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Periodic Trends
Section 6.3
Periodic Trends

The size of an atom is determined by x-ray
diffraction
X-rays bend around atoms in molecules and
create a pattern
 Computers generate an image from x-rays

X-ray diffraction of NaCl
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.
Atomic Radius

Atomic Radius- ½ of the distance between
the nuclei of two atoms of the same
element in a diatomic molecule
Atomic Radius
Periodic Trends

Trend for atomic size
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
Ionization Energy


Ion- atom that gains or loses electrons
Ionization Energy- energy required to remove an
electron.




Easiest to remove 2 electrons from 2A


Because there are 2 valence
Easiest to remove 3 electrons from 3A


E 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
Because there are 3 valence
Outer shell electrons are easier to remove than other
electrons!
Ionization Energy
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)

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)
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
 Within period- size decreases
 Down a group – size increases

Forming cations
Forming anions
Atom versus Ion
Periodic Trends

Electronegativity- tendency for the atoms
of the element to attract electrons when
the atoms are part of a compound


F is most electronegative
Noble gases- no electronegativity valuesdon’t form compounds
Periodic Trends

Electronegativity TrendsDown a group – decreases- electron shielding
results in less attraction for electrons by the
nucleus
 Across a period- increases- higher atomic
number and consistent electron shielding
result in more attraction for electrons


Electronegativity allows you to predict
bond type: covalent- polar vs. nonpolar
and ionic
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.

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.

Electron Affinity

http://chemed.chem.purdue.edu/genchem/
topicreview/bp/ch7/ie_ea.html#ea
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/
6.3
Summary of Trends
Increases
Increases
Decreases
Ionization
Ionicof
size
energy
Size
Size
Electronegativity
Atomic
Nuclear
Shielding
of
anions
cations
Size
Charge
DecreasesConstant
6.3 Section Quiz

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

2.
 gain
Metals tend to
electrons to form cations.
 gain electrons to form anions.
 lose electrons to form anions.
 lose electrons to form cations.
6.3 Section Quiz

3.
Which of the following is the most
electronegative?
 Cl
 Se
 Na
I