The Periodic Table - Notes
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Elements in the periodic table are arranged in order of increasing atomic number.
Atomic number indicates the number of protons, and if the element is neutral the number
of protons is equal to the number of electrons.
The periodic table is arranged in four blocks: s, p, d, and f that group together elements
with similar electron configurations.
The horizontal rows are called periods.
The vertical columns are called groups or families. Elements in the same column are
chemically similar to each other because each element in the family has the same number
of valence electrons.
The number of valence electrons (outer shell electrons) is equal to the number of s and p
electrons in the highest principal energy level.
The Periodic Table consists of nonmetals, metalloids and metals, and elements become
more “metallic” as you move from the right to the left.
Periodic Trends. Make sure that in each case you can describe the trend (up/down and
left/right) and also explain why the trend occurs.
Atomic Size—the distance from the nucleus of an atom to the outermost electrons. It
depends entirely on the outer shell electrons. Notice that though the nucleus makes up most of
the mass of the atom, it takes up very little space. In a typical atom, the nucleus would be about
the size of a marble on the 50-yard line of a football stadium with the electrons orbiting like
crazed fans in the upper stands.
 Vertical trend: Atomic size increases moving down the periodic table
 Explanation: Atomic size increases going down because the principal energy level
increases (2p, 3p, 4p for example). This means that the outer shell electrons have more
energy and can move farther from the nucleus.
 Horizontal trend: Atomic size decreases when moving left to right.
 Explanation: As you move left to right along a period, the atomic size decreases
because outer shell electrons (valence electrons) are added to the same principal energy
level or lower (ex: 4s then 3d then 4p in period 4) while the number of protons increases.
In other words, as the atomic number increases, the number of protons increases.
Remember that protons attract electrons because protons are positive and electrons are
negative. The fact that there are more protons causes the electrons to be drawn in
towards the nucleus.
Ionic size -- the distance from the nucleus of an ion to the outermost electrons.
The trends in atomic radius are different from the trends in ionic radius. It is important to keep
in mind that the number of electrons is different for ions than it is for neutral elements. It is
possible to predict how much bigger or smaller an ion will be by comparing its relative number
of protons and electrons and comparing that number to a neutral element with the same number
of protons or electrons.
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Ionization energy—the energy required to remove an electron.
In equation form, this looks like: X  X+ + eNotice that this equation looks reasonable for metals:
Na  Na+ + eMg  Mg+ + eSn  Sn+ + eAnd should look unreasonable for nonmetals:
Cl  Cl+ + eO  O+ + e Vertical trend: ionization energy decreases moving down the periodic table
 Explanation: as with ionic radius, electrons are in higher principal energy levels moving
down the PT. This means that they are of higher energy and farther from the nucleus.
This makes them easier to remove. There is another factor at work here: inner shell
electrons (the ones that aren’t the outer shell or valence electrons) get in the way of the
protons’ ability to hang on to the electrons. This is called the shielding effect.
 Horizontal trend: ionization energy increases going left to right () on the PT
 Explanation: this occurs for the same reason as the horizontal trend in atomic size. More
protons, same principal energy level—electrons are held more strongly and harder to
remove.
Electronegativity: the tendency for an atom to attract electrons to itself when forming a
bond with another element.
The vertical and horizontal trends for electronegativity (and their explanations) are the SAME as
those for ionization energy. There is an exception: fluorine, not helium has the highest
electronegativity on the periodic table and similarly the halogens are the group with the highest
electronegativity, not the noble gases. This is because the noble gases have a full outer shell of
electrons and do not tend to attract additional electron density. The halogens, on the other hand
strongly attract electrons in order to fill their outer shell (remember that they have seven valence
electrons, but would be most stable with eight)
Electron affinity: the energy change that occurs when an atom gains an electron.
This is how electron affinity can be expressed as an equation:
Ne (g) + e-  Neelectron affinity = 29 kJ/mol
F (g) + e  F
electron affinity = -328 kJ/mol
Notice that the reaction is highly exothermic (releases energy, which you know because the heat
change is negative) in the case of fluorine and endothermic in the case of neon. This makes
sense because fluorine is made much more stable when it gains an electron, so it releases heat as
it becomes more stable. The gain of an electron is favored for fluorine.
In some ways, electron affinity can be viewed as the opposite of ionization energy, but there are
some ways in which they are not completely opposite. For instance, the noble gases have a high
ionization energy but low and positive electron affinity.
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