Download Structure of the atom

Structure of the atom
The ancient Greeks were the first to
postulate that matter consists of
indivisible constituents.
Later scientists realized that the atom
consisted of charged entities.
EARLY 1900s
CRT (Cathode Ray Tubes) JJ Thompson
1. Rays are same no material what material is
used to generate them.
2. Rays are stream of negatively charged
particles.
Charge of electron determined later by Milliken
Structure of atom
Studies of radioactivity: matter is
composed of negative, positive and
neutral material
J.J. Thompson’s
interpretation:
cookie dough model
Structure of Atom
• ~1910 Earnest Rutherford
tested Thompson’s Model
Nuclear Atom
Rutherford
concludes:
massive nucleus
with electrons
orbiting around it.
Modern View of atom
• An atom is almost entirely
empty space.
1nm = 10-9 m
10Å = 1nm
Å= angstrom
Atom Diameter = 0.1-0.5 nm
Nuclear diameter = 10-5 nm
Ratio of nuclear size to atomic radius: 10-4
Carbon atom diameter = 0.154 nm
Modern View of Atom
COMPONENTS OF ATOMS
mass
charge
(a.m.u.)
(a.u.)
proton
p
electron e
neutron n
~1
~0
~1
Neutral
19
9
Ion
39
19
F
K
+
+1
−1
0
ISOTOPES
• Number of protons = atomic number
• DEFINES identity of element
For a given element, number of protons is
fixed, but number of neutrons can vary.
35
17
Cl
37
17
Cl
Isotopes: atoms of a given element with a
different number of neutrons.
The Atomic Mass Scale
• We define:
mass of 12C = exactly 12 amu.
1 amu = 1.66054 x 10-24 g
1 g = 6.02214 x 1023 amu
• Using atomic mass units:
1H
weighs 1.6735 x 10-24 g
16O 2.6560 x 10-23 g.
Average Atomic Masses
Relative atomic mass: average masses of isotopes:
Naturally occurring C: 98.892 % 12C + 1.108 % 13C.
Average mass of C:
(0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu.
Atomic weight (AW) is also known as average atomic
mass (atomic weight).
Atomic weights are listed on the periodic table.
The Mole and Avogadro’s Number
Mole: connection between microscopic
(what we don’t see: atoms) and
macroscopic (what we see)
1 mole of anything (atoms, molecules, green peas) =
Avogadro’s number of them
1 mole 12C atoms has 6.02 × 1023 12C atoms.
1 mole CO2 molecules = 6.02 × 1023 CO2 molecules
1 mole peas = 6.02 × 1023 peas
Experimentally, 1 mole of 12C has a mass of 12 g.
CRT
Cathode Rays and Electrons
• A cathode ray tube (CRT) is a
hollow vessel with an electrode at
either end.
• A high voltage is applied across
the electrodes.
• The voltage causes negative
particles to move from the
negative electrode to the positive
electrode.
• The path of the electrons can be
altered by the presence of a
magnetic field.
CRT
Consider cathode rays leaving the positive
electrode through a small hole.
• If they interact with a magnetic field
perpendicular to an applied electric field,
the cathode rays can be deflected by
different amounts.
• The amount of deflection of the cathode
rays depends on the applied magnetic and
electric fields.
• In turn, the amount of deflection also
depends on the charge to mass ratio of
the electron.