Electron Configuration Notes

Compounds

A compound is a group of atoms with a specific number and type of atoms arranged in a specific way. Exactly the same elements in exactly the same proportions are in every bit of the compound.
Example: Water is a compound composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is attached to an oxygen atom by a chemical bond. H2O is the formula for the compound, water.

If any other elements are attached, it is not water. For example, H2S is hydrogen sulfide. Hydrogen sulfide does not have the same types of atoms as water, so it is a different compound.
If a different number of atoms of hydrogen or oxygen are attached, it is not water. H2O2 is the formula for hydrogen peroxide. It might have the right elements in it to be water, but it does not have them in the right proportion. A molecule is a single formula of a compound joined by covalent bonds.

The Law of Constant Proportions states that a given compound always contains the same proportion by weight of the same elements.

Electron Configuration and Valence Electrons
In a stable atom, the number of electrons is equal to the number of protons.
Electrons in atoms are present in discrete orbits or "shells" around the nucleus of the atom.
There is a ranking or heirarchy of the shells, with the shells further from the nucleus having a higher energy.
The innermost electron shell holds only two electrons.
The outermost shell contains the valence electrons. The maximum number of electrons that can occupy the outer shell is eight. When there are eight electrons in the outer shell, it is said to have an octet of electrons.
The valence of an atom is the likely charge it will take on as an ion.
A valence is the amount of positive or negative charge on an ion of an element.
Example: Hydrogen only has one electron. It can lose an electron to become H+, a hydrogen ion, or it can gain an electron to become H-, a hydride ion.

The Octet Rule
The octet rule states that atoms are most stable when they have a full shell of 8 electrons in the outside electron shell.
Octet = 8
An atom with eight electrons in the outer shell is more stable than an atom which as fewer electrons in the outer shell.

The exception to this is Helium (atomic number 2) which only has two electrons in its outer shell. It has a full shell, so it is a stable inert element.

Valence electrons are the only electrons involved in chemical bonds.

Atoms will form chemical bonds with other atoms by either sharing electrons, or by transferring electrons in order to complete their octet and get 8 electrons in the outer shell.

Ions
In a stable atom, the number of electrons is equal to the number of protons.
An atom which has a different number of electrons than it does protons is called an ion.

Ions are charged particles. Types of ions:

Cation - A positively charged ion.A cation is an atom or group of atoms with a net positive charge, caused by the loss of one or more electrons. Examples: Na+, NH4+, Mg+2

Anion - a negatively charged ion.An anion is an atom or group of atoms with a net negative charge, caused by the gain of one or more electrons.Examples: F-, S2-, NO3-

Polyatomic ion - a group of atoms which function as a group and which has a net positive or negative charge (cation or anion).Examples: NH4+ or NO3-

The Periodic Chart can show how the octet rule works. All of the Group I elements have one electron in the outside shell and they all have a valence of plus one. Group I elements will lose that one electron in the outside shell, to become a single positive ion with a full electron shell of eight electrons (an octet) in the s and p subshells under it.

Bonding
A bond is an attachment among atoms. Atoms may be held together for any of several reasons, but all bonds have to do with the electrons (particularly the outside electrons) of atoms.
There are several types of bonds:

Ionic bonds occur due to a full electrical charge difference attraction.

Covalent bonds occur due to sharing electrons.
There are bonds that come about from partial charges or the position or shape of electrons about an atom.

Ionic Bonds
The attraction between a positive ion and a negative ion is an ionic bond.

Some atoms (such as metals) tend to lose electrons to make the outside ring of electrons more stable. When an atom loses electrons it becomes a positive ion (or cation) because the number of protons exceeds the number of electrons.

Other atoms tend to gain electrons to complete the outside electron ring. The non-metal ions tend to gain electrons to fill out the outer shell. When the number of electrons exceeds the number of protons, the ion is negative. (Non-metal ions and most of the polyatomic ions have a negative charge.)

Ionic compounds - composed of cations and anions which are ionically bonded to each other due to attractions of opposite charges
1. Cations and anions combine in a ratio that produces a neutral compound; smallest whole number ratio is used for formula of an ionic compound.
e.g., Na+ + Cl- --> NaCl (one of each is needed to balance the charges: +1 and -1)
Mg+2 + Cl- ---> MgCl2(two Cl's are needed to balance the charges since Cl is -1 and Mg is +2 charge)
2. Cation is listed first, then anion in the formula