Engineering Science 1021A/B Lecture Notes - Lecture 2: Unified Atomic Mass Unit, Valence Electron, Metallic Bonding
11 May 2018
School
Department
Professor
Atomic Structure
Composed of a nucleus surrounded by electrons
•
Nucleus contains neutrons and positively charged protons and carries a net positive
charge
•
Negatively charged electrons are held to the nucleus by an electrostatic attraction
•
Nucleus surrounded by tightly bound inner elections & loosely bound by valance electron
•
Atomic number: # of protons in each atom
•
An atom is electrically neutral because the numbers of protons and electrons are equal
•
Atomic mass unit (amu):
!
"#
the mass of a atom of
$!"
Carbon atom with twelve nucleons: 6 protons and 6 neutrons
○
•
Atomic Mass (M): total mass of the average number of protons and neutrons in the atom
in amu
•
1 mole =
%&'(()*'"+
atoms
•
Atomic Bonding
Metallic Bonding (2 metals)
Metallic elements have electropositive atoms that donate valance electrons to form a
"sea" of electrons that surrounding the atoms
not tightly bound to any particular nucleus
○
•
Valence electrons move freely within the electron sea and become associated with
several atom cores
•
Positively charged ion cores are held together by mutual attraction to the electrons,
producing a strong metallic bond
•
Good electrical conductors because their valence electrons are not fixed in any one
position
•
Net charge on each atom is 0
•
Ductile because the metallic bonds are non-directional
•
Relatively high melting points
•
•
Covalent Bonding (2 non-metals)
Valence electrons are shared with a small # of near neighbours•
Net charge on each atom is 0•
Fixed, directional relationship between atoms•
Bonds are strong and hard•
Not good conductors since the valence electrons are locked in bonds between atoms•
Very high melting points•
•
Ionic Bonding (metal & non-metal)
Requires 2 different species of atoms•
Atoms with an almost empty valence shell donates its valence electrons to an atom with
an almost full valence shell
•
Net charge on each atom is positive or negative
Positive: cation
○
Negative: anion
○
•
Non-directional•
•
Van der Waals Bonding
Secondary bonds due to electrostatic attraction between polarized molecules•
When a neutral atom is exposed to an internal or external electric field, the atom may
become polarized
Centers of positive and negative charges separate
○
•
•
Strength
Bond Strength Binding Energy
Ionic Bonding Very Strong• Requires 150-370 ,-
./0
Covalent Bonding Strong• Requires 125-300 ,-
./0
Metallic Bonding Medium• Requires 25-200 ,-
./0
Vander Waals Bonding Weak• Requires <10 ,-
./0
Atoms
Atomic Structure
Composed of a nucleus surrounded by electrons
•
Nucleus contains neutrons and positively charged protons and carries a net positive
charge
•
Negatively charged electrons are held to the nucleus by an electrostatic attraction
•
Nucleus surrounded by tightly bound inner elections & loosely bound by valance electron
•
Atomic number: # of protons in each atom
•
An atom is electrically neutral because the numbers of protons and electrons are equal
•
Atomic mass unit (amu):
!
"#
the mass of a atom of
$!"
Carbon atom with twelve nucleons: 6 protons and 6 neutrons
○
•
Atomic Mass (M): total mass of the average number of protons and neutrons in the atom
in amu
•
1 mole =
%&'(()*'"+
atoms
•
Atomic Bonding
Metallic Bonding (2 metals)
Metallic elements have electropositive atoms that donate valance electrons to form a
"sea" of electrons that surrounding the atoms
not tightly bound to any particular nucleus
○
•
Valence electrons move freely within the electron sea and become associated with
several atom cores
•
Positively charged ion cores are held together by mutual attraction to the electrons,
producing a strong metallic bond
•
Good electrical conductors because their valence electrons are not fixed in any one
position
•
Net charge on each atom is 0
•
Ductile because the metallic bonds are non-directional
•
Relatively high melting points
•
•
Covalent Bonding (2 non-metals)
Valence electrons are shared with a small # of near neighbours•
Net charge on each atom is 0•
Fixed, directional relationship between atoms•
Bonds are strong and hard•
Not good conductors since the valence electrons are locked in bonds between atoms•
Very high melting points•
•
Ionic Bonding (metal & non-metal)
Requires 2 different species of atoms•
Atoms with an almost empty valence shell donates its valence electrons to an atom with
an almost full valence shell
•
Net charge on each atom is positive or negative
Positive: cation
○
Negative: anion
○
•
Non-directional•
•
Van der Waals Bonding
Secondary bonds due to electrostatic attraction between polarized molecules•
When a neutral atom is exposed to an internal or external electric field, the atom may
become polarized
Centers of positive and negative charges separate
○
•
•
Strength
Bond Strength Binding Energy
Ionic Bonding Very Strong• Requires 150-370 ,-
./0
Covalent Bonding Strong• Requires 125-300 ,-
./0
Metallic Bonding Medium• Requires 25-200 ,-
./0
Vander Waals Bonding Weak• Requires <10 ,-
./0
Atoms
