Valence Electrons

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Valence Electrons
valence electrons outer electrons that
participate in chemical reactions neutral
atoms have a characteristic number of valence
electrons in fact, the periodic table is
arranged according to the number of valence
electrons of the atoms
s-block
p-block
d-block
f-block
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Valence Electrons
Counting valence electrons of atoms
1
2
1st row
1 2
3 4 5 6 7 8
2nd row 3rd row
4th row
1 2
s-only
3 4 5 6 7 8 9 10 1 2
transition metals
1 2
3 4 5 6 7 8
p-block
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Noble Gas Electron Configurations
The noble gases are chemically inert. Their
electronic configurations are very stable.
1s2 He He2s22p6 Ne Ne3s23p6 Ar Ar4s23d10
4p6 Kr Kr5s24d105p6 Xe
Notice, that for all of the noble gases, there
are eight valence electrons, and that they
completely fill the ns and np orbitals. This is
called a stable octet of electrons.
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Ions
For some atom, Z, the number of protons is equal
to the number of electrons such that the atom is
electroneutral. It is possible for an atom to
lose or gain electrons, whereupon it becomes
charged. Atoms that have more, or fewer,
electrons than protons is called an ion a
positively charged ion is called a cation a
negatively charged ion is called an anion If a
neutral atom loses an electron, it becomes a
cation. If a neutral atom gains an electron, it
becomes an anion.
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Ions Relation to the Octet of Electrons
As s- or p-block elements become ions, their
electronic configurations will be identical to
the electronic configuration of the nearest noble
gas element. Ex Predict the most stable ion of
potassium. K electronic configuration
1s22s22p63s23p64s1 Nearest noble gas element Ar
1s22s22p63s23p6 In order for the electronic
configuration of potassium to be equivalent to
that of argon, the potassium atom must lose one
electron. K ? K e- The most stable ion
of potassium is its 1 ion, or potassium ion.
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Ions Relation to the Octet of Electrons
Second Example Oxygen, predict its most stable
ion Electronic configuration of oxygen
1s22s22p4 An oxygen atom has 6 valence
electrons, and requires 8 to complete a stable
octet of electrons. Therefore, oxygen will gain
two electrons to form an anion, O2- Electronic
configuration of O2- 1s22s22p6 The nearest
noble gas element to oxygen is neon,
Ne. Electronic configuration of Ne 1s22s22p6
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Atoms relative size (from atomic radius)
100 pm
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Energies of Ion Formation
Ionization energy energy required to remove an
electron from an atom in the gaseous state Atom
Energy ? Cation Electron Electron affinity
energy released on adding an electron to an
atom in the gaseous state Atom Electron ?
Anion Energy Comparisons Small ionization
energy electron easily lost Small electron
affinity electron not easily gained (cation
formation is favored) Large ionization energy
electron not easily lost Large electron affinity
electron easily gained (anion formation is
favored)
Alkali metals
Halogens
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Ionization energy and Electron Affinity
It is difficult to remove an electron from noble
gases (stable octet)
It is energetically favorable to add an electron
to the halogens (stable octet)
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Reaction of Sodium and Chlorine
Oppositely charged ions organized to form an
ordered lattice. These are crystals.
Why is Cl- larger than Na ?
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Nomenclature of ions
If an s- or p-block element is ionized to form
its cation, then the cation takes the name of the
element, followed by the word ion Ca ? Ca2
2 e- calcium is ionized to calcium ion If an
element is ionized to form its anion, then the
anion take the first part of the name of the
element, followed by ide S 2 e- ?
S2- sulfur is ionized to sulfide N 3 e- ?
N3- nitrogen is ionized to nitride
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Transition Metal Ions
Transition metals have low ionization energies,
and tend to form cations. Transition metals can
form more than one cation Ex iron can lose
two electrons to form iron 2 or lose three
electrons to form iron 3 There are two ways to
name transition metal ions First method name
of the element (roman numeral of its positive
charge) Ex iron(II) is the ion that forms as
iron loses two electrons Second method use the
latin name of the element follow by ous or
ic -ous indicates the cation with lower chare,
where ic indicates the cation with higher
charge. Ex iron(II) ferrous ion Fe2 and
iron(III) ferric ion Fe3
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Polyatomic Ions
Some ions are made up of several atoms, and the
overall group has a charge. These are classified
as polyatomic ions.