periodic trends
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Objectives
• Define atomic and ionic radii, ionization energy, electron affinity, and electronegativity.
• Compare the periodic trends of atomic radii, ionization energy, and electronegativity, and state the reasons for these variations.
• Define valence electrons, and state how many are present in atoms of each main-group element.
Section 2 Periodic Trends
Atomic Radii
• The boundaries of an atom are fuzzy, and an atom’s radius can vary under different conditions.
• To compare different atomic radii, they must be measured under specified conditions.
• Atomic radius - one-half the distance between the nuclei of identical atoms that are bonded together.
Section 2 Periodic Trends
Atomic Radii
Section 2 Periodic Trends
Atomic Radii
• Atoms tend to be smaller as you go from left to right on the periodic table.
• This is due to increasing positive charge in the nucleus, pulling the electrons closer to the center.
• Atoms tend to be larger down a group.
• This trend is due to the increasing size of the electron cloud as electrons fill up larger energy levels.
Section 2 Periodic Trends
Atomic Radii
Section 2 Periodic Trends
Ion definition
Section 2 Periodic Trends
• Watch the video on entitled “Ion definition” found in this section of the course
Ionic Radii trends
Section 2 Periodic Trends
Ionic Radii trends
Section 2 Periodic Trends
• Positive ions tend to be smaller than neutral atoms.
• The larger the positive charge, the smaller the ion.
• Again, this is due to increasing positive charge in the nucleus, pulling the electrons closer to the center.
Ionic Radii trends
Section 2 Periodic Trends
• Negative ions tend to be larger than neutral atoms.
• The more negative the charge, the larger the ion.
• This is due to the repelling forces between electrons, causing them to occupy a larger space around the atom
Ionization Energy
Section 2 Periodic Trends
• The process to form an ion is called ionization.
• The energy required to remove one electron from a neutral atom is called ionization energy.
• Sometimes refered to as IE1 or first ionization energy.
Ionization Energy
Section 2 Periodic Trends
• Ionization energies increase across a period.
• Caused by increasing effect nuclear charge
• Higher positive charge more strongly attracts electrons in the same energy level
Ionization Energy
Section 2 Periodic Trends
• Ionization energies decrease down a group.
• Electrons removed from larger atoms are at higher energy levels, and are farther away from the nucleus.
• Electrons are removed more easily due to their distance from the nucleus.
Ionization Energy
Section 2 Periodic Trends
Ionization Energy
Section 2 Periodic Trends
• Watch the “Ionization” in this lesson when you get to this slide before continuing.
Electron affinity
Section 2 Periodic Trends
• The energy change that occurs when an electron is acquired by a neutral atom is called electron affinity.
• Electron affinity generally increases across periods.
• Electron affinity generally decreases down groups.
• All these can be explained by effective nuclear charge and by the distance of an electron from the nucleus.
Electron Affinity
Section 2 Periodic Trends
• Watch the video on “Electron Affinity” that is included in this lesson.
Valence electrons
Section 2 Periodic Trends
• Compounds form because electrons are lost, gained, or shared between atoms.
• Electrons involved in this behavior are called valence electrons.
• Valence electrons are the outermost energy level electrons in an atom.
• Atoms tend to want to have a full set of 8 valence electrons to be stable.
Valence electrons
Section 2 Periodic Trends
• Watch the video on “Valence electrons” included in this lesson.
Electronegativity
Section 2 Periodic Trends
• Valence electrons hold atoms together in compounds.
• In many compounds, the negative charge of the electrons is concentrated closer to one atom than another.
Electronegativity
Section 2 Periodic Trends
• Electronegativity is the ability of an atom in a compound to attract electrons from another atom in the compound.
• Electronegativity generally increases across rows, and decreases down a group
Electronegativity
Section 2 Periodic Trends
• Watch the video on “Electronegativity” found in this lesson.
Summary of periodic trends
Section 2 Periodic Trends
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