Chapter 4: Atoms and Elements

 Determine ion charge from numbers of protons and electrons.  Determine the number of protons and electrons in an ion.

 In chemical reactions, atoms often lose or gain electrons to form charged particles called ions.  Positive ions are called cations.  Negative ions are called anions.

 The charge of an ion is shown in the upper right corner of the symbol.  Ion charges are usually written with the magnitude of the charge first, followed by the sign of the charge.  Examples: Mg 2+, O 2-

 The charge of an ion depends on how many electrons were gained or lost and is given by the following formula where p + stands for proton and e − stands for electron.

 In reactions, lithium atoms lose one electron (e − ) to form Li + ions.  For the Li + ion with 3 protons and 2 electrons, the charge is  Ion charge = 3 − 2 = 1 +

 In reactions, fluorine atoms gain one electron (e − ) to form F − ions:  For the F − ion with 9 protons and 10 electrons, the charge is  Ion charge = 9 – 10 = 1 –

 The location of the elements on the periodic table gives clues to the number of valence electrons an atom has.  Valence electrons: the electrons found in the outermost shell of an atom. There cannot be any more than 8 valence electrons in an atom.

 In order to be stable, elements want to have a full outer shell.  They want 8 electrons.  Atoms form ions in order to become more stable.

 The key to predicting the charge acquired by an element is its position in the periodic table relative to the noble gases.  Main-group elements tend to form ions that have the same number of valence electrons as the nearest noble gas.

Gain Electrons +1 e - +2 e - +3 e - Lose Electrons -1 e - -2 e - -3 e - +4 e - or -4 e -

 Alkali Metals (group 1) will form 1 + ions  Alkaline Earth Metals (group 2) will form 2 + ions  The Boron Group (group 3) will form 3 + ions

 Transition metals commonly form 2 + or 3 + ions, but can form greater than 3 + ions.

 Halogens (group 17) will form 1 - ions  Chalcogens (group 16) will form 2 - ions  Pnictogens (group 15) will form 3 - ions

 Determine the charge of each ion:  Mg with 10 electrons  S with 18 electrons  Fe with 23 electrons

 Determine the number of protons and electrons in the Ca 2+ ion.  Determine the number of protons and electrons in the S 2- ion.

 Based on their position in the periodic table, what ions do barium and iodine tend to form?