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Atomic Mass Trends ReasoningWhy does atomic mass increase across a period?Moving left to right, the number of protons, neutrons and electrons all increase.M b t i ti l l d t hi hMore subatomic particles leads to higher mass.
Why does atomic radii decrease across a period?Moving left to right, the number of protons, neutrons and electrons all increase.
n ep
n
ep
n ep
n
e
np
p
e
p
n
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e
e
Move across the periodic table
Lithium atom Beryllium atom
Radius decreases
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Lithium atom Beryllium atom
As the # of protons and electrons increase, the attraction between the positive nucleus and negative electron cloud increases.This attraction “pulls” in on the electrons.
Why does electronegativity increase across a period?Moving left to right, the radius of the atom decreases as more protons pull on more electrons.
n ep
n
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n ep
n
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np
p
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Move across the periodic table
Radius decreases
Electronegativity increases
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Lithium atom Beryllium atom
When an atom is smaller, the electrons are closer to the nucleus, and therefore feel the pull more strongly.
Smaller atoms will have a higher electronegativity.
Electronegativity Trends Reasoning - 2
Why does electronegativity decrease down a group?As you move down a group, the radius increases as more electrons shells are addedelectrons shells are added.
Move down the periodic table
Radius increases + e
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e
e
e
e
e
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+
Electronegativity decreases
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Lithium atomSodium atom
As the outer electrons (those involved in bonding) are farther from the nucleus, they will feel the “pull” of the nucleus less.
Why does electronegativity decrease down a group?As you move down a group, the radius increases as more electrons shells are addedelectrons shells are added.
Move down the periodic table
Radius increases + e
ee
e
e
e
e
e
e
eee e
+
IE decreases
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Lithium atomSodium atom
As the outer electrons (those involved in bonding) are farther from the nucleus, they will feel the “pull” of the nucleus less.
It is easier to remove an electron from a larger atom.
Don’t memorize them all… understand the reasoning behind the atomic radii trend, and know the definitions of the other terms...then you can reason out their trends as well.
Example - 1
Example: List Li, Cs and K in increasing order of:
A: Atomic radii
These elements are in the same group.
Radii of the atomA: Atomic radii
B: Electronegativity
C: Ionization Energy
As you move down a group, electron shells are added.This makes a larger atom. The element at the top will be the smallest.
Li < K < Cs
As you move down a group, atoms get larger.The larger the atom, the less pull on the electrons.Smallest atom will have the highest electronegativity.
Cs < K < Li
Radii of the atom
Pull an atom has on electrons it shares in a bond.
Energy needed to remove the outermost electron
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C: Ionization Energy
D: Electron Affinity
As you move down a group, atoms get larger.The larger the atom, the less pull on the electrons.Smallest atom will have the highest ionization energy.
Cs < K < Li
As you move down a group, atoms get larger.The larger the atom, the less pull on the electrons.Smallest atom will have the highest electron affinity.
Example: Put in order of increasing size: Ca, Ca2+ and Ca+.
Cations are formed by removing electronsCations are formed by removing electrons.When electrons are removed, there are more protons per electron.The pull on each electron from the nucleus is now greater.The cation will be smaller than the parent atom.
Another Multiple Choice QuestionSome questions are “definition” type questions.
E lExample: The energy needed to remove the outermost electron is ____.
A. Electron AffinityB. Activation EnergyC. Ionization EnergyD. Enthalpy of FormationE. Electronegativity
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Answer: C. Ionization Energy
Free Response QuestionsThe Free Response questions often ask for you to explain certain behavior.
Example: Explain the following in terms of atomic or molecular structure:
A. The second ionization energy of Mg is much lower than the second ionization energy of Na
B. The atomic radius of N is greater than OC. The boiling point of H2O is higher than H2SD. The bond angle of NH3 is smaller than NH4
+
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Periodicity questions are often combined with sub-questions asking about properties dependent on bonding theory or intermolecular forces (covered in future tutorials).
Answering Free Response QuestionsBe sure to thoroughly EXPLAIN…not just rephrase!
Explain the following in terms of atomic or molecular structure:
A Th d i i ti f M i h l th th dA. The second ionization energy of Mg is much lower than the second ionization energy of Na
Removing the second electron from Na requires removing an electron from a full, more stable electron shell. Removing the second electron from Mg does not. Therefore, it takes more energy to remove the 2nd electron from Na than Mg.
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B. The atomic radius of N is greater than O
N has fewer protons and electrons than O, although both use the same highest energy subshell. The fewer protons and electrons in the N atom have less of an attraction as the greater number of protons and electrons in the O (the electrons are in the same energy level and thus no more shielded). Therefore, the radius of the N is larger.
Other periodic trends (Electronegativity, Ionization Energy
Other periodic trends (Electronegativity, Ionization Energy
The periodic t bl i
The periodic t bl i
Ions gain or lose electrons to form a
charge. Cations
Ions gain or lose electrons to form a
charge. Cations
Learning Summary
Ionization Energy and Electron Affinity)
can be reasoned through using the atomic radii trend.
Ionization Energy and Electron Affinity)
can be reasoned through using the atomic radii trend.
table organizes the elements by atomic number.
table organizes the elements by atomic number.
gare smaller and
anions are larger than the “parent”
atom.
gare smaller and
anions are larger than the “parent”
atom.
The periodic table canThe periodic table can
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Radii decreases across the periodic table and increases down the
periodic table.
Radii decreases across the periodic table and increases down the