Transcript Periodic Table Trends - Magoffin County Schools

Periodic Table Trends
Atomic Radius
Atomic Radius (Radii)
• A trend is a prevailing tendency or a
general course.
• We commonly discuss weather
trends and fashion trends.
• Trends also occur in the
arrangement of the periodic table.
• Atomic size is measured according to the
radius of the atom, or the atomic radius.
• Remember that atoms are spherically shaped
and are made up of protons and neutrons
within the nucleus and electrons in the
electron cloud. The negatively-charged
electrons are held within the atom by their
attraction to the positively-charged nucleus.
• An atom doesn’t have a well-defined
boundary, and its radius cannot be directly
measured.
• *Atomic size can be estimated by measuring
the distance between the nuclei of two atoms
of the same type that are joined together.
• Therefore, the atomic radius is defined as one
half the distance between the nuclei of two
atoms of the same element that are joined.
Atomic Radius
**LOOK AT THE ATOMIC RADIUS
CHART ON THE BACK OF THE
NOTESHEET**
Question - What general trends or
patterns do you see among the
elements?
• On the periodic table, atomic radii tend to
decrease left to right along a period, so group 1
atoms are generally larger than group 18 atoms.
• This is because, within a period, the number of
principle energy levels (PELs) in each element
generally remains constant.
• For example, all elements in Period 3 have three
energy levels. However, the nucleus gains protons
as atomic number increases, so it becomes more
attractive to the electrons and pulls them in
tighter.
• This growing attraction causes the atomic radii to
decrease as protons are added.
• Moving down a group on the periodic table
atomic radii tend to increase.
• Atomic number increases as protons are
added; however, the number of occupied
orbitals increases as layers of electrons are
added.
• As the number of electrons located between
the nucleus and the outer shell increases,
valence electrons are increasingly shielded
from the full attraction of the positive nucleus.
• This is called the SHIELDING EFFECT.
• Electron shielding has a greater influence on
the atom than the increased attraction of the
nucleus; therefore, atomic radii generally
increase moving down the periodic table.
• The largest elements are in the lower left
corner of the periodic table. The smallest
elements are in the upper right corner.
IONIC RADIUS
• The radius of an ion is different from the
radius of a neutral atom.
• An ion is an atom with a charge.
• A positive ion, called a cation, forms
when an atom loses electrons. Metals,
such as sodium, tend to lose electrons to
form cations. Cations are always smaller
than the neutral atom.
• When an atom loses electrons, the ionic
radius decreases for two reasons.
• First, lost electrons will almost always be
valence electrons, so they can leave an
empty outer shell, which makes the radius
smaller.
• Second, the positively-charged nucleus
exerts the same amount of pull but on
fewer electrons, so it pulls the remaining
electrons closer to the nucleus.
• A negative ion, called an anion,
forms when electrons are gained.
• Nonmetals, such as chlorine, tend to
gain electrons to form anions.
• Negative ions are always larger than
the neutral atom.
• When an atom gains electrons, the ionic
radius increases because of the identical
charge of all electrons.
• Remember that only opposites attract—
negatively-charged electrons are attracted to
the positively-charged nucleus.
• Electrons are not attracted to each other but
instead repel and push away from each other.
• Adding electrons to the outer shell forces
them to push farther apart, which increases
the radius.
• On the periodic table, the trend in ionic
radii is similar to the trend in atomic
radii.
• Ionic radii generally decrease from left to
right across a period and increase down a
group.
• However, groups that form cations will
have smaller radii than groups that form
anions.
• Elements in groups 1, 2, 13 and (14) lose
electrons to form cations, so the ionic radii are
smaller than the neutral atom.
• Within these groups, the ionic radii decrease
left to right along a period and decrease down
the group.
• Most elements in groups 15, 16, and 17 gain
electrons to form anions, so the ionic radii are
larger than the neutral atom.
• Within these groups, the ionic radii generally
increase left to right along a period and
increase down the group.
• Recall that group 18, the noble gases, do not
form ions. Noble gases have a full valence
energy level so they do not gain or lose
electrons.