Download - Atomic Structure
Atomic Structure Timeline
Democritus (400 B.C.)
• Proposed that matter was composed of tiny indivisible particles
• Not based on experimental data
• Greek: atomos
Alchemy (next 2000 years)
• Mixture of science and mysticism. • Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists.
John Dalton (1807)
• British Schoolteacher–based his theory on
others’ experimental data
• Solid Sphere Model–atom is a
uniform, solid sphere
John Dalton
Dalton’s Four Postulates1. Elements are composed of small
indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of different elements are different.
3. Compounds contain atoms of more than one element
4. In a compound, atoms of different elements always combine the same way.
Henri Becquerel (1896)
• Discovered radioactivity–spontaneous emission of
radiation from the nucleus• Three types:
–alpha () - positive–beta () - negative–gamma () - neutral
J. J. Thomson (1903)• Cathode Ray Tube
Experiments–beam of negative particles
• Discovered Electrons–negative particles within
the atom
• Plum-pudding Model
Thomson’s ExperimentThe Cathode Ray Tube
screen with hole Anode +
High Voltage
Cathode -
Gas at very low pressure(almost vacuum)
Direction of Cathode Rays
to vacuum pump
screen with hole Anode +
High Voltage
Cathode -
Gas at very low pressure(almost vacuum)
to vacuum pump
-
+
Negatively charged plate
Positively charged plate
Direction of cathode ray
screen with hole Anode +
High Voltage
Cathode -
to vacuum pump
Shadow formed
screen with hole Anode +
High Voltage
Cathode -
to vacuum pump
Thomson Model• Thomson studied the passage of an electric current through a gas.
• As the current passed through the gas, it gave off rays of negatively charged particles.
J. J. Thomson (1903)
Plum-pudding Model–positive sphere
(pudding) with negative electrons (plums) dispersed throughout
Eugen Goldstein (1886)
• Discovered proton (component of canal rays)– Positive
particles within the atom
screen with hole Anode +
High Voltage
Cathode -
Cathode rays (Electrons)
Direction of Cathode Rays
to vacuum pump
Direction of canal rays
Canal rays (positive particles
Robert Milikan (1909)
• Determined the electric charge of an electron
• The charge on a single electron: 1.602 × 10−19 coulomb
Milikan’s Oil Drop Experiment
Ernest Rutherford (1911)
• Gold Foil Experiment
• Discovered the nucleus–dense, positive
charge in the center of the atom
• Nuclear Model
Rutherford’s ExperimentGold Foil
Source ofalpha particles
Fluorescent screen
Gold foil
Atoms of gold
Alpha particles
NUCLEUS
Ernest Rutherford (1911)
• Nuclear Model–dense, positive nucleus surrounded by negative
electrons
James Chadwick (1932)
Discovered the neutron– neutral particles
within the atom
Subatomic Particles
POSIT IVECHARG E
PROTONS
NEUTRALCHARG E
NEUT RONS
NUCLEUS
NEG ATIVE CHARGE
ELECT RONS
ATOM
NUCLEUS ELECTRONS
PROTONS NEUTRONS NEGATIVE CHARGE
POSITIVE CHARGE
NEUTRAL CHARGE
ATOM
Atomic Structure
C126Mass
Number
Atomic Number
XAZMass
Number
Atomic Number
Element
Atomic Number (Z)
• Number of protons in an atom• Unique for each element
Z = p+
Mass Number (A)
• Number of nucleons in an atom– Nucleon is the numerical sum of the
protons and neutrons
A = p+ + n0
Subatomic Particles
POSIT IVECHARGE
PROT ONS
NEUT RALCHARG E
NEUTRONS
NUCLEUS
NEG ATIVE CHARG E
ELECTRONS
AT OM
Most of the atom’s mass. Atomic Numberequals the # of...
in a neutral atom
NUCLEUS ELECTRONS
PROTONS NEUTRONS NEGATIVE CHARGE
POSITIVE CHARGE
NEUTRAL CHARGE
ATOM
How do we compute for the number of protons, electrons and neutrons
in a neutral atom?
• Protonsp+ = Z
• Electronse- = p+
• Neutronsn0 = A – Z or n0 = A – p+
ExampleDetermine A, Z, p+, e-, n0 and net charge of
the neutral atom carbon.
C126
A = 12Z = 6p+ = 6e- = 6n0 = 6 Net Charge = 0
Ions
• Charged particle• Exist when an atom transfers or
gains one or more electrons• May be positive (cation) or negative
(anion)
Example
4126C
A = 12Z = 6p+ = 6e- = 6 – 4 = 2n0 = 6 Net Charge = +4
Determine A, Z, p+, e-, n0 and net charge of the charged atom carbon.
Isotopes
• Atoms that have the same number of protons but different numbers of neutrons.Ex. 1
1H – hydrogen2
1H – deuterium3
1H – tritium
Atomic Mass
• The mass of an atom in atomic mass unit (amu).
• Atomic mass unit is defined as the mass exactly equal to 1/12 the mass of one carbon-12 atom
Example
The element copper has naturally occurring isotopes with mass numbers 63 and 65. The percent abundance and atomic masses are 69.2% for atomic mass=62.93 amu, and 30.8% for atomic mass=64.93. Calculate the relative atomic mass of copper.
Example
Calculate the relative/average atomic mass of bromine. The two isotopes of bromine have atomic masses and percent abundance of 78.92 amu (50.69%) and 80.92 amu (43.91%).
ExampleCompute for X.
IsotopeAtomic mass (amu)
% Abundance
Relative Atomic Mass (amu)
3517Cl 34.969 75.53
35.4537
17Cl X 24.47
ExampleCompute for X.
IsotopeAtomic mass (amu)
% Abundance
Relative Atomic Mass (amu)
32He X 0.0001
4.002642He 4.0026 99.9999
Modern Atomic Theory
Niels Bohr (1913)• Bright-Line Spectrum
–tried to explain presence of specific colors in hydrogen’s spectrum
• Energy Levels–electrons can only exist in
specific energy states
• Planetary Model
Niels Bohr (1913)
• Planetary Model
–electrons move in circular orbits within specific energy levels
Bright-line spectrum
Erwin Schrödinger (1926)• Quantum mechanics
–electrons can only exist in specified energy states
• Electron cloud model –orbital: region around the
nucleus where e- are likely to be found
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)• dots represent probability of finding an e-
not actual electrons
Wave Model
James Chadwick (1932)• Discovered neutrons
–neutral particles in the nucleus of an atom
• Irene and Frederic Joliot-Curie –Chadwick based his
theory on their experimental evidence
James Chadwick (1932)
Neutron Model• revision of Rutherford’s Nuclear Model
Atomic Orbitals
• Orbital–Region of space
around the nucleus where an electron is likely to be found
–More energy = more orbitals
The Wave Model
• Today’s atomic model is based on the principles of wave mechanics.
• According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.
The Wave Model
• In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
• According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.
Electron Cloud:
• A space in which electrons are likely to be found.
• Electrons whirl about the nucleus billions of times in one second
• They are not moving around in random patterns.
• Location of electrons depends upon how much energy the electron has.
Electron Cloud:
• Depending on their energy they are locked into a certain area in the cloud.
• Electrons with the lowest energy are found in the energy level closest to the nucleus
• Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.
Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X
RADIOACTIVITYDefinition: spontaneous emission of
particles and.or radiationTYPES:• Alpha (α) Ray – consists of
positively charged particles • - has a low penetrating
ability (can be stopped by paper)
• Beta (β) rays – consists of electrons.
• has a medium penetrating ability (can be stopped by
heavy clothing)
• Gamma (γ) Rays – no charge and not affected by an external field.
• high penetrating ability (can be
stopped by lead)
Fundamental Subatomic ParticlesParticle Symbol Relative Mass Mass
Electrical (g) (amu)Charge
ELECTRON e- -1 9.11 x 10-28 0.00060
PROTON p+ +1 1.67 x 10-24 1.00731
NEUTRON n0 0 1.67 x 10-24 1.00871
Electron Configuration• Arrangement of electrons within the orbitals
of the atom–The most stable configuration is the one in
which electrons are in their lowest possible orbitals. This is called their Ground State.
• If energy is added, electrons can move to a higher energy orbital. The atom is then considered to be in an excited state.
Example• Lithium
–When lithium reacts with water, the atom goes to an excited state.
–When the electron returns to its ground state, it gives of energy in the form of fire.