periodic trends mrs. coyle chemistry. part i – atomic size
TRANSCRIPT
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Periodic Trends
Mrs. CoyleChemistry
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Part I – Atomic Size
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Atomic Radius
Measures as distance from nucleus to nucleus and divided by 2.
Unit commonly used is pm picometer= 10-12m
Example: iodine atomic radius 140pm
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How does atomic radius change across a period?
It is smaller to the right.
Why?
More protons in the nucleus higher electrical force pulls electrons closer to nucleus.
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How does atomic radius change down a group?
It is larger down the group.
Why?
Valence electrons are at higher energy levels and are not bound as tightly to the nucleus because they are screened or shielded ( pushed away) by other electrons in inner levels.
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Note
There are some exceptions. Example column 13.
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The Periodic Table and Atomic Radius
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Example:
Which is larger: a lithium atom or a fluorine atom?
A lithium atom
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Example:
Which is larger: an arsenic atom or a sulfur atom?
An arsenic atom
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Part II – Ionization Energy
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Ionization energy
Ionization energy is the amount of energy needed to remove an electron from a gaseous atom. First ionization energy – 1+ Second ionization energy – 2+
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Ion
Positive ion ---removal of electron
Negative ion--- addition of electron
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How does ionization energy change down a group?
The first ionization energy decreases as you move down a group.
Why? The size of the atom increases. Electron is further from the nucleus.
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The first ionization energy increases as you move from left to right across a period.
Why? Nuclear charge increases while
shielding is constant. Attraction of the electron to the
nucleus increases.
How does ionization energy change across a period?
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Ionic size
Metallic elements easily lose electrons.
Non-metals more readily gain electrons.
How does losing or gaining an electron effect the size of the atom
(ion) ?
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Positive ions
Positive ions are always smaller that the neutral atom. Loss of outer shell electrons.
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Negative Ions
Negative ions are always larger than the neutral atom. Gaining electrons.
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Ion size trends in periods.
Going from left to right there is a decrease in size of positive ions.
Starting with group 5, there is sharp increase followed by a decrease in the size of the anion as you move from left to right.
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Ion size trends in columns.
Ion size increases as you move down a column for both positive and negative ions
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Electronegativity: the ability of an atom in a bond to pull on the electron. (Linus Pauling)
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Electronegativity When electrons are shared by two
atoms a covalent bond is formed.
When the atoms are the same they pull on the electrons equally. Example, H-H.
When the atoms are different, the atoms pull on the electrons unevenly. Example, HCl
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Trends in Electronegativity
Electronegativity generally decreases as you move down a group.
Electronegativity of the representative elements (Group A elements) increases as you move across a period.
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Electronegativities of Some Elements
Element Pauling scale F 4.0 Cl 3.0 O 3.5N 3.0 S 2.5C 2.5H 2.1Na 0.9Cs 0.7
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Note
Most electronegative element is F (EN 4.0)
Least electronegative stable element is Cs
(EN 0.7)
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SummaryShielding is constantAtomic Radius decreasesIonization energy increasesElectronegativity increasesNuclear charge increases
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