TRENDS IN THE PERIODIC TABLE
1. Atomic Radius
2. Ionization Energy
3. Electronegativity
4. Melting/Boiling Point
5. Reactivity
1. Atomic radii (atomic size)
Ø Atomic radii decrease from left to right along a given period (row) of the periodic table.
Explanation: The outer electrons of the atoms in a single period have the same
number of occupied energy levels (same number of shells). But as you move from left to right along a period, the nuclei are gaining protons, that is, the nuclear charge is increasing. So with the nuclei getting more and more positive, the outer electrons are being pulled closer to the core of the atom.
Ø Atomic radii increase from top to bottom within one family/group (column) of the periodic table.
Explanation: As you move down within the same group, the valence electrons
are in higher and higher energy levels. Simply put, more shells = bigger atoms.
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Figure 1: Atomic radii
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2. Ionization Energy
Ionization energy is the energy required to remove one valence electron from a neutral atom.
Ø Ionization energy increases from left to right along a given period (row) of the periodic table.
Explanation:
· If it’s hard to remove an electron from an atom, then that atom will have a high ionization energy.
· As the nuclear charge increases across a period (row), the valence electrons are pulled in tighter and tighter making it harder to remove an electron, thus ionization energy increases.
· Also, it is important to know that all atoms want to have the same electron configuration as the nearest Noble Gas. That is, they want to have a full outer shell!
· Halogens (group VIIA), which have 7 electrons in their outer shell, would much rather gain 1 electron to fill their outer shell than lose one. So, they have high ionization energies. Noble gases have the highest ionization energy.
· On the other hand, alkali metals (group IA), which only have one valence electron, are more than happy to give up their lone electron to have a full outer shell. So they have very low ionization energies.
Ø Ionization energy decreases from top to bottom within one family/group (column) of the periodic table.
Explanation: As you move down within a group, the valence electrons are in higher and
higher energy levels, further and further away from the nucleus. The further an electron is from the nucleus, the less it feels the pull from the positively charged core.
So, helium (He) has the highest ionization energy and francium (Fr) has the lowest.
Figure 2: Ionization energy
3. Electronegativity
Electronegativity is the ability of atoms to attract electrons from other nearby atoms.
Ø Electronegativity increases from left to right along a given period (row) of the periodic table.
Explanation: As the nuclear charge increases across a period (row), the nucleus
becomes stronger and stronger, thus increasing the atom’s ability to attract nearby electrons.
Ø Electronegativity decreases from top to bottom within one family/group (column) of the periodic table.
Explanation: The more energy levels an atom has, the more its valance electrons are
shielded from the attractive force of the positive nucleus. So as you move down in a group, the number of energy levels increases, so the ability the atom to attract nearby electrons decreases due to the shielding affect of all the energy levels.
Fluorine (F) has the highest electronegativity (4.0), while Cesium (Cs) and Francium (Fr) have the lowest (0.7).
Figure 3: Electronegativity
4. Melting/Boiling Points
Ø Melting and boiling points increase from left to right along a given period of the periodic table until the middle and then begin to decrease.
· So, alkali metals have the lowest melting and boiling points of the metals and the noble gases have the lowest melting and boiling points of the non-metals.
· However, melting and boiling points are generally higher for metallic elements than for nonmetallic elements. At room temperature, most metals are solid, whereas many non-metals are gases.
Figure 4: Melting points
5. Reactivity
Ø FOR METALS:
· Reactivity decreases from left to right along a given period (row) of the periodic table.
Explanation: When metals react, they tend to lose their valence electrons. So the
fewer valence electrons a metal has the more reactive it will be. This is why alkali metals (only 1 valence electron) are the most reactive metals.
· Reactivity increases from top to bottom within one family/group (column) of the periodic table.
Explanation: The further the valence electrons are from the nucleus, the more reactive a
metal will be. This is why francium (Fr) which has its valence electron in the 7th energy level, is the most reactive metal.
Ø FOR NON-METALS:
· Reactivity increases from left to right along a given period (row) of the periodic table.
Explanation: When non-metals react, they tend to gain electrons in order to fill their
outer shell. So the higher their electronegativity, the more reactive they are. This is why the halogens (7 valence electrons – only have to gain1e-) are the most reactive non-metals.
· Reactivity decreases from top to bottom within one family/group (column) of the periodic table.
Explanation: (See explanation for decreasing electronegativity within a group. P.3)
Fluorine (F) is the most reactive non-metal.
SUMMARY OF PERIODIC TRENDS
By: Jerry Maycock (www.learnchem.net/tutorials/trends.shtml)
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