Transcript L41 - Atomic Structure

ATOMIC STRUCTURE
Atomic Structure
All matter is composed of atoms.
Understanding the structure of atoms is
critical to understanding the properties
of matter
HISTORY OF THE ATOM
1808
John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS
DALTONS ATOMIC THEORY
16 X
+
8Y
8 X2Y
Subatomic Particles
Particle
Mass
(g)
Charge
(Coulombs)
Electron (e-) 9.1 x 10-28 -1.6 x 10-19
Proton (p)
1.67 x 10
-24
Neutron (n) 1.67 x 10-24
+1.6 x 10
0
-19
Charge
(units)
-1
+1
0
mass p = mass n = 1840 x mass e-
HISTORY OF THE ATOM
1898
Joseph John Thompson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
A = alpha
B = gamma
C = beta
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
CHARGE OF AN ELECTRON
gold foil
helium nuclei
Millikan oil drop
experiment
HISTORY OF THE ATOM
1910
Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them
passed through. About 1 in 10,000 hit
Rutherford’s experiment.
Plum Pudding model of an atom.
Results of foil experiment if Plum
Pudding model had been correct.
Actual Results.
A nuclear atom viewed in cross
section.
Rutherford’s Model of the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
Atomic Structure
Atoms are composed of
-protons – positively charged particles
-neutrons – neutral particles
-electrons – negatively charged particles
Protons and neutrons are located in the
nucleus. Electrons are found in orbitals
surrounding the nucleus.
HELIUM ATOM
Shell
proton
+
electron
N
N
+
-
neutron
Atomic Structure
Every different atom has a characteristic
number of protons in the nucleus.
atomic number = number of protons
Atoms with the same atomic number
have the same chemical properties and
belong to the same element.
Atomic Structure
Each proton and neutron has a mass of
approximately 1 dalton.
The sum of protons and neutrons is the atom’s
atomic mass.
Isotopes – atoms of the same element that
have different atomic mass numbers due to
different numbers of neutrons.
ATOMIC STRUCTURE
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
2
4
He
number of electrons = number of protons
ATOMIC NUMBER (Z) = number of protons in nucleus
MASS NUMBER (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
ISOTOPS are atoms of the same element (X) with different numbers of
neutrons in the nucleus
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
U
(T)
Atomic Structure
Atomic Structure
Two isotopes of sodium.
HISTORY OF THE ATOM
1913
Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits. Rather
like planets orbiting the sun. With each
orbit only able to contain a set number of
electrons.
MULTIELECTRON ATOMS
ELECTRONS IN ORBIT ABOUT
THE NUCLEUS
Bohr’s Model of
the Atom (1913)
1. e- can have only specific
(quantized) energy values
2. light is emitted as e- moves
from one energy level to a
lower energy level
En = -RH (
1
n2
)
n (principal quantum number) = 1,2,3,…
RH (Rydberg constant) = 2.18 x 10-18J
The Bohr Model of the Atom
Atomic Structure
The Bohr Model of the Atom:
Ground and Excited States
• In the Bohr model of hydrogen, the lowest amount
of energy hydrogen’s one electron can have
corresponds to being in the n = 1 orbit. We call this
its ground state.
• When the atom gains energy, the electron leaps to a
higher energy orbit. We call this an excited state.
• The atom is less stable in an excited state and so it
will release the extra energy to return to the ground
state.
– Either all at once or in several steps.
Line Emission Spectrum of Hydrogen Atoms
Every element has a unique emission spectrum
The Bohr Model of the Atom:
Hydrogen Spectrum
• Every hydrogen atom has identical orbits, so every
hydrogen atom can undergo the same energy
transitions.
• However, since the distances between the orbits in
an atom are not all the same, no two leaps in an
atom will have the same energy.
– The closer the orbits are in energy, the lower
the energy of the photon emitted.
– Lower energy photon = longer wavelength.
• Therefore, we get an emission spectrum that has a
lot of lines that are unique to hydrogen.
The Bohr Model of the Atom:
Hydrogen Spectrum
Bohr showed the energy a H
atom can have is equal to:
En = -RH (
1
n2
)
Ephoton = DE = Ef - Ei
1
Ef = -RH ( 2
nf
1
Ei = -RH ( 2
ni
1
DE = RH( 2
ni
RH is the Rydberg constant
n is the principal quantum number
)
)
1
n2f
)
Line spectrum of
some elements
LIGHT EMISSION OF SODIUM ATOM
Line spectrum
Atomic Structure
Neutral atoms have the same number of
protons and electrons.
Ions are charged atoms.
-cations – have more protons than
electrons and are positively charged
-anions – have more electrons than
protons and are negatively charged
An ion is formed when an atom, or group of atoms, has a
net positive or negative charge (why?).
If a neutral atom looses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons