 400 BC)
• Atoms from the Greek word atomos, which means uncut” or “indivisible
Dalton’s Atomic Theory -1803
• Each element comprised of atoms
• All atoms of an element are identical
• Atoms are neither created or destroyed in a chemical reaction
• A compound has the same relative number and kinds of atoms ( H2O)
Dalton
J. J. Thomson 1897
Thomson discovered that an atom was neutral in charge but contained a positive
charged mass and negatively charged particles spread through out.
Thomson
Thomson’s experiment used a cathode ray tube that projects a glowing beam
that bent when a charge was introduced
.
Ernest Rutherford 1908
Rutherford's model proposed that an atom is mostly empty space.
There is a small, positive nucleus with the negative electrons scattered
around the outside edge
.
• The nucleus is a dense, positively charged mass located in the
center of the atom.
Rutherford
Neils Bohr (1913)
• Bohr agreed with Rutherford’s model that the nucleus was
surrounded by a large volume of space
• Bohr’s model focused on the arrangement of electrons based on
energy levels
.
• Energy levels are the possible energies that electrons in an atom
can have.
• electrons can move up or down energy levels
Bohr
James Chadwick (1891 – 1974)
In 1932 –
Suggested radiation from a Beryllium (Be) atom consisted of
neutral particles with a mass equal to a proton.
This neutral subatomic particle is called a “neutron”.
Nuclear reaction that resulted in the discovery of a neutron:
Be - 9 + alpha particle → C -12 + 1n
0
Radiation emitted by alpha particle bombardment of a Be-atom
was not deflected by electric or magnetic field forces.
Therefore, the particle has no electric charge (neutral).
Electron Cloud
• An electron cloud is a visual model of the most likely locations for
electrons in an atom.
• The cloud is denser at locations where the probability of finding an
electron is high.
Modern Atomic Theory
Atoms comprised of 3 subatomic particles
Protons, neutrons, and electrons
Atom’s nucleus – positively charged central core comprised of protons(+)
and neutrons(0)
Negatively charged electrons move around the nucleus
Fundamental Subatomic Particles
Particle
Location
Charge
Mass (amu)
Proton
Nucleus
+1
1.00
Neutron
Nucleus
0
1.00
0.00
Periodic Table Atomic Information
Element Name →
Atomic Mass →
Symbol →
Atomic Number →
Nitrogen
14.00
N
7
Atomic Number = # of Protons
Mass # = # protons + # neutrons
Atoms are electrically neutral (0 charge),
Atomic # = # protons = # electrons
Hydrogen Atom: ( atomic # 1 )
1 proton (+)
1 electron ( - )
IONS
Charged particles from atoms that lost or gained electron(s)
Ion Charge = # Protons - # electrons
Negative Ion = Anion = # electrons > # protons
Positive Ion = Cation = # electrons < # protons
Calcium atom loses 2 electrons, = ? ion
Calcium atom (atomic # 20) = # protons = 20 ; # electrons = 20
Atom lost 2 electrons, now has 18 electrons
ion Charge = # protons - # electrons
20 - 8
= +2
Calcium cation = Ca+2
Isotopes
Atoms with same # protons, but a different # neutrons
3 Hydrogen Isotopes:
Symbol
Hydrogen
1 proton
0 neutrons
H-1
Deuterium
1 proton
1 neutron
H-2
Tritium
1 proton
2 neutrons
H-3
An element’s isotopes:
 Differ in Mass (protons + neutrons)
 Have identical chemical properties, but differ in physical properties
 Natural Elements are an isotope mixture
Atomic Weight of an element posted on the periodic Table is a weighted
average of the atomic weights of all that element’s isotopes weights.
Express mass of an atom in a.m.u.
atomic mass units, a.m.u.
1 amu ~ proton mass
Mass of a Cu-65 atom is ~ 65 amu
Mass of a N -14 atom is ~ 14 amu
1 amu 1/12 (mass C-12 atom) = 1.66 x 10 – 24 g
Atomic weight ≠ Mass #
Mass # = # protons + # neutrons
Chlorine – 37 atom = 36.966 amu
Chlorine – 35 atom = 34.969 amu
What is the atomic weight of Chlorine?
Chlorine exists in nature as 2 isotopes:
Cl – 35 and Cl – 37
Isotope
Mass # Isotope
Mass
(amu)
Cl – 35
Cl – 37
35
37
34.969
36.966
Fractional
Abundance
(%)
75.53
24.47
Chlorine – _____? is the most abundant isotope of chlorine
Atomic weight of an element is the Weighted Average of its isotopes
Chlorine’s atomic weight = 35.45 amu
Atomic weight is listed on Periodic Table
Calculate the atomic weight of Chlorine
For each isotope of that element,
Sum of: % abundance x isotope mass
Cl - 35: 0.7553
x
34.969 amu = 26.41 amu
Cl – 37: 0.2447
x 36.966 amu = 9.04 amu
Atomic Weight = average mass =
(Periodic Table)
(weighted)
35.45 amu
Magnesium occurs as 3 isotopes,
Mg – 24,
Mg – 25, Mg – 26
Calculate the atomic weight of magnesium
Isotope
Mg – 24
mass (amu)
23.985
% abundance
78.70
Mg – 25
24.986
10.13
Mg – 26
25.983
11.17