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Bohr’s Atom
electrons in orbits
nucleus
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What is an atom?
• Atom: the smallest unit of matter that retains the identity of the substance
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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
NucleusElectron
Cloud
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What’s in the Nucleus?
• The nucleus contains 2 of the 3 subatomic particles:– Protons: positively charged subatomic
particles– Neutrons: neutrally charged subatomic
particles
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HELIUM ATOM
+N
N
+-
-
proton
electron
neutron
Shell
What do these particles consist of?
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ATOMIC STRUCTUREATOMIC STRUCTURE
Particle
proton
neutron
electron
Charge
+ charge
- charge
No charge
1
1
nil
Mass
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What’s in the Electron Cloud?
• The 3rd subatomic particle resides outside of the nucleus in the electron cloud–Electron: the subatomic particle with a negative charge and relatively no mass
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How do these particles interact?
• Protons and neutrons live compacted in the tiny positively charged nucleus
• They account 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
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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;
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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
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• Mass number: the number of protons and neutrons in the nucleus– Ex: oxygene can have a mass of 16. Since it has 8 proton it must have 8
neutrons
How do we know the number of subatomic particles in an
atom?
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What about the electrons?• The electrons are equal to the
number of protons– Ex: O has a mass
of 16 and an atomic number of 8 and has 8 electrons.
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What about the electrons?
• Model of the atom pictures the electrons moving around the nucleus in a region
called an electron cloud.
• The electron cloud is a cloud of varying density surrounding the nucleus.
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What about the electrons?
• Electrons are found in specific circular paths (orbits) around the nucleus.
• The electrons have fixed energies called energy levels. (like rungs on a ladder)
• The amount of energy required to move an electron to another energy level is
called a quantum (shell).• The energy levels of electrons are
labeled by principal quantum numbers (n)
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ATOMIC STRUCTUREATOMIC STRUCTURE
Electrons are arranged in Energy
Levels or Shells around the
nucleus of an atom.• first shell a maximum of 2 electrons
• second shell a maximum of 8 electrons
• third shell a maximum of 8 electrons
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How exactly are the particles arranged?
• Bohr Model of the atom:All of the protons and the neutrons
The 1st ring can hold up to 2 e-
The 2nd ring can hold up to 8 e-
The 3rd ring can hold up to 18 e-
The 4th ring and any after can hold up to 32 e-
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What does carbon look like?
Mass number = 12 Atomic number = 6
p+ = 6 no = 6 e- = 6
6 p and 6 n live in the nucleus
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ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION
With electronic configuration elements are
represented numerically by the number of
electrons in their shells and number of shells.
For example;
N
Nitrogen
7
14
2 in 1st shell
5 in 2nd shell
configuration = 2 , 5
2 + 5 = 7
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ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION
Write the electronic configuration for the following elements;
Ca O
Cl Si
Na20
40
11
23
8
17
16
35
14
28B
11
5
a) b) c)
d) e) f)
1s2,8,8,2 2,8,1
2,8,7 2,8,4 2,3
2,6
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DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS
With Dot & Cross diagrams elements and
compounds are represented by Dots or Crosses to
show electrons, and circles to show the shells. For
example;
Nitrogen N XX X
X
XX
X
N7
14
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DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS
O Cl8 17
16 35a) b)
O
X
XX
X
X
X
X
X
Cl
X
X
X
X X
X
XX
X
X
X
X
X
XX
X
X
X
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How is the dispertion of electrons in shell?
• The outer energy level or “shellshell” is considered full when it has the maximum number of electrons that can fit into that level.
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How is the dispertion of electrons in shell?
• The outer energy level or “shellshell” is considered full when it has the maximum number of electrons that can fit into that level.
• For example, the 1st level is full when it has 2 electrons, but the 2nd and 3rd levels are full when it has 8 electrons.)
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How is the dispertion of electrons in shell?
• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.
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How is the dispertion of electrons in shell?
• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.
• If the outside energy level is incomplete (not full), it is considered reactive.
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How is the dispertion of electrons in shell?
• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.
• If the outside energy level is incomplete (not full), it is considered reactive.
• It will either lose or gain electrons and chemically react or bond with another atom or molecule.
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Orbitals
• The electrons of an atom are in constant motion .
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Orbitals
• The electrons of an atom are in constant motion .
• The region of space that an electron occupies is known as an ORBİTAL.
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Orbitals
• The electrons of an atom are in constant motion .
• The region of space that an electron occupies is known as an ORBİTAL.
• Orbital comprise sublevels of a principal energy level.
• The sublevels are named as s, p, d, f.
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Orbitals
• Each sublevels containes a different number of electrons.
s = 1 orbital d = 5 orbitals
p = 3 orbitals f = 7 orbitals
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Orbitals
• Each sublevels containes a different number of electrons.
s = 1 orbital d = 5 orbitals
p = 3 orbitals f = 7 orbitals• s and p orbitals most important in organic
and biological chemistry
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Orbitals• Each sublevels containes a different number
of electrons.
s = 1 orbital d = 5 orbitals
p = 3 orbitals f = 7 orbitals• s and p orbitals most important in organic
and biological chemistry• s orbitals: spherical, nucleus at center• p orbitals: dumbbell-shaped, nucleus at
middle• d orbitals: elongated dumbbell-shaped,
nucleus at center
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Sublevels continued
• The maximum number of electrons allowed in each sublevel are as follows:
• s = max 2 electrons• p = max 6 electrons• d = max 10 electrons• f = max 14 electrons
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Orbitals
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Orbitals• Orbital are grouped in shells of increasing
size and energy
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Orbitals• Orbital are grouped in shells of increasing
size and energy• Different shells contain different numbers
and kinds of orbitals• Each orbital can be occupied by two
electrons.
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Orbitals• Orbital are grouped in shells of increasing
size and energy• Different shells contain different numbers
and kinds of orbitals• Each orbital can be occupied by two
electrons.
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SUMMARYSUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
2. The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
3. The number of Protons = Number of Electrons.
4. Electrons orbit the nucleus in shells.
5. Each shell can only carry a set number of electrons.
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