Chapter 9, AP Chemistry

1. 1. • s orbital + p orbital two “sp” hybrid orbitals
        Be in BeF₂

        2s	2px	2py	2pz

      • s orbital + two p orbitals three “sp²” hybrid orbitals
        B in BF₃

        2s	2px	2py	2pz

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First, a little about definitions.
Orbital states can be described with several terms:

• Filled – An orbital that contains the maximum number of electrons it can hold.
• Empty – An orbital that contains no electrons.
• Occupied – An orbital that contains at least one electron.
• Unoccupied – An orbital that contains at least one open space for an electron.

	Filled/Occupied
	Occupied/Unoccupied
	Unoccupied/Empty

With these definitions in mind, we can see that a filled orbital is also occupied, but an occupied orbital is not necessarily filled. Also, an orbital can be both occupied and unoccupied. A good way to remember this is that occupied means that one space is occupied by an electron, and unoccupied measn at least one space is free to accept an electron. Thus, it is better to think of filled and empty in terms of the entire orbital, but of occupied and unoccupied in terms of the spaces for electrons.

So, the best way to remember the terms would be as follows:

• Filled – An orbital that is filled with the maximum number of electrons it can hold.
• Empty – An orbital that is empty of electrons.
• Occupied – An orbital that has at least one space occupied by one electron.
• Unoccupied – An orbital that has at least one unoccupied space for an electron.

So, turning now to Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), we can understand what is meant by these terms better with our definitions form above. These orbitals are called the frontier orbitals, and determine the way the molecule interacts with other species. The HOMO is the orbital that could act as an electron donor, since it is the outermost(highest energy) orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, since it is the innermost(lowest energy) orbital that has room to accept electrons. In accordance with the above definitions, a single orbital may be both the LUMO and the HOMO.

The polarity of water
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an “electronegative” or electron “loving” atom compared with hydrogen.
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Chemistry is the study of matter and the interactions between different types of matter and energy. The fundamental building block of matter is the atom. An atom consists of three main parts: protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge. Protons and neutrons are found together in what is called the nucleus of the atom. Electrons circle around nucleus.

Chemical reactions involve interactions between the electrons of one atom and the electrons of another atom. Atoms which have different amounts of electrons and protons have a positive or negative electrical charge and are called ions. When atoms bond together, they can make larger building blocks of matter called molecules.