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Dr. Vatsala Soni
PGGC Sector 11
Chandigarh
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
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.
The Periodic Table and Atomic Radius
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 –
 Second ionization energy –
1+
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: the ability of an atom
in a bond to pull on the electron. (Linus
Pauling)
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 representative elements
(Group A elements) increases as you move across a
period.
Electronegativities of Some
Elements
Element
F
Cl
O
N
S
C
H
Na
Cs
Pauling scale
4.0
3.0
3.5
3.0
2.5
2.5
2.1
0.9
0.7
Note
 Most electronegative element is F
(EN 4.0)
 Least electronegative stable element is Cs
(EN 0.7)
Nuclear charge increases
Shielding increases
Atomic radius increases
Ionic size increases
Ionization energy decreases
Electronegativity decreases
Summary periods
Groups
Shielding is constant
Atomic Radius decreases
Ionization energy increases
Electronegativity increases
Nuclear charge increases
Thank You