3. Periodic Trends
The arrangement of the periodic table reveals trends in
the properties of the elements.
• A trend is a predictable change in a particular direction.
Understanding a trend among the elements enables you to
make predictions about the chemical behavior of the
elements.
• We will look at six trends:
– Atomic Number
– Atomic Mass
– Atomic Radius
– Electronegativity
– Ionization Energy
– Reactivity
3
4. Periodic Trends:
Atomic Number
Atomic Number
• As you move across a period the atomic number increases.
Similarly, as you move down a group the atomic number
increases. In this way, the atomic number represents exactly
where in the periodic table an element stands.
• More importantly, and the reason why the ordering of the
elements according to atomic number yields elements in
groups with similar chemical and physical properties, the
atomic number is the same as the number of protons in the
nucleus of an atom of an element, and also the same as the
number of electrons surrounding the nucleus in a neutral
state. 4
5. Periodic Trends:
Atomic Mass
Atomic Mass
• The atomic mass of the elements generally increases
as you move down a group and across a period.
• There are some instances when this trend does not
hold true, however. For instance, because it has a
high percentage of isotopes with many neutrons, the
atomic weight of tellurium (Te) is higher than that for
iodine (I), even though iodine has a higher atomic
number.
5
6. Periodic Trends:
Atomic Radius
• Atomic Radius - The distance from the center of an
atom’s nucleus to its outermost electron.
• Since an electron cloud’s edge is difficult to define, an
atom’s atomic radius is determined as half the
distance between the nuclei of 2 bonded atoms of the
same element.
6
7. Periodic Trends:
Atomic Radius
• Group Trend: Atomic radii increases as you move
down a group. Recall that as you move down the
column, additional energy levels (layers of electrons)
are being added, thus taking up more space.
• Period Trend: Leaving the noble gases out, atoms get
smaller as you go across a period. Each step across
adds a proton and an electron. Electrons are added to
existing atomic orbitals, so are not increasing the
atom’s radius. The effect is that the more positive
nucleus has a greater pull on the electron cloud,
pulling the cloud in and making the atom smaller. 7
9. Periodic Trends:
Electronegativity
• Electronegativity – the ability of an atom to attract
electrons from another atom toward itself to form a
chemical bond with that atom.
• The difference between the electronegativities of two
atoms will determine what kind of bond they form (ionic
or covalent). Noble gases do not form bonds and
therefore have no electronegativity value!
• Generally, metals are electron givers and have low
electronegativities. Nonmetals are electron takers and
have high electronegativities. The atom with the higher
electronegativity will pull on the electrons more strongly
than the other atom will.
9
10. Periodic Trends:
Electronegativity
• Group Trend: Electronegativity decreases going
down a group because the bonding electrons are
increasingly distant from the attraction of the nucleus
(recall that energy levels are being added).
• Period Trend: Electronegativity increases going
across a period as a result of the atom’s smaller size,
which make it hold onto bonding electrons more
tightly. Atomic radii
10
11. Periodic Trends:
Electronegativity
• Fluorine (F) is the
most electronegative
(upper right) and will
always win the battle
for electrons in a
chemical reaction.
(HOT)
• Francium (Fr) is the
least electronegative
(bottom left) and
always loses. (NOT) 11
12. Periodic Trends:
Ionization Energy
• Ionization Energy - The amount of energy needed to
remove one (the outer-most) electron from an atom.
• Consider two magnets: The first is the nucleus, the
second the electrons in the outer energy level. When
the magnets are close together it is hard to pull them
away from each other (requires more energy). When
the magnets are far apart it is easier to pull them
further apart, because of a weaker force of attraction.
12
13. Periodic Trends:
Ionization Energy
• The larger the atom is, the easier its electrons are to
remove (thus requiring less energy).
• Group Trend: Ionization energy decreases as you
move down a group since atomic radii increases.
• Period Trend: Ionization energy increases as you
move across a period since atomic radii decreases.
Atomic radii
13
14. Periodic Trends:
Reactivity
• Reactivity - The ability or tendency of an element to go
through a chemical change (or react with another
element).
• Metals are losers, when it comes to electrons.
• Nonmetals are bullies, which it comes to electrons.
****Can’t Compare Metals to Nonmetals!!!!
14
15. Periodic Trends:
Reactivity
Metals (recall: the most reactive metal is Francium and
metals tend to LOSE electrons)
• Group Trend: Reactivity increases as you move down
a group since atomic radii increases and electrons are
less tightly held.
• Period Trend: Reactivity decreases as you move
across a period since atomic radii decreases and
increases the charge of the nucleus to pull the
electrons closer towards them.
15
16. Periodic Trends:
Reactivity
Nonmetals (recall: the most reactive nonmetal is
Fluorine and nonmetals tend to GAIN electrons)
• Group Trend: Reactivity decreases as you move
down a group since the atomic radii increases and has
less nuclear charge to attract electrons from
neighboring atoms.
• Period Trend: Reactivity increases as you move
across a period (until you get to the noble gases)
since atomic radii decreases and increases the charge
of the nucleus to pull the electrons of neighboring
atoms closer to them. 16
Notas do Editor
Integral for students to understand the molecular/atomic basis for the formation of compounds and the reaction of those compounds (later in the year)
We will probably mention atomic radius again when we talk about ionic compounds, in our next unit. The size changes as a result of gaining or losing electrons.
The GREEDINESS of an atom for electrons….like the Grinch!
STINGY with electrons, like Scrooge!
JOKE: Sorry for all the negative electron terminology…Get it?
