Transcript Trends of the Periodic Table

Trends of the Periodic Table
Background
• Electrons can jump between
shells (Bohr’s model supported
by line spectra)
•The electrons can be pushed so
far that they escape the attraction
of the nucleus
Losing an electron is called ionization
Ions
• An electrically charged particle formed when
an atom or molecule loses or gains an
electron.
• Q: what would the charge be on an atom that
lost an electron?
• A: +1 (because your losing a -ve electron) called a
cation
• Q:Gained two electrons?
• A: -2 (because you gain 2 -ve electrons) called an anion
Ionization energy
• Ionization energy is the energy required to remove
one outer electron from an atom.
Most weakly held
11p+
12n°
Na
Ionization
+ Energy
+
E ionization
+1
11p+
12n°
Na+1 (g)
+
+
1e-
1e-
Predict…
Is there an IE trend in the PT of E?
• Down a group?
I.E. DECREASES because:
o More energy levels, resulting in a decrease in
attractive forces between the nucleus and valence
electrons. Removing one electron becomes easier.
• Across a period?
I.E. INCREASES because:
o Electrons are added along the same energy level,
increasing the attractive forces between protons and
valence electrons requiring more IE to remove them.
Ionization energy vs. atomic number
He
Ionization energy (kJ/mol)
2500
Ne
2000
Ar
F
1500
N
H
Cl
C
Be
1000
O
P S
B
500
Mg Si
Al
Li
Ca
Na
K
0
0
2
4
6
8
10
12
Element
14
16
18
20
Electrostatic Force Theory
(EFT)
• The degree of attraction or repulsion experienced by
two charged particles depends on TWO main factors:
1. The distance between them 2. The magnitude of the charge.
• As the # of protons in the nucleus increases, the
electrons will experience a greater electrostatic
attraction for the nucleus.
• As electrons are positioned in energy shells further
from the nucleus, they will experience less
electrostatic attraction from the positive nucleus.
What does this mean?
Look at the arrangement of electrons
Is being ‘pulled’ in by the 11 protons
11p+
12n°
BUT is ‘shielded’ by 10 electrons in
front of it
Chlorine
Sodium
Is being ‘pulled’ in by the 17 protons
BUT is ‘shielded’ by 10 electrons in
front of it
17p+
18n°
More protons means greater attraction between nucleus
and outer electron thus higher ionization energy.
Electron Affinity
• The energy released when a free electron is added to the
lowest available energy shell of an atom in the gas phase.
-1
17p+
18n°
Cl
+
+
1e-
1e-
17p+
18n°
Cl-1 (g)
Electron
+ Affinity
+
E.A.
Predict…
Is there an EA trend in the PT of E?
• Down a group?
EA. DECREASES because:
o More energy levels are added increasing the distance
between the nucleus and valence electrons.
• Across a period?
EA. INCREASES because:
o More energy is released as atoms become more
stable. For example, elements in group 7 are very
stable with the addition of 1 e-, thus a higher EA.
Elements in group 1 become less stable with the
addition of 1 e-, thus a lower EA.
Atomic Radius
• the estimated distance from the centre
of the nucleus to the outer edge of the
atom.
• Typically radii = 100 pm (picometer)
» 1 pm = 1 x 10-12 m
Predict…
Is there a trend in Atomic Radii in the PT of E?
Down a group?
• Atomic size INCREASES because:
o Energy levels are added, therefore electrons are
at a greater distance from the nucleus.
o There is an increase in shielding effect due to the
inner electrons.
Across a period?
• Atomic size DECREASES because:
o Electrons are added along the same energy level,
so are protons, therefore overall attraction
increases, atomic radius decreases.
Atomic radius vs. atomic number
Atomic Radius (pm)
250
K
200
Na
Li
150
Mg
Al Si
Be
100
Ca
P S Cl
B C N
O F
Ar
Ne
50
H
0
0
He
2
4
6
8
10
12
Element
14
16
18
20
• Handout
• Follow directions on sheet and answer
questions (you can use textbook for help)
Hint: Ignore Hydrogen when looking at
trends.
Look at many periods/groups when
summarizing trends.
Periodic table trends
Answers
Answers
2 a) – He, Ne, Ar, Noble gases
2 b) – Li, Na, K, Alkali metals
3 a) – Li, Na, K, Alkali metals
3 b) – He, Ne, Ar, Noble gases
4. As one increases, the other decreases
5. Ionization energy increases
Atomic radius decreases
6. Ionization energy decreases
Atomic radius increases
7.
11p+
12n°
11p+
12n°
10p+
10n°
Na has 11
electrons
Na+ has 10
electrons
Ne has 10
electrons
electron configuration of Na+ resembles Ne
Alkali metals become like noble gases
8. Radius increases because shells are added
Increased radius will make it easier to lose an
electron because of greater distance between
positive and negative charges
9. Proton # increases. More protons means
greater attraction between nucleus and outer
electron thus higher ionization energy.
The greater attraction also means that outer
electrons are brought closer to the nucleus,
thus smaller atomic radius results.
Li (enc = 1)
++
+
Be (enc = 2)
++
+
+
B (enc = 3)
+++
++
10. Noble gases are ignored
11. Electron affinity is energy associated with an
atom gaining an electron. It is highest in the
top right where atoms are smallest with the
greatest number of protons
12. Electronegativity is a number that describes
the relative ability of an atom (when bonded)
to attract electrons. The trend is the same as
affinity for the same reason