Prince George’s Community College
CHM1010 (Shah)
Chapter 8: Periodic Properties of the elements
Outlines:

Ground state electron configuration: Aufbau principle

Valence electrons

Periodic properties: Atomic size, Ion size, IE, EA, and EN
Electron configuration:
The distribution or arrangement of electrons among the orbitals of an atom is
called electron configuration.
Electron configurations are built up by filling lowest energy orbitals first.
Electron configurations can be abbreviated using noble gas core configurations
(condensed form).
Three things are very important in looking at the way the electron density
distributes itself in an atomic orbitals: The shape of the orbital, size of the orbitals
and its orientation in space relative to other orbitals.
Pauli Exclusion Principle: It states that no two electrons in an atom have the
same four quantum numbers. In another word, “An orbital can hold at most two
electrons, and then only if the electrons have opposite spin.” (The orbital can only
hold two electrons, and they must have opposite spins.)
Aufbau Principle: Aufbau principle: electrons in an atoms will occupy the lowest
energy orbital available first. The smaller the quantum number, the lower the
energy is.
Hund’s rule: It states that orbital of the same n and l quantum numbers are each
occupied by one electron, before any pairing occurs.
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Practice:
1.
What is the electron configuration of (a) Mn (b) Bi (c) Ge (d) Cd
2.
Draw orbital diagrams for (a) Na (b) S, and (c) Fe (d) Mg (e) Ge
Magnetic properties of Atoms: An electron in an atom behaves like a small
magnet, but the magnetic attractions from two electrons that are opposite in spin
cancel each other. As a result, an atom that has only doubly occupied orbital has
no spin magnetism.
A paramagnetic substance: It is a substance that is weakly attracted by a
magnetic field, and this attraction is generally the result of unpaired electrons.
Example: sodium vapor
A diamagnetic substance: It is a substance that is not attracted by a magnetic
field or is very slightly repelled by such a field. (The substance has generally
paired electrons). Example: mercury vapor
Elecronegativity: The tendency of an atom to attract bonding electrons to itself
when it bonds with another atom.(The ability of an atom in a molecule to attract
shared electrons to itself).
An electronegativity is related to ionization energy (IE) and electron affinity (EA).
In general, the lower its EN, the more metallic an element is. The higher the EN.
More nonmetallic it is.
Remember: General trends- > It increases from left to right along a period and
decreases down a group.
→
↑
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Among Rb, F, S, Ba  F has the highest and Ba has the lowest EN.
Electronegativity difference
Type of bond
Between 0 to 0.5
Nonpolar covalent
Between 0.5 to 2.1
Polar covalent
Between 2.1 to 3.3
Usually Ionic
Isoelectronic ions: ions containing the same number of electrons but different
number of protons.
Ions
# of protons
# of electrons
O --
8
10
F-
9
10
Na +
11
10
Mg ++
12
10
Al +++
13
10
Al +++ is the smallest and O - - is the largest in above series. Greater # protons
pulls electron closer to nucleus.
For isoelectronic cations, more positive the ionic charge, the smaller the ionic
radius, while for anions, the more negative the charge, the larger the ionic radius.
Examples
Mg+2 < Na+
S-2 > Cl-
Remember: In general, for a series of isoelectronic ions the size decreases as
the nuclear charge Z increases (with increasing atomic number).greatest the Z
smallest the ion and smallest the Z largest the ion.
Practice: arrange the ions Se 2-, Br - , Rb+, Sr 2+ in order of increasing size.
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Atomic size(Atomic radius): It is defined as half the distance between the nuclei
in a molecule consisting of identical atoms. (General trends in periodic table – top
to bottom increases and left to right decreases).
←
↓
Among Al, Cl, Ge, Rb  Rb has the largest atomic radius.
Ionic radius: Ions are formed by gaining or losing electrons and the resulting
radius is called the ionic radius.
Remember: general trends- atomic radius (ionic radius) or atomic size---->
increase down a family and decreases across a row.
Be2+< Mg2+< Ca2+ < Sr2+
Electron affinity: Ionization Energy concerns the loss of electrons. Electron
affinity is a measure of the energy change when a gaseous atom gains an
electron.
X(g) + e - - ----> X - (g)
if the reaction is exothermic the sign for EA is negative
F(g) + e - - ----> F - (g)
Where EA = -328 kJ/mol
Remember: EA is usually not considered for inert gases. Generally it decreases
down a family/group and increases across a row/period.
→
↑
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EA decreases as atomic radii increases.
Example: O, F, Ne (they are in one row with increasing atomic number) among
these elements F has the highest electron affinity
Ionization energy: Electrons are attracted to the nucleus due to opposite charges.
Ionization energy is the energy required to remove an electron from a gaseous
atom or ion.
X(g) - ----> X + (g) + e -
where an atom or ion is assumed to be in its
ground state.
Al(g) - ----> Al + (g) + e -
l1 = 580 kJ/mol
Al+(g) - ----> Al 2+ (g) + e - l2 = 1815 kJ/mol
Al2+(g) - ----> Al 3+ (g) + e - l3 = 2740 kJ/mol
Al3+(g) - ----> Al 4+ (g) + e - l4 = 11,600 kJ/mol
Al+ has the lowest IE in the series of Al+, Al2+, Al3+
Remember: general trend IP or IE (Ionization potential energy) decreases down
a family and increases across a period. (like EA)
(a) Na, Mg, Al in this series Na has the lowest IP
(b) K, Ca, Mn in this series K has the lowest IP
As n increases, the size of the orbital increases and the
→
↑
electron is easier to remove.
Practice
1. Which element in each series has the highest electron affinity?

O, F, Ne
S, O Se

Na, Mg, Al
Fe, Co, Ni
Ca, Sr, Ba
5. Which element in each series has the largest ionic radius?

Li+2, B+3, Be+2
Be+2, Mg+2, Ca+2

O-2, Se-2, S-2
O-2, F-1
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
Na+, K+, Rb+
6. Which element in each series has the lowest Ionization potential energy?

Al, Mg, Na
Al+, Al++, Al+++

Ca, K, Mn
Be, Mg, Ca

Ca, Sr, Ba
7. Determine maximum and minimum size for the following isoelectronic
species and arrange them in increasing or decreasing order.

O-2, F-, Ne, Na+, Mg+2
Se-2, Rb+, Br -, Sr+2
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