Transcript Periodic Trends PowerPoint - Atomic Size & Ionization Energy

Periodic table trends
Trends in the periodic table:
Ionization Energy
Atomic Radii
Ionic Radii
Electron Affinity
Electronegativity
Ionization Energy
Shielding
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
• An ion is an atom that has either a net
positive or net negative charge
• Q: What would the charge be on an atom
that lost an electron? Gained two electrons?
– A: +1 (because your losing a -ve electron)
– A: -2 (because you gain 2 -ve electrons)
IONIZATION ENERGY
• Ionization energy is the energy required to remove
one outer electron from an atom
– increase across, decrease down ( ,  )
• increase across due to the increasing nuclear charge – takes
more energy to remove electrons since they are closer to full
valence shells
• decrease down due to electrons being further from the nucleus
– 1st ionization energy = energy required to remove 1st
outermost electron
– 2nd ionization energy = energy required to remove 2nd
outermost electron, etc.
– once electrons are removed from a neutral atom, atom is
now an ion
• ion = atom or group of bonded atoms that has either a positive
or negative charge
ATOMIC RADIUS and IONIC RADIUS
• Atomic radius is the distance from the nucleus to
the outer electron shell
– decrease across, increase down (  ,  )
• decrease across - atom has valence shell that is becoming full and
electrons are being pulled closer to nucleus
• increase down - has a lot more electrons so is a larger atom
• Ionic radius is the distance from the nucleus to the
outer electron shell
– decrease across, increase down (  ,  )
– cations are always SMALLER than the neutral atoms from
which they are formed due to the loss of the outer shell
electrons resulting in increased attraction by the nucleus
– anions are always LARGER than the neutral atom because
the nuclear attraction is less for increased number of electrons
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
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
IONIZATION ENERGY
vs.
ATOMIC RADIUS
• As one increases, the other decreases
– Ionization energy increases
– Atomic radius decreases
And. . .
– Ionization energy decreases
– Atomic radius increases
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
• 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
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)
+++
++
ELECTRON AFFINITY
•
Electron affinity is energy associated with an atom
gaining an electron.
– increase across, decrease down (, )
•
–
–
–
–
–
decrease down due to a larger size of atom (they already
have lots of electrons)
energy that accompanies the addition of an electron
to a gaseous atom
F(g) + e- 
F-(g)
negative means the gain of an electron is favorable
since energy is released
positive means the gain of an electron is unfavorable
since energy is absorbed
it is highest in the top right where atoms are smallest
with the greatest number of protons
ELECTRO-NEGATIVITY
• Electro-negativity 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.
– increase across, decrease down ( ,  )
– The tendency for an atom to attract electrons to itself
when chemically combined
– Cs is the least and F is the most
– noble gases are omitted because they do not form
compounds
SHIELDING
• this is how well the nucleus is surrounded by electrons
– remains constant across, increases down
(constant  ,  )
*****NOTE:
• Ignore H when looking at trends, look at many
periods/groups when summarizing trends
• Noble gases are ignored since they already have a full
outer, valence, shell.