atomic structure theory for best preparation of jee advanced

Dalton’s Early Atomic Model
►“Billiard Ball” model
►he envisioned atoms as solid, hard spheres,
like billiard(pool) balls, so he used
wooden balls to model them

J.J. Thomson
1897
Discovered the electron
He was the first scientist to show
the atom was made of even smaller
things

JJ Thomson
►Used the Cathode ray tube to discover
electrons

Thomson’s Experiment
Voltage source
+
-
Vacuum tube
Metal Disks

Voltage source
+
-

 Passing an electric current makes a
beam appear to move from the
negative to the positive end
Voltage source
+
-

Voltage source
►By adding an electric field

Voltage source
 By adding an electric field
+
-

Voltage source
 Adding an electric field cause the beam to
move toward the positive plate.
 Thomson concluded the beam was made of
negative moving pieces.
+
- -

Thomson’s “Plum Pudding” Atom Model

Democritus, Dalton, Thomson Video

Eugen Goldstein 1850-1930
Using a cathode ray tube he discovered
canal rays which are beams of positively
charged particles.
He is credited with the discovery of
protons in an atom.
Canal Rays discovery

Millikan’s Oil Drop Experiment
• Determined the charge on an electron
• Used Thomson’s charge to mass ratio to
calculate the mass of an electron
Millikan’s Oil Drop
Experiment
Millikan’s Oil Drop
Experiment – another
video

Ernest Rutherford - 1913
• discovered the
nucleus of a
gold atom with
his “gold foil”
experiment

atomic structure theory for best preparation of jee advanced

Using J.J Thomson’s Plum Pudding
atomic model, Rutherford predicted the
alpha particles would pass straight
though the gold foil. That’s not what
happened.

Gold Foil Experiment Results
most alpha particles go straight through
the gold foil
A few alpha particles are sharply deflected

Rutherford’s Conclusion
►The atom is mostly
empty space.
►There is a small,
dense center with a
positive charge.
►Rutherford
discovered the
nucleus in atoms

Rutherford’s Contribution to the
Atomic Theory
►The atom is
mostly empty
space.
►The nucleus is
a small, dense
core with a
positive charge.

Structure of the Atom
► Atom – smallest particle of an element that can
exist alone
 Two regions of an atom
►Nucleus
 Center of atom
 Protons and neutrons
►Electron “cloud”
 Area surrounding nucleus
containing electrons

Structure of the Atom
►Proton – Positive charge (+), 1 atomic
mass unit (amu); found in the nucleus
 amu -Approximate mass of a proton or a
neutron
►Neutron – Neutral charge (0), 1 amu;
found in the nucleus
►Electron – Negative charge (-), mass is
VERY small

Counting Atoms
►Atomic Number
 Number of protons in nucleus
 The number of protons determines identity
of the element!!
►Mass Number (Atomic Mass)
 Number of protons + neutrons
 Units are g/mol

Counting Atoms
►Isotopes
 Atoms of the same element with varying
number of neutrons
 Different isotopes have different mass numbers
because the number of neutrons is different

Composition of the atom video
Start at 3:25

Counting Atoms
 Nuclear Symbol Notation

Atoms
►Protons have a positive (+) charge and
electrons have a negative (-) charge
►In a neutral atom, the number of protons
equals the number of electrons, so the
overall charge is zero (0)
 Example/ Helium, with an atomic number of 2,
has 2 protons and 2 electrons when stable

Ions
►In a neutral atom
 Atomic number = # of protons = #of electrons
►Sometimes atoms will gain or lose electrons
and form IONS
►Because an electron has a negative charge:
 When an atom GAINS electrons it becomes
NEGATIVE
 When an atom LOSES electrons it becomes
POSITIVE

Ions
Cation = a positive ion
Anion = a negative ion

Let’s Practice
►Aluminum (Al) (no periodic table)
 Protons = 13
 Electrons =
 Neutrons = 14
 Atomic Number =
 Atomic Mass =

Let’s Practice w/ nuclear symbol
notation
►Nuclear Symbol notation
(no periodic table)
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
Ag
108
47

Let’s Practice w/ isotopes
►use the periodic table
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
Uranium-235

Let’s Practice with Ions
Use the periodic table
 Charge =
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
K
39 1+

atomic structure theory for best preparation of jee advanced

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