+380.000 andere samenvattingen
Een unieke studietool
Een oefentool voor deze samenvatting
Studiecoaching met filmpjes
Onthoud sneller, leer beter. Wetenschappelijk bewezen.
Samenvatting - Fundamentals of Materials Science and Engineering
2 Atomic Structure and Interatomic Bonding
Name the two atomic models cited, and note the differences between them.Bohr and wave mechanical. Whereas the Bohr model assumes electrons to be particles orbiting the nucleus in discrete paths, in wave mechanics we consider them to be wavelike and treat electron position in terms of probability distribution.
Describe the important quantum-mechanical principle that relates to electron energies.The energies of electrons are quantized, only specific values of energy are allowed.
What bonds contain Polymers?Covalent bonds
What are the four quantum numbers?N, l, ml, and ms.
What is the Pauli Exclusion principle?Each electron state can accommodate no more than two electrons which must have opposite spins.
What bonds contain metals?Metallic bonds
What bonds contain ceramics?Ionic bonds/mixed ionic-covalent bonds
What bonds contain molecular solids?Van der Waals bonds
What bonds contain Semi-metals?Mixed covalent-metallic bonds
What bonds contain intermetallics?Mixed metallic-ionic bonds
Properties of solid materials can depend on geometric atomic arrangements and also by interactions that exist among constituent atoms or molecules.
2.2 Fundamental Concepts
Each atom consists of a nucleus composed of protons and neutrons and is encircled by moving electrons. Electrons and protons have an electrical charge of 1.602x10^-19 C. Neutrons are electrically neutral.
The atomic number is equal to the amount of protons in the nucleus.
The atomic mass of a specific atom is the sum of the masses of protons and neutrons within the nucleus. The number of neutrons may vary for atoms of the same element, these atoms are called isotopes.
The atomic mass unit (amu) may be used to compute atomic weight.
2.3 Electrons in Atoms
Quantum mechanics is a set of laws and principles that govern systems of atomic and subatomic entities.
Bohratomic model assumes that electrons revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less defined in terms of its orbital.
Electrons are permitted to have only specific values of energy, which are called levels or states.
In the wave-mechanical model, the electron is considered to exhibit both wavelike and particlelike characteristics. The position of the electron is described by a probability distribution or electron cloud.
Every electron is
characterizedby four parameters called quantum numbers. The size, shapeand spatial orientationof an electron's probability density (or orbital) are specifiedby three of these quantum numbers.
Shells are specified by a principle quantum number n, which is related to the size of an electron's orbital.
l designates the subshell, which are restricted by the number of n. L can take on values from l=0 to l=(n- 1).
L=0 --> s
L=1 --> p
L=2 --> d
L=3 --> f
Electron orbital shapes depend on l.
The number of electron orbitals is determined by the third quantum number, ml. Ml can take values between -l and +l, including 0.
In the absence of an external magnetic field, all orbitals within each subshell are identical in orbital assuming a slightly different energy. However, when a magnetic field is applied, these subshell states splitt, with each orbital assuming a slightly different energy.
Associated with each electron is a spin moment, which must be oriented either up or down. Related to this spin moment is the fourth quantum number, Ms, for which two values are possible: +1/2 or -1/2.
The smaller the principal quantum number, the lower the energy.
The Pauli exclusion principle stipulates that each electron state can hold no more than two electrons which must have opposite spins.
s --> 2 electrons
p --> 6 electrons
d --> 10 electrons
f --> 14 electrons
When all the electrons occupy the lowest possible energies, an atom is said to be in its ground state. Transitions to higher states are possible. The electron configuration or structure of an atom represents the manner in which these states are occupied.
For example: Sodium = 1s12s22p63s1
Valence electrons are the electrons in the outermost shell, they participate in the bonding between atoms to form atomic and molecular aggregates. Many properties are based on these electrons.
Stable electron configurations are states within the outermost shell that are completely filled. This corresponds to the occupation of just the s and p states for the outermost shell by a total of 8 electrons. These atoms are inert.
Lees volledige samenvatting
Deze samenvatting. +380.000 andere samenvattingen. Een unieke studietool. Een oefentool voor deze samenvatting. Studiecoaching met filmpjes.