Atomic Radius
The atomic radius is the distance from the electron from the proton. The size of the electron cloud increases
as the principal quantum number increases. So...as you look down the periodic table, the size of atoms in each
group is going to increase. When you look across the periodic table, you see that all the atoms in each group
have the same principal quantum number. However, for each element, the positive charge on the nucleus
increases by one proton. This means that the outer electron cloud is pulled in a little tighter. One periodic
property of atoms is that they tend to decrease in size from left to right across a period of the table.
OVERALL: Atomic radii increase as you go down and left on the periodic table.
Ionization Energy
Ionization energy is the energy needed to remove the most loosely held electron from an atom. Ionization
energies are periodic. The ionization energy tends to increase as atomic number increases in any horizontal row
or period. In any column or group, there is a gradual decrease in ionization energy as the atomic number
increases. Metals typically have a low ionization energy. Nonmetals typically have a high ionization energy.
OVERALL: Ionization energy increases as you go up and right on the periodic table.
Electron Configuration
A way in which electrons are distributed among the various orbitals.
Hund's Rule: The most stable arrangement of electrons is that with the maximum number of unpaired electrons,
all with the same spin direction. (One electron goes in a orbital at a time before doubling up)
Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. (Only
2 electrons can be in an orbital and they must have opposite spins)
Core electrons: Inner electrons of an atom (all electrons except the valence electrons)
Valence electrons: Outermost electrons of an atom (in highest energy level). These determine the chemical
properties of an element.
fill subshells in order by counting across periods, from hydrogen up to the element of interest:
1.Orbital box notation- this is the way shown in the orbital diagram below
Protons, Electrons, Neutrons in a atom, ion, or
isotope
Step 1 - Gather Information
The first thing you will need to do is find some information about your element. Go to the Periodic Table and
find your element. Then, find your element's atomic number. The atomic number is the number located in the
upper left corner.
Step 2 - The Number of Protons is...
The atomic number is the number of protons in an atom of an element. In our example, krypton's atomic
number is 36. This tells us that an atom of krypton has 36 protons in its nucleus.
What is cool is that is that every atom of krypton contains 36 protons. If an atom doesn't have 36 protons, it
can't be an atom of krypton. Adding or removing protons from the nucleus of an atom creates a different
element. For example, removing one proton from an atom of krypton creates an atom of bromine.
Step 3 - The Number of Electrons is...
By definition, atoms have no overall electrical charge. That means that there must be a balance between the
positively charged protons and the negatively charged electrons. Atoms must have equal numbers of protons
and electrons. In our example, an atom of krypton must contain 36 electrons since it contains 36 protons.
Electrons are arranged around atoms in a special way.
An atom can gain or lose electrons, becoming what is known as an ion. An ion is nothing more than an
electrically charged atom. Adding or removing electrons from an atom does not change which element it is, just
its net charge.
For example, removing an electron from an atom of krypton forms a krypton ion, which is usually written as
Kr+. The plus sign means that this is a positively charged ion. It is positively charged because a negatively
charged electron was removed from the atom. The 35 remaining electrons were outnumbered by the 36
positively charged protons, resulting in a charge of +1.
Step 4 - The Number of Neutrons is...
The atomic weight is basically a measurement of the total number of particles in an atom's nucleus. In
reality, it isn't that clean cut. The atomic weight is actually a weighted average of all of the naturally occurring
isotopes of an element. The mass number is a count of the number of particles in an atom's nucleus. Remember
that the nucleus is made up of protons and neutrons. So, if we want, we can write:
Mass Number = (Number of Protons) + (Number of Neutrons)
For krypton, this equation becomes:
84 = (Number of Protons) + (Number of Neutrons)
If we only knew how many protons krypton has, we could figure out how many neutrons it has. Wait a minute...
We do know how many protons krypton has! We did that back in Step 2! The atomic number (36) is the
number of protons in krypton. Putting this into the equation, we get:
84 = 36 + (Number of Neutrons)
What number added to 36 makes 84? Hopefully, you said 48. That is the number of neutrons in an atom of
krypton.
The interesting thing here is that adding or removing neutrons from an atom does not create a different element.
Rather, it creates a heavier or lighter version of that element. These different versions are called isotopes and
most elements are actually a mixture of different isotopes.
If you could grab atoms of krypton and count the number of neutrons each one had, you would find that most
would have 48, others would have 47, some would have 50, some others would have 46, a few would have 44
and a very few would have 42. You would count different numbers of neutrons because krypton is a mixture of
six isotopes.
In Summary...
For any element:
Number of Protons = Atomic Number
Number of Electrons = Number of Protons = Atomic Number
Number of Neutrons = Mass Number - Atomic Number
EXAMPLE:
1. H = 1 electron (atomic number from the periodic table = 1)
We will start with the lowest energy subshell, the 1s, which has 1 orbital. The single electron is placed in this
orbital.
Box notation: We can explicitly show each electron in each orbital. Each orbital is shown as a box. Each
electron is shown as an arrow. An arrow point up indicates spin up, and an arrow point down indicates spin
down. Each subshell of orbitals is labeled underneath the grouping of orbitals for that subshell.
The orbital is named, and the number of electrons inside the orbital is shown as a superscript.
1s1
Hydrogen atoms are paramagnetic with one unpaired electron.
2. Helium:
The atomic number is 2, so He has 2 electrons to place.
He = 1s2
Box notation:
3. Carbon:
Atomic number = 6, so C has 6 electrons to place.
The 1s subshell is filled with just 2 electrons. We see that the next subshell to fill is the 2s. It will also be filled
with 2 electrons, so we will have to move to the 2p subshell for the remaining 2 electrons.
C = 1s22s22p2
Box notation: Each s subshell has one orbital, but the p subshell has 3 orbitals. Hund's rule tells us that we have
to put the 2p
electrons in separate orbitals since there is room to do so.
4. Neon:
Atomic number = 10, so there are 10 electrons to place.
Ne = 1s22s22p6
Noble gas core notation: [Ne]
Box notation:
The six electrons in the p subshell completely fill 2p.
5. ClAtomic number = 17 for Cl; Add 1 for the anion (negatively charged) = 18 electrons in ClCl = 1s22s22p63s23p6
Box notation: