Periodic Trends

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# Periodic Trends - PowerPoint PPT Presentation

Periodic Trends. Mrs. Coyle Chemistry. Part I – Atomic Size. Atomic Radius. Measures as distance from nucleus to nucleus and divided by 2. Unit commonly used is pm picometer= 10 -12 m Example: iodine atomic radius 140pm. How does atomic radius change across a period?.

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### Periodic Trends

Mrs. Coyle

Chemistry

• Measures as distance from nucleus to nucleus and divided by 2.
• Unit commonly used is pm
• picometer= 10-12m
• Example: iodine atomic radius 140pm
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.
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.
Note
• There are some exceptions. Example column 13.
Example:

Which is larger: a lithium atom or a fluorine atom?

A lithium atom

Example:
• Which is larger: an arsenic atom or a sulfur atom?
• An arsenic atom
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+
Ion
• Positive ion ---removal of electron
• Negative ion--- addition of electron
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.
How does ionization energy change across a period?
• 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.
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) ?

Positive ions
• Positive ions are always smaller that the neutral atom. Loss of outer shell electrons.
Negative Ions
• Negative ions are always larger than the neutral atom. Gaining electrons.
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.
Ion size trends in columns.
• Ion size increases as you move down a column for both positive and negative ions
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
Trends in Electronegativity
• Electronegativity generally decreases as you move down a group.
• Electronegativity of the representativeelements (Group A elements) increases as you move across a period.
Electronegativities of Some Elements

Element Pauling scale

F 4.0

Cl 3.0

O 3.5

N 3.0

S 2.5

C 2.5

H 2.1

Na 0.9

Cs 0.7

Note
• Most electronegative element is F

(EN 4.0)

• Least electronegative stable element is Cs

(EN 0.7)

Summary

Nuclear charge increases

Shielding increases

Ionic size increases

Ionization energy decreases

Electronegativity decreases

Shielding is constant