Periodic Trends

The Periodic LawMendeleev understood the ‘Periodic Law’

which states:When arranged by increasing atomic number,

the chemical elements display a regular and repeating pattern of chemical and physical properties.

The Periodic LawAtoms with similar properties appear in

groups or families (vertical columns) on the periodic table.

They are similar because they all have the same number of valence (outer shell) electrons, which governs their chemical behavior.

There are two aspects about how atomic structure changes from atom to atom that impact/cause these trends:

1. effective nuclear charge2. shielding

Atomic Structure Impacts on Trends

Effective Nuclear Charge

What keeps electrons from simply flying off into space?

Effective nuclear charge is the pull that an electron “feels” from the nucleus.

The closer an electron is to the nucleus, the more pull it feels.

As effective nuclear charge increases, the electron cloud is pulled in tighter.

ShieldingAs more energy levels are added to

atoms, the inner layers of electrons shield the outer electrons from the nucleus.

The effective nuclear charge on those outer electrons is less, and so the outer electrons are less tightly held.

Radius decreases across a periodIncreased effective nuclear charge due to

addition of protonsRadius increases down a group

Each row adds another “shell” or energy level to the atom so valence electrons are that much further from nucleus

Atomic Radius

The Periodic Table and Atomic Radius

Period Trend:Atomic Radius

Tends to increase across a periodAs radius decreases across a period, the

electron you are removing is closer to the nucleus and harder to remove.

Tends to decrease down a groupOuter electrons are farther from the

nucleus and easier to remove.

Ionization EnergyThe energy required to remove an electron from an atom.

Ionization EnergyIf an electron is given enough energy (in the

form of a photon) to overcome the effective nuclear charge holding the electron in the cloud, it can leave the atom completely.

The atom has been “ionized” or charged.The number of protons and electrons is no

longer equal. The larger the atom is, the easier its

electrons are to remove.Ionization energy and atomic radius are

inversely proportional.

Periodic Trend:Ionization Energy

Electron AffinityWhat does the word ‘affinity’ mean?Electron affinity is the energy change that

occurs when an atom gains an electron (also measured in kJ).

This is directly related to Ionization Energy; the more energy it takes to remove an electron, the more the atom would rather gain an electron.

Electronegativity tends to increase across a periodAs radius decreases, electrons get closer to the

bonding atom’s nucleus so the nuclear charge exerts more influence

Electronegativity tends to decrease down a group or remain the sameAs radius increases, electrons are farther from

the bonding atom’s nucleus so its nuclear charge has less influence

ElectronegativityA measure of an atom’s attraction for another atom’s electrons

ElectronegativityIt is an arbitrary scale that ranges from 0 to

4.The units of electronegativity are Paulings.Generally, metals are electron givers and

have low electronegativities.Nonmetals are electron takers and have high

electronegativities.

Periodic Table of Electronegativities

Periodic Trend:Electronegativity

Positively charged ions form when an atom of a metal loses one or more electrons

Smaller than the corresponding atom

Negatively charged ions form when an atom of a nonmetal gains one or more electrons

Larger than the corresponding atom

Ionic Radii

Cations

Anions

Graphic courtesy Wikimedia Commons user Popnose

Overall ReactivityThis ties all the previous trends together

in one package.However, we must treat metals and

nonmetals separately.The most reactive metals are the largest

since they are the best electron givers. The most reactive nonmetals are the

smallest ones, the best electron takers.

SummaryShielding is constantAtomic Radius decreasesIonization energy increasesElectronegativity increasesNuclear charge increases

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