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6.3 Periodic Trends

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Laura Wommack

on 19 January 2017

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Transcript of 6.3 Periodic Trends

Objectives
Know the trends among the elements for atomic size.
Know how ions form.
Know the trends among elements for ionization energy, ionic size and electronegativity.
6.3 Periodic Trends
How Ions Form
Ions are atoms (or group of atoms) with positive or negative charges.
Positive and negative ions form when electrons are
transferred
.
A cation is an ion with a positive charge.
An anion is an ion with a negative charge.
In the representative elements, the first three columns tend to LOSE electrons and form cations.
Columns V, VI and VII tend to GAIN electrons and form anions.
Whether other elements gain or lose electrons is based on ionization energy.
Trends in Ionization Energy
Energy required to REMOVE an electron from an atom.
First ionization energy: Energy required to remove the first electron.
Ionization energy helps predict what ions will form.
Trends in Ionic Size
Ionic size: Increases from top to bottom within a group; decreases from left to right across a period.
Cations are always smaller than their atoms; anions are always larger than their atoms.
For representative elements, when an atom loses an electron, the attraction between the nucleus and remaining electrons is greater, so size decreases.
For nonmetals, the ion is larger than the atom; as the number of electrons increases, the attraction of the nucleus for any one electron decreases.
Trends in Electronegativity
Electronegativity: Ability of an atom to attract electrons.
Electronegativity helps predict types of bonds that will form.
Electronegativity decreases from top to bottom within a group; for representative elements, values tend to increase from left to right across a period.
The least electronegative element is Cs, the most is F.
Your Assignment
P 182, #18-25
Trends in Atomic Size
Size of atoms is expressed as atomic radius - one-half the distance between nuclei of two atoms of the same element when they are joined.
Distances are very small so the unit is in picometers (pm) (10 to the 12 pm in a meter).
Atomic size (atomic radius) increases from top to bottom within a group (down a group) and decreases from left to right across a period.
As atomic number increases within a group, the charge on the nucleus increases and the number of occupied energy levels increases; the positive charge draws electrons closer to the nucleus but the shielding effect from the # of electrons has a greater effect and radius increases.
With increasing atomic number, each element has another proton and electron.
Increasing nuclear charge pulls electrons closer and atomic radius decreases across a period.
In general, first ionization energy decreases from top to bottom within a group - atomic radius increases, electrons are further away from the positive nucleus, so it's easier to remove an electron.
In general first ionization energy increases from left to right across a period; the nuclear charge increases (increasing nuclear attraction) but shielding effect remains relatively constant so it takes more energy to remove electrons across a period.
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