Atomic Radius –Trend within a group
Hydrogen (1s1)
Atoms get larger
going down a
group
Lithium (1s2 2s1)
Sodium (1s2 2s2 2p6 3s1)
Shielding Effect
WHAT CAUSES THE ATOMIC RADIUS TO
INCREASE AS YOU MOVE DOWN A GROUP?
H
Li
As we increase the
atomic number (or go
down a group). . .
 each atom has another
energy level,
 so the atoms get

bigger.
Na
K
Rb
Atomic Radius-Trend within a period
1s2
2s2
2p1
1s2 2s2 2p2
5p+
Atoms get smaller as
you move left-to-right
across a period.
6p+
1s2 2s2 2p3
7p+
Shrinking Effect
What causes the atomic radius to decrease
as you move across a period left-to-right?
 Going
from left to right across a period,
the size gets smaller.
 Electrons are in the same energy level.
 But, there is more nuclear charge.
 Outermost electrons are pulled closer.
Na
Mg
Al
Si
P
S Cl Ar
Atomic Radii of Representative Elements (nm)
1A
2A
3A
4A
Li
Be
B
C
5A
6A
7A
O
F
N
Na
Mg
Al
Si
P
S
Cl
K
Ca
Ga
Ge
As
Se
Br
Rb
Sr
In
Sn
Sb
Te
I
Cs
Ba
Tl
Pb
Bi
Po
At
Atomic Radius
Atomic Radius decreases
H
Li
F
Na
K
At
Fr
Decreases
going up
CHALLENGE
1.
a.
b.
c.
d.
e.
Arrange the following
groups of atoms in order of
decreasing atomic radius:
B, Al, Ga
Sn, Sb, Te
Cd, Si, Ga
As, P, Cl
O, Cl, F
ANSWERS
1.
a. Ga, Al, B
b. Sn, Sb, Te
c. Cd, Ga, Si
d. As, P, Cl
e. Cl, O, F
WHAT ARE IONS?
 Atoms
can gain or lose electrons. When
they do, they form ions.
 An ion is an atom (or group of atoms)
that has a positive or negative charge.
IONS
 Metals
tend to LOSE electrons, from their
outer energy level.
 Removal of the electron form positive ions
(atoms with a “+” charge) are called
“cations.”
(pronounced “cat-eye-on”)
 Nonmetals
tend to GAIN one or more
electrons.
 Adding electrons form negative ions (atoms
with a “-” charge) are called “anions.”
(pronounced “an-eye-on”)
IONS
Sodium loses one: there are now more protons (11)
than electrons (10).
 The charge is written as a number followed by a
plus sign: Na1+
 Now named a “sodium ion”




Chlorine will gain one electron
Protons (17) no longer equals the electrons (18), so
a charge of -1
Cl1- is re-named a “chloride ion”
WRITE THE SYMBOLS AND CHARGES FOR
THE FOLLOWING ELEMENTS:
Element
Calcium
Bromine
Lithium
Chemical
Symbol
Charge
WRITE THE SYMBOLS AND CHARGES FOR
THE FOLLOWING ELEMENTS:
Element
Chemical
Symbol
Charge
Calcium
Ca
Ca2+
Bromine
Br
Br1-
Lithium
Li
Li1+
Ionization Energy
Energy needed to remove one of atom’s
electrons from its outermost shell
A+E
A+ +
e-
H
Li
F
Na
K
At
Fr
Ionization energy increases
Ionization energy increases
Ionization Energy
H
Li
F
Na
K
At
Fr
Electronegativity increases
Ionization energy increases
Electron Affinity increases
Electron affinity increases
Ionization energy increases
Atomic Radius increases/ Ionic size
Electronegativity increases
Summary of Periodic Trends