Transcript File - Riverside Rebel Science

Periodic
Trends
SCH 3U
SECTION 1.3
Atomic Size (Atomic Radius)
 The
atomic size or radius of an refers
to the distance between an atom's
nucleus and its valence electrons.
 Measured
in Å, angstroms
Moving Across a Period
 Moving
from left to right across a
period, the atomic radius
decreases.
 The nucleus of the atom gains
protons moving from left to right,
increasing the positive charge of
the nucleus and increasing the
attractive force of the nucleus
upon the electrons.
Moving Down a Group
 The
atomic radius increases moving
down a group.

Moving down a group, new energy shells
of electrons are added. The new energy
shells provide shielding, allowing the
valence electrons to experience only a
minimal amount of the protons' positive
charge.
Cations and Anions

The atomic radius of a cation is
smaller than in the normal neutral
atom


the positive charge of the nucleus is distributed
over a smaller number of electrons and
electron-electron repulsion is decreased,
meaning that the electrons are held more
tightly.
The atomic radius of an anion is
greater than the neutral atom

in atoms that have gained electrons, electronelectron repulsion increases and the positive
charge of the nucleus is distributed over a
larger number of electrons, meaning the
electrons are held less tightly.
Trend in Atomic Radii
Which atom is the largest?
Which atom is the smallest?
Ionization Energy and
Electron Affinity

The process of gaining or losing an
electron requires energy. There are two
common ways to measure this energy
change: ionization energy and electron
affinity.

Remember, the closer an electron is to
the nucleus, the lower its energy and
the more tightly it is held.
Ionization Energy


The ionization energy is the energy it
takes to fully remove an electron from
the atom.
When several electrons are removed
from an atom, the energy that it takes
to remove the electrons are called
successive ionization energies:
 first
electron is called the first ionization
energy, the energy it takes to remove
the second electron is the second
ionization energy, and so on.
Ionization Energy
First Ionization Energy Vs. Atomic Number
Across a Period

Ionization energy increases moving
across the periodic table from left to
right.

From left to right, as the number of
protons are increasing, the electrons
therefore become more tightly held
meaning it takes more energy to pry
them loose.
Down a Group

Ionization energy decreases
moving down a group.

The atomic radius increases, as new
energy shells are added. This
means it takes less energy to
remove an electron, as they
become more shielded from the
attraction of the nucleus by core
electrons.