Periodic TrendsPeriodic Trends

Chapter 14 Notes, part IIChapter 14 Notes, part II

Atomic SizeAtomic Size

• Atomic size increases down a Atomic size increases down a group.group.

• Atomic size decreases from left Atomic size decreases from left to right across a period.to right across a period.

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Why increasing down a Why increasing down a group?group?

• The increasing trend occurs because as you The increasing trend occurs because as you go down a group more energy levels cause go down a group more energy levels cause the atom to be larger.the atom to be larger.

• Shielding also affects the size; the electrons Shielding also affects the size; the electrons in lower energy levels cause less pull to those in lower energy levels cause less pull to those electrons outside of it.electrons outside of it.

• Atomic size increases down a Atomic size increases down a group.group.

• Atomic size decreases from left Atomic size decreases from left to right across a period.to right across a period.

Why decreasing across a Why decreasing across a period?period?

• Across a period no new energy levels are Across a period no new energy levels are added, but more protons are. The added added, but more protons are. The added positive charge gives more pull to the nucleus, positive charge gives more pull to the nucleus, so it can pull electrons towards the center. so it can pull electrons towards the center.

• Atomic size increases down a Atomic size increases down a group.group.

• Atomic size decreases from left Atomic size decreases from left to right across a period.to right across a period.

Which is bigger?

• Li or Na?Li or Na?• Se or As?Se or As?• N or Si?N or Si?• What is the biggest element?What is the biggest element?• What is the smallest element?What is the smallest element?

• Atomic size increases down a Atomic size increases down a group.group.

• Atomic size decreases from left Atomic size decreases from left to right across a period.to right across a period.

Valence ElectronsValence Electrons

• Valence electrons increase from Valence electrons increase from left to right across a period.left to right across a period.

• Valence electrons are the same Valence electrons are the same in a group.in a group.

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Before we go on...Before we go on...

• An An ionion is an atom that loses or gains electrons is an atom that loses or gains electrons to have a full outside energy level.to have a full outside energy level.

• In doing so, it becomes charged--ceasing to be In doing so, it becomes charged--ceasing to be electrically neutral.electrically neutral.

• A positively charged ion is called a A positively charged ion is called a cationcation, a , a negatively charged ion is called an negatively charged ion is called an anionanion..

• We will look at trends in:We will look at trends in:– Ion sizeIon size– Ionization energyIonization energy

Oxidation Number

Oxidation number is the charge that an element has after it becomes an ion.

+1+2 +3 ±4 -3 -2 -1 0

Varies

Ion SizeIon Size

• Essentially the same trend as Essentially the same trend as atomic size, except anions are atomic size, except anions are always bigger than cations.always bigger than cations.

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Why are anions bigger?Why are anions bigger?

• Anions have more electrons than the atom it Anions have more electrons than the atom it is made from, so there will be more repulsion is made from, so there will be more repulsion between them, causing increase in radius.between them, causing increase in radius.

• Cations have less electrons, so they will have Cations have less electrons, so they will have one less energy level than the element made one less energy level than the element made from.from.

• Essentially the same trend as Essentially the same trend as atomic size, except anions are atomic size, except anions are always bigger than cations.always bigger than cations.

Which is bigger?Which is bigger?

• Cations or anions?Cations or anions?• NN3-3- or F or F--??• GaGa3+3+ or As or As 3-3-??• CC4+4+ or C or C4-4-??

• Essentially the same trend as Essentially the same trend as atomic size, except anions are atomic size, except anions are always bigger than cations.always bigger than cations.

Ionization energyIonization energy

• Ionization energyIonization energy is the amount of energy it is the amount of energy it takes to pull an electron from the outside shell takes to pull an electron from the outside shell of an atom.of an atom.

• 11stst Ionization energy is the amount of energy Ionization energy is the amount of energy to pull the first electron away, the 2to pull the first electron away, the 2ndnd ionization energy is the amount of energy to ionization energy is the amount of energy to pull away a second electron, etc.pull away a second electron, etc.

• We will look at the trend in 1We will look at the trend in 1stst and and 22ndnd ionization energies. ionization energies.

11stst Ionization Energy Ionization Energy

• Ionization energy increases up a Ionization energy increases up a group.group.

• Ionization energy increases left to Ionization energy increases left to right across a period.right across a period.

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Why decreasing down a group?Why decreasing down a group?

• As you go down a group, the valence electrons As you go down a group, the valence electrons are farther away from the nucleus, and so it are farther away from the nucleus, and so it takes less energy to pull them away from the takes less energy to pull them away from the atom.atom.

• Ionization energy increases up a Ionization energy increases up a group.group.

• Ionization energy increases left to Ionization energy increases left to right across a period.right across a period.

Why increasing across a period?Why increasing across a period?

• The more electrons in the outside energy level, The more electrons in the outside energy level, the less eager an atom will be to lose an the less eager an atom will be to lose an electron.electron.

• This is because nuclear charge increases while This is because nuclear charge increases while shielding remains constant.shielding remains constant.

• Ionization energy increases up a Ionization energy increases up a group.group.

• Ionization energy increases left to Ionization energy increases left to right across a period.right across a period.

Which has a higher I.E.?

• K or Ca?• Mg or Ba?• Br or Sb?• I or Rb?

• Ionization energy increases up a Ionization energy increases up a group.group.

• Ionization energy increases left to Ionization energy increases left to right across a period.right across a period.

22ndnd Ionization Energy Ionization Energy

• The second ionization energy follows the same The second ionization energy follows the same general trend as the first, except that all the general trend as the first, except that all the elements in the first group have the highest IE.elements in the first group have the highest IE.

• This is because after the first row loses one This is because after the first row loses one electron it has a full valence, making as stable electron it has a full valence, making as stable as possible. It takes much more energy to as possible. It takes much more energy to take away a second electron.take away a second electron.

• Ionization energy increases up a Ionization energy increases up a group.group.

• Ionization energy increases left to Ionization energy increases left to right across a period.right across a period.

Electronegativity

• The The electronegativityelectronegativity of an element is the of an element is the tendency for the atoms of the element to tendency for the atoms of the element to attract an electron when combining with attract an electron when combining with other elements.other elements.

• The scale is based on several The scale is based on several factors, including the ionization factors, including the ionization energy of the element. It can be energy of the element. It can be used to predict the type of bonds.used to predict the type of bonds.

ElectronegativityElectronegativity

• Electronegativity increases up a Electronegativity increases up a group.group.

• Electronegativity increases from Electronegativity increases from left to right across a period.left to right across a period.

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ElectronegativityElectronegativity

• The exception to the general trend for The exception to the general trend for electronegativity is that theelectronegativity is that the noble gases have a noble gases have a value of zerovalue of zero!!

• This is because they do not attract electrons, since This is because they do not attract electrons, since they already have a full outside shell.they already have a full outside shell.

• Electronegativity increases up a Electronegativity increases up a group.group.

• Electronegativity increases from Electronegativity increases from left to right across a period.left to right across a period.

Which has a higher Which has a higher electronegativity?electronegativity?

• S or P?S or P?• Al or Ga?Al or Ga?• Cl or Ar?Cl or Ar?• C or Mg?C or Mg?• Cs or He?Cs or He?

• Electronegativity increases up a Electronegativity increases up a group.group.

• Electronegativity increases from Electronegativity increases from left to right across a period.left to right across a period.