ATOMS AND IONS What’s the difference?

Atoms vs. Ions  Atoms  Have no overall charge.  The number of protons equals the number of electrons.  Ions  Have an overall charge.  The numbers of protons and electrons are NOT equal.  Ions are formed by adding or removing electrons.

Valence Electrons  These are the electrons in the outermost energy level.  Valence electrons are important because they are usually the ones involved in ion formation and chemical reactions.  For groups 1 and 2, the group number is the same as the number of valence electrons.  For the groups on the periodic table subtract 10 from the group number to get the number of valence electrons.

Determining the Number of Valence Electrons Na (sodium)  Group 1A  Electron configuration: 1s 2 2s 2 2p 6 3s 1  Number of valence electrons = ______ N (nitrogen)  Electron configuration: 1s 2 2s 2 2p 3  Group =  Number of valence electrons = ______

Octet Rule  Atoms are (usually) most stable when there are eight (8) electrons in the valence shell.  Atoms tend to lose, gain, or share electrons in order to have a complete set of eight (8) valence electrons. This is one of the most important rules for understanding chemistry!!!

Types of Ions  If electrons are removed, a positive ion is formed.  Positive ions are called cations.  (pronounced cat-i-on)  If electrons are added, a negative ion is formed.  Negative ions are called anions.

Ions  Here is a simple way to remember which is the cation and which the anion: This is a cat-ion. This is Ann Ion. He’s a “plussy” cat! She’s unhappy and negative. +

Why do ions form?  Ions form in order to satisfy the octet rule.  For atoms with 1, 2, or 3 valence electrons, the easiest way to get to 8 valence electrons is to lose all of the outer layer. This results in positive ions.  For atoms with 5, 6, or 7 valence electrons, the easiest way to get to 8 valence electrons is to add electrons until there are 8. This results in negative ions.  Elements with 4 valence electrons tend to satisfy the octet rule by sharing electrons.

Forming a Na Ion

Forming a Nitrogen Ion

Ion Trends by Group