Atoms, Electrons and Orbitals

Chapter 1: Chemical Bonding

Atoms:

All matter is composed of atoms.   Atoms are a collection of various subatomic particles.

You should be familiar with the subatomic particles how to determine how many of each a specific atom has and what the atomic mass of an atom represents.  Try some questions.  If you are not comfortable perhaps you need to look a bit more in depth.

In organic chemistry, as far as the compounds are concerned, the atoms of greatest interest are located the first three rows of the periodic table.  Later when one probes deeper into synthetic methods, one needs to be familiar with some of the heavier metals as well.

Electrons and Orbtials:

The subatomic particle of greatest interest with respect to chemical bonding, is the electron.

Electrons are 'said' to occupy orbitals in an atom.  The term orbital is somewhat 'erroneous' as electrons do not 'orbit' around a nucleus.  These orbitals, also, do not really exist.   They are mathematical constructs which make use of a wavefunction, y, that describes the motion of an electron.

An orbital is, more correctly, a mathematical function, 4pr2Y2, which describes a region of high probability in 3D space, about a nucleus, where an electron may be found.   In organic chemistry one needs to be most familiar with the s- and p-type orbitals.   These orbitals can be described by using quantum numbers.  The orbitals for the electrons in carbon, and the associated quantum numbers are given below.  The nucleus of the carbon atom would reside at the center of the x,y,z coordinate.

Atomic orbitals

You should be familiar with the meaning of quantum numbers, how to name an orbital using quantum numbers or what orbital a set of quantum numbers defines. Try some questions.  If you're struggling you'll need to review the meaning of quantum numbers.

The electron configuration of an atom describes the number of electrons that atom possesses and the orbitals in which those electrons are to be found.

e.g.  electron configurations of carbon
 
simple 1s2 2s2 2p2
short form [He]2s2 2p2
orbital energy diagram

You should be familiar with how to determine an electron configuration for an atom and identify the valence electrons.  You should be able to identify both ground and excited state electron configurations.  Try some questions.  If you are having trouble you should review how to determine electron configurations.

 

Atoms:

Lets review some key aspects of atomic structure:

The above characteristics can easily be figured out by looking at a periodic table.   It is important to note that when obtaining the mass number from a periodic table, A, is the isotopic mass number or the weighted average of isotopic masses for a collection of atoms.
 
Z
Symbol 
A
1
H
1.008
In introductory organic chemistry the vast majority of the atoms involved in the organic compounds we will be interested in will be based on only a small group of elements, H, C, N, O, and the halogens Cl, Br and I.
 
Orbitals:

The properties of orbitals (energy, size, shape etc.) and the electrons within the orbitals (location, spin) can be determined using quantum numbers:
 

quantum number value range describes
Principal, n 1, 2, 3, .... energy level
Angular momentum, l,   0 to n-1 orbital shape
Magnetic, ml - l to +  l spatial orientation and degeneracy
Spin, Ms ± 1/2 electron spin

Given an orbital, the quantum numbers that describe that orbital can be determined:
 
 

For a given set of quantum numbers one can also determine the type of orbital:
 

n
l
ml
orbital
1
0
0
1s
2
0
0
2s
1
-1
2px
0
2py
+1
2pz

Electron Configurations:

e.g. Consider the electron configuration of Al.

A simple electron configuration lists all the orbitals and their electron occupancy
       1s22s22p63s23p1
A short form of the above collapses the noble gas configurations into the atomic symbol for the noble gas
       [Ne]3s23p1
Orbital energy diagrams are usually written with a collapsed core.  They are used to indicate how exactly each individual valence orbital is being filled with electrons.
Electron configurations can be determined using a periodic table and by considering the following:
Aufbau (Building-Up) Principle.  When one considers the electrons within an atom they are placed in orbitals from lowest to highest energy.  The order of these orbitals is determined using the Schrodinger equation and can be easily determined by looking at the periodic table or using a simple mnemonic.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p etc......
Pauli Exclusion Principle.  No two electrons may have the same four quantum numbers, i.e. no more than two electrons are allowed to occupy the same orbital and to do so they must possess equal and opposite spin.
Pauli exclusion principle
Hund's rule of Maximum Multiplicity.  When degenerate orbitals are being filled, electron are placed into each degenerate orbital singly before they are paired with another electron in the same orbital.
Hund's rule
Important distinction:
Valence Electrons.  These are the electrons which are involved in bonding between atoms.  They possess the highest energy and are referred to as the 'outermost' electrons (keep in mind that does not mean they cannot travel close to the nucleus at anyone time but they will be the electrons which travel furthest away from the nucleus).
Core electrons.  These are electrons which under 'normal' reaction conditions are chemically inert.  They are the electrons of an atom which are located in filled energy levels.