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Periodic Trends
3/11/16 SLO
Students will be able to describe trends
among elements for atomic size,
ionization energy, ionic size, and how
ions are formed.
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How can we further use the
Periodic Table to make
predictions about the elements?
Recognize trends in the elements as we
move across a period or down a group.
We can determine the following characteristics
of an element using the periodic table:
Atomic Radius
Ionization Energy
Electronegativity
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2 Trends to explain all other trends
Electron Shielding- the reduction of the
attractive force of the nucleus for the outer
electrons
caused by electrons in energy levels between
the nucleus and the outer electrons
Nuclear charge- the number of protons in
the nucleus.
More protons = increased nuclear charge and
increased attraction between the nucleus and
electrons.
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Atomic Radius
Atomic Radius- ½ of the distance
between the nuclei of two atoms of the
same element in a diatomic molecule
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Atomic Radius
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Atomic Radius
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Independent Practice
Using your periodic table and the atomic
radius chart, determine which of the elements
in each pair has a larger atomic radius:
1. Cesium (Cs) and Potassium (K)
2. Calcium (Ca) and Gold (Au)
3. Rubidium (Rb) and Strontium (Sr)
4. Oxygen (O) and Sulfur (S)
5. Xenon (Xe) and Neon (Ne)
6. Aluminum (Al) and Tin (Sn)
7. Helium (He) and Fluorine (F)
8. Boron (B) and Bromine (Br)
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Why do elements have different
reactivities? It all depends on valence electrons
Almost every atom is stable (not reactive)
if it has 8 valence electrons
Exceptions: those that only need 2 electrons
to fill the outer electron shell: H, He, Li, Be, B
If an atom has less than 8 valence
electrons, it will gain, lose, or share
electrons to become stable
Ions= When atoms gain or lose electrons,
they become charged
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Becoming Stable
Atoms that have 1-3 valence electrons will
LOSE (or share) electrons to become
stable
When electrons are lost, this causes a charge
More protons than electrons results in a
positively charged ion called a cation
Example: potassium has 1 valence electron.
It loses an electron to become K+
Calcium has 2 valence electrons. It loses 2
electrons to become Ca2+
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Becoming Stable
Atoms that have 5-7 valence electrons will
GAIN (or share) electrons to become
stable
When electrons are gained, this causes a
charge
More electrons than protons results in a
negatively charged ion called an anion
Example: Chlorine has 7 valence electrons. It
gains 1 electron to be stable. Cl-
Oxygen has 6 valence electrons. It gains 2
electrons to be stable O2-
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Forming cations
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Forming anions
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In addition to Atomic Radius, we
have an Ionic Radius
Where do ions or ionic radius come from?
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Periodic Trends
Trend for atomic size (atomic radius)-
Down a group, size increases
Occurs because # of E levels increases &
Electrons shielding reduces amount of attraction
between nucleus and outer electrons
Across a period, size decreases
# of protons increases (nuclear charge increases),
pulling electrons closer
Electron shielding doesn’t change because
electrons are added to the same energy level
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Atomic Radius Decreases
Atomic Radius Increases
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Ionization Energy Ion- atom that gains or loses electrons
Ionization Energy- energy required to remove an electron. Energy must be added to overcome the attraction of the
positive charge of the nucleus
X(g) X+(g) + e- 1st ionization
X+(g) X2+(g) + e- 2nd ionization
Easiest to remove 2 electrons from 2A Because there are 2 valence
Easiest to remove 3 electrons from 3A Because there are 3 valence
Outer shell electrons are easier to remove than other electrons!
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Ionization Energy
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Periodic Trends
Ionization energy
Down a group- decreases because electrons
are held more loosely due to increased
electron shielding
Across a period- increases because electrons
are held more tightly due to increased nuclear
charge (increased # of protons in the nucleus)
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Ionization Energy Increases
Ionization Energy Decreases
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Periodic Trends
Metals form positive ions- Cations
More likely to lose electrons (lower ionization
energy)
Nonmetals form negative ions- Anions
More likely to gain electrons (higher ionization
energy)
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Periodic Trends
Ionic Radii Trends
Cations- smaller than neutral atom because
fewer electrons result in greater attraction by
nuclei
Anions- larger than neutral atom because
more electrons result in less attraction by
nuclei
Across a period- size decreases
Down a group – size increases
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Atom versus Ion
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Ion Size Decreases
Ion Size Increases
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CLASSWORK!
Reading Assignment: Sec 6.3
Written Assignment: pg. 182, #18-21, 24, 25
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Periodic Trends Part 2
Do Now
Complete the half sheet of paper relating
to the Periodic Trends that we have
already discussed in class.
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Periodic Trends Part 2
SLO
Students will be able to describe trends
in electronegativity and electron affinity
on the periodic table.
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Homework Check!
18. Atomic size generally increases as you move
down a group, and decreases from left to right
across a period.
19. Ions form when electrons are transferred
between atoms.
20. First ionization energy generally decreases as
you move down a group and increases from left
to right across a period.
21. Anions are larger and cations are smaller than
the atoms from which they form.
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Vocabulary Review
Ion
Electronic Shielding
Alkaline Earth Metals
Nuclear Charge Metal Period Nonmetal Metalloid
Cation
Ionization Energy
Anion
Transition Metals
Inner Transition Metals
Periodic Law
Noble Gases
Group
Representative Elements
Alkali Metals
Halogens
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Periodic Trends
Electronegativity- tendency for the atoms
of the element to attract electrons when
the atoms are part of a compound
Fluorine (F) is most electronegative
Noble gases- no electronegativity values-
don’t form compounds
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Periodic Trends
Electronegativity Trends-
Down a group – decreases- since electron shielding results in less attraction for electrons by the nucleus
Across a period- increases- since there is a higher atomic number and consistent electron shielding result in more attraction for electrons
Electronegativity allows you to predict bond type: covalent (includes polar vs. nonpolar) and ionic
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Electronegativity Increases
Electronegativity Decreases
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Electron Affinity
Electron affinity of an element is the
energy given off when a neutral atom in
the gas phase gains an extra electron to
form a negatively charged ion
Example: F(g) + e- F-(g)
Ho (ENERGY) = -328.0 kJ/mol
Think of it like electronegativity without the
need to bond… It still has to do with attraction
for electrons.
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Trends in Electron Affinity
Down a group, it decreases because
electron shielding blocks some of the
attraction from the nucleus
Across a period, it increases because
nuclear charge increases, attracting
electrons more strongly.
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Electron Affinity Increases
Electron Affinity Decreases
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Periodic Trends
Knowledge of trends in electron shielding
and nuclear charge explain all other trends
http://www.teachersdomain.org/resource/ls
ps07.sci.phys.matter.graphperiodic/
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1. Which of the following sequences is
correct for atomic size?
Mg > Al > S
Li > Na > K
F > N > B
F > Cl > Br
6.3 Section Quiz
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6.3 Section Quiz
2. Metals tend to
gain electrons to form cations.
gain electrons to form anions.
lose electrons to form anions.
lose electrons to form cations.
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6.3 Section Quiz
3. Which of the following is the most
electronegative?
Cl
Se
Na
I
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CLASSWORK!
Written Assignment: pg. 186, #38, 40, 41,
43, 44, 45
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Summary of Trends
6.3
Atomic Size Increases
Incre
ases
Decreases
Decre
ases
Size of cations Shielding Nuclear Charge Electronegativity Ionization energy Size of anions Ionic size Constant
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Periodic Trends Part 2
Do Now
Arrange the following elements in order of
increasing atomic radius:
Radon (Rn) Xenon (Xe)
Nickel (Ni) Cobalt (Co)
Sodium (Na) Potassium (K)
Tellurium (Te) Antimony (Sb)