Atomic Theory
&
Atomic Structure
What is an atom?
• Atom: The smallest unit of matter that retains the identity of the substance
• First proposed by Democratus
Atomic Structure
• Atoms are composed of 2 regions:
• Nucleus: the center of the atom that contains the mass of the atom
• Electron cloud: region that surrounds the nucleus that contains most of
the space in the atom
Nucleus Electron
Cloud
What’s in the Nucleus?
• The nucleus contains 2 of the 3 subatomic particles:
• Protons: positively charged subatomic particles
• Neutrons: neutrally charged subatomic particles
What’s in the Electron Cloud?
• The 3
rdsubatomic particle resides outside of the nucleus in the electron cloud
• Electron: the subatomic particle with a negative charge and relatively no
mass
Electron Cloud:
•Electrons orbit the nucleus.
Nucleus or “Core”:
•Protons and Neutrons are found in the nucleus
.Subatomic particles
How do these particles interact?
• Protons and neutrons live compacted in the tiny positively charged nucleus accounting for most of the mass of the atom
• The negatively charged electrons are small and have a relatively
small mass but occupy a large volume of space outside the nucleus
How do the subatomic particles balance each other?
• In an atom:
• The protons = the electrons
• If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral
• The neutrons have no charge; therefore they do not have to equal the
number of protons or electrons
How do we know the number of subatomic particles in an atom?
• Atomic number: this number indicates the number of protons in an atom
• Ex: Hydrogen’s atomic number is 1
• So hydrogen has 1 proton
• Ex: Carbon’s atomic number is 6
• So carbon has 6 protons
**The number of protons identifies the atom.
Ex. 2 protons = He, 29 protons = Cu
How do we know the number of subatomic particles in an atom?
• Mass number: the number of protons and neutrons in the nucleus
• Ex: hydrogen can have a mass of 3.
Since it has 1 proton it must have 2 neutrons
• # of neutrons = mass # - atomic #
The APE MAN!
Atomic Number = Protons = Electrons Mass Number = Atomic Number
+Neutrons
Isotopes
• Atoms that have the same number of protons, but have different numbers of neutrons
• Examples
+ -
+
-
+
-
Hydrogen (Protium) Hydrogen (Deuterium) Hydrogen (Tritium)
Notice that each of these atoms have one proton;
therefore they are all types of hydrogen. They just have a different mass number (# of neutrons).
Symbol = O2+
Ion
• Charged particle that typically
results from a loss or gain of electrons
• Two types:
• Anion = negatively charged particle
• Cation = positively
charged particle +
+ +
+ +
+ + -
-
-
-
- - -
- +
-
Now that this atom of oxygen just gained an electron, it is no longer
neutral or an atom. It is now considered an ion (anion). This ion has
more electrons (9) than protons (8).
+
-
= 8
= 8
= 896
Symbol = O1-
Now that three electrons were lost, the number of electrons (6) and
protons (8) is still unbalanced;
therefore, it is still an ion, but now it is specifically referred to as a cation.
Currently, this atom of oxygen is neutral because it has an equal number of electrons (8) and protons
(8). Symbol = O
Atomic Theory Timeline
Scientist Information Model
John Dalton
All matter is made of atoms.
Atoms are too small to see,
indivisible and indestructible. All
atoms of a given element are
identical.
Scientist Information Model
J.J
Thompson
Discovered the negative
electron, and predicted that
there also must be a positive
particle to hold the electrons in
place.
Scientist Information Model
Ernest
Rutherford
Discovered the nucleus of an atom and named the positive particles in the nucleus “protons”. Concluded that electrons are scattered in
empty space around the nucleus.
Scientist Information Model
James Chadwick
Discovered that neutrons were also located in the nucleus of an atoms and that they contain no charge.
Neutrons
Scientist Information Model
Neils Bohr
Concluded that electrons are
located in planet-like orbits
around the nucleus in certain
energy levels.
Scientist Information Model
(Many Scientists!) The Modern Atomic Theory
Electrons do not orbit the nucleus in neat planet-like
orbits but move at high speeds
in an electron cloud around the
nucleus.
John Dalton (1766-1844)
• Dalton is the “Father of Atomic Theory”
• Dalton’s ideas were so brilliant that they
have remained essentially intact up to
the present time and has only been
slightly corrected.
Dalton’s Atomic Theory (1803) 5 Postulates
1. All matter is composed of extremely small particles called atoms. (I agree with Democritus!)
2. All atoms of a given element are identical, having the same:
- size - mass
- chemical properties.
3. All atoms of different elements are
different.
4. Atoms cannot be created, divided into smaller particles, or destroyed.
**In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element.
5. Atoms combine in definite whole number ratios to make compounds
(
you can’t have a ½ of a Carbon bonding with Oxygen; it’s a whole atom or no atom)
Rutherford ‘Scattering’
• In 1909 Rutherford undertook a series of experiments
• He fired (alpha) particles at a very thin sample of gold foil
• According to the Thomson model the particles would only be slightly deflected
• Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source
particle source
Lead collimator Gold foil
1. Atom has a lot of space inside it, which could explain why lost of the particles went through the gold paper with no deviation from their path.
2. Atom has a dense center, which gives the mass of the atom.
This center, called nucleus, is positively charged, too. This could explain why the alpha particles suffered deviation from their path when they approach to a nucleus.
3. Due to the atom is a electrically neutral particle, the electrons must be around the nucleus, going round it in orbits like planets do around the Sun. The number of electrons is equal (but with opposite sign) to the charge present in the nucleus.
Postulates of Rutherford atomic model
Rutherford’s Apparatus
Rutherford received the 1908 Nobel Prize in Chemistry
for his pioneering work in nuclear chemistry.
Drawbacks of Rutherford’s model of atom
1.Rutherford proposed that electrons revolve at a high speed in circular orbits around the positively charged nucleus. When a charged particle i. e. electron revolves around
positively charge nucleus, it needs to be accelerated so as to keep it moving in circular orbits. However, according to electromagnetic theory, whenever a charged particle such as an electron is accelerated around another charged center ( nucleus ) which are under force of attraction, there will be continuous radiation of energy. This loss of energy would slow down the speed of the electron. This would reduce the radius of the electron–orbit.
Eventually the electron would fall into the nucleus. The result would be that the atom would collapse. But this does not happen. Thus Rutherford’s atom could not explain the stability of the atom.
2.Rutherford proposed that electrons revolve around the nucleus in the fixed orbits.
However, he did not specify the orbits and the number of electrons in each orbit.
Rutherford’s Apparatus
beam of alpha particles
radioactive substance
fluorescent screen circular - ZnS coated
gold foil
Bohr's Model of the Atom Bohr's model:
-electrons orbit the nucleus like
planets orbit the sun
Bohr's Model of the Atom Bohr's model:
-electrons orbit the nucleus like planets orbit the sun
-each orbit can hold a specific
maximum number of electrons
Bohr's Model of the Atom Bohr's model:
-electrons orbit the nucleus like planets orbit the sun
-each orbit can hold a specific maximum number of electrons
orbit maximum # electrons
1 2
2 8
3 8
4 18
Bohr's Model of the Atom
-electrons orbit the nucleus like planets orbit the sun
-each orbit can hold a specific maximum number of electrons
-electrons fill orbits closest to the
nucleus first.
1. Electrons revolve around the nucleus in circular path, which are known as
"ORBITS" or "ENERGY LEVEL".
2. Energy of an electron in one of its allowed orbits is fixed.As long as an electron remains in one of its allowed orbit, it can not absorb or radiate energy.
3. If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of energy.
4. If an electron jumps form higher energy level to a lower energy level, it radiates a definite amount of energy
5. Energy released or absorbed by an electron is equal to the difference of energy of two energy levels.
6. Spectrum of light emitted from an electron is a "LINE SPECTRUM".
7. 7. Angular momentum of an electron is given by:
