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6.3 Periodic Trends

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Laura Wommack

on 11 December 2013

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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
An ion is an atom (or group of atoms) with a positive or a negative charge.
Positive and negative ions are formed 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 will tend to LOSE electrons and form cations.
Columns V, VI and VII will tend to GAIN electrons and form anions.
Whether other elements gain or lose electrons is based on ionization energy.
Trends in Ionization Energy
The energy required to REMOVE an electron from an atom is ionization energy.
First ionization energy is the energy required to remove the first electron.
First ionization energy decreases from top to bottom within a group and increases from left to right across a period.
Ionization energy helps predict what ions will form.
Trends in Ionic Size
Ionic size tends to increase from top to bottom within a group; generally, the size of cations and anions decrease 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 is ability of an atom to attract electrons.
Electronegativity helps predict the type of bond that will form.
In general, electronegativity values decrease from top to bottom within a group; for representative elements, values tend to increase from left to righ across a period. The least electronegative element is Cs, the most is F.
Your Assignment
P 182, #18-25

For the representative elements, create flashcards showing the ION they tend to form.
For example, Li form Li+, Be forms Be2+
Where you have them, use your old flashcards and add the ion; for new ones, create them.
Trends in Atomic Size
Size of atoms is expressed as atomic radius.
The atomic radius is 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) (there are 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. Across a period then, the shielding effect is constant.
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; this is explained by nuclear charge and shielding effect; nuclear charge increases but shielding effect remains constant so it takes more energy to remove electrons across a period.
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