l41 - atomic structure
TRANSCRIPT
ATOMIC STRUCTUREATOMIC STRUCTURE
Atomic StructureAtomic Structure All matter is composed of atoms.
Understanding the structure of atoms is critical to understanding the properties of matter
HISTORY OF THE ATOMHISTORY 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 themATOMSATOMS
8 X2Y16 X 8 Y+
DALTONS ATOMIC THEORY
Subatomic Particles
Particle Mass (g)
Charge (Coulombs)
Charge (units)
Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1
Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1
Neutron (n) 1.67 x 10-24 0 0
mass p = mass n = 1840 x mass e-
HISTORY OF THE ATOMHISTORY OF THE ATOM
1898 Joseph John Thompson
found that atoms could sometimes
eject a far smaller negative
particle which he called an
ELECTRONELECTRON
J.J. Thomson, measured mass/charge of e-
(1906 Nobel Prize in Physics)
A = alphaB = gammaC = beta
gold foil
helium nuclei
CHARGE OF AN ELECTRON
Millikan oil drop experiment
HISTORY OF THE ATOMHISTORY 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.
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
Rutherford’s Model of the Atom
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
+N
N
+-
-
proton
electron
neutron
Shell
Atomic StructureAtomic StructureEvery 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 StructureAtomic StructureEach 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 STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and neutrons in an atom
HeHe
22
44Atomic mass
Atomic number
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
XAZ
H11 H (D)21 H (T)3
1
U23592 U238
92
Mass Number Atomic Number Element Symbol
Atomic Structure
Atomic StructureAtomic Structure
Two isotopes of sodium.
HISTORY OF THE ATOMHISTORY 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 ATOMSMULTIELECTRON ATOMS
ELECTRONS IN ORBIT ABOUT ELECTRONS IN ORBIT ABOUT
THE NUCLEUSTHE NUCLEUS
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
Bohr’s Model of the Atom (1913)
En = -RH ( )1n2
n (principal quantum number) = 1,2,3,…RH (Rydberg constant) = 2.18 x 10-18J
The Bohr Model of the Atom
Atomic StructureAtomic 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
Ephoton = E = Ef - Ei
Ef = -RH ( )1n2f
Ei = -RH ( )1n2i
i fE = RH( )
1n2
1n2
RH is the Rydberg constant
n is the principal quantum number
En = -RH ( )1n2
Bohr showed the energy a H atom can have is equal to:
Line spectrum ofsome elements
LIGHT EMISSION OF SODIUM ATOM
Line spectrum
Atomic StructureAtomic StructureNeutral 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.
If a neutral atom gains one or more electronsit becomes an anion.
Na 11 protons11 electrons Na+ 11 protons
10 electrons
Cl 17 protons17 electrons Cl-
17 protons18 electrons