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Atomic Theory
Atoms
• Matter is made of atoms
•Atoms are the basic building block of matter
History of the Atom400 BC DemocritusTHE ATOM IS AN INDIVISIBLE PARTICLE• the atom is the smallest particle of matter1808 DaltonTHE SOLID SPHERE MODEL• atoms are solid , indestructible spheres (like billiard balls)
1897 J.J. ThompsonTHE RAISIN BUN MODEL• atoms are solid spheres made up of solid positive mass with tiny negative particles embedded in the positive
History of the Atom1911 RutherfordTHE PLANETARY MODEL• the atom is composed of a positive nucleus and negative electrons which surround the nucleus• atom is mostly empty space• Gold foil experiment
The Gold Foil Experimentby Rutherford
PredictionIn accordance to the J.J. Thomson model of an atom, the alpha particles should have passed directly through the gold foil for all instances. Observation
most of the alpha particles passed straight through the foil, a small percentage of them were deflected at an angles and some were even backscattered. Conclusion two conclusions: one, an atom is mostly
empty space and scattered electrons and two, an atom must have a positively charged centre that contains most of its mass
History of the Atom
1932 Sir James Chadwick• the nucleus contains particles called neutrons and positively charge protons
1913 Niels Bohr • electrons orbit the nucleus in different energy levels• electrons can move from one level to another by gaining or losing packets of energy• electrons are more stable when they are closer to the nucleus
History of the Atom
Structure of the Atom
Consists of sub-atomic particles. •protons•neutrons•electrons
•Protons and neutrons are located in the nucleus•Electrons orbit the nucleus
Determining the number of subatomic particles
Each element has an atomic number and a mass number
Atomic number = no. of protons = no. of electrons
Mass number = no. of protons + no. of neutrons
Therefore, no. of neutrons = mass number – atomic number
Example: How many protons, neutrons and electrons does carbon have?
Atomic number = 6Mass number = 12
Number of protons: 6Number of neutrons: 12-6 = 6Number of electrons: 6
A
ZE
mass number
chemical symbol
atomic number
An international recognized system that allows anyone to commuicate information about the atom
Atomic Symbol
Isotopes and ions
Isotope is an atom of an element with a different number of neutrons and hence a different mass number.
An Ion is an atom that has lost or gained electron/s.
The Bohr Model• Electrons move around the nucleus in fixed energy levels called shells.•Shells close to the nucleus are lower in energy while shells farther away are high in energy. • Shells are number outwards from the nucleus (1,2,3..) and are also lettered (K,L,M,N...)
Number of electrons in shells
Different shells hold different numbers of electrons. The maximum number of electrons that can fit into a shell is 2n2 (n = shell number)
Shell number (n) Representative letter Maximum
number of electrons in this shell (2n2)
1 K 2 X 1 2 = 22 L 2 X 2 2 = 83 M 2 X 3 2 =
184 N 2 X 4 2 =
32
Drawing Atoms
Bohr diagram number of protons and neutrons written in the center•electrons drawn in circular orbits around the nucleus
10 P10 N
17 P18 N
Example: Chlorine
Atomic number is 17.17 protons and electrons
Mass number is 35.35 - 17 = 18 neutrons
Subshells
subshell Number of electrons
s 2
p 6
d 10
f 14
Subshells are energy levels found within shells.
There are four different types of subshells
Named- s, p, d, f .
Subshell energy: s<p<d<f
Shells and subshells
Electron configuration
The way in which electrons are arranged around the nucleus of an atom is called the electron configuration of the atom.
Aufbau Principle
Electrons occupy orbitals of lower energy first.
Order of electron filling
Electrons fill shells and subshells of lowest energy first.
Write the full electronic configuration of:
Aluminum – 13 electrons
Calcium – 20 electrons
Iron – 26 electrons
Bromine – 35 electrons
Answers
Aluminum – 13 electrons 1s22s22p63s23p1
Calcium – 20 electrons 1s22s22p63s23p64s2
Iron – 26 electrons 1s22s22p63s23p64s23d6
Bromine – 35 electrons 1s22s22p63s23p64s23d104p5
Timberlake LecturePLUS 2000
Orbital is the 3 dimensional space
around a nucleus in which electrons
are most likely to be found
Each orbital can hold up to 2
electrons.
ShellSubshell Orbital
Orbitals
s subshell: spherical1 orbital-2 electrons
p subshell: pair of lobes3 orbital- 6electronsx
y
z
d subshell: double dumpbells5 orbital-10 electrons
xy
z2
xz
x
y
yz
x2-y2
Ground state and excited state
At ground state the electron is at its lowest possible energy level (electrons are as close to the nucleus as possible).
Example: Sodium (Z=11)
Ground state electron configuration: 2, 8, 1
Evidence for the Bohr Model
If atoms are given large amounts of energy (heat, light or electricity), the electrons can jump to an orbit further away from the nucleus. The atom is then in an excited state. Fluorine
Ground state F: 1s2 2s2 2p5
Exited stateF*: 1s2 2s1 2p5 3s1
Evidence for the Bohr Model
When the electrons return to the ground state, they release this energy in the form of light. The light released has a specific wavelength corresponding to the difference between the energy levels.
Electromagnetic Energy
Evidence for the Bohr Model
Emission Spectra are emissions of light from atoms that have been heated or provided with energy.
Evidence for the Bohr Model
Emission Spectra are different for each material.
Fe
They can be used to determine the identity of an unknown sample of an element.
Practise
1) Draw 2 Bohr-Rutherford diagrams for lithium; one in the ground state and one in the excited state. Make sure to include the energy put into the system, and energy released.
Hydrogen
Helium
2) Explain why different elements have different emission line spectrums.
Practise
A
B
C
D
Identify each of the unknown gases based on the emission line spectrums provided. The 4 possibilities are oxygen, hydrogen, helium and neon.