Periodic TrendsChapter 6 Section 3

pp.163-169

What is a trend?

1. The general direction in which something tends to move.

2. A general tendency or inclination. See Synonyms at tendency.

Things to remember…An element’s # of protons does not change

Isotopes- atoms of an element with different masses. Different number of neutrons

Nuclear charge- over all charge obtained from the attraction of all the protons and all e- in an atom Positively charged nucleus pulls electrons

towards it

Ions An atom or bonded group of atoms that has a

positive or negative charge Occurs when an atom gains or loses an electron

Atoms that gain e- have a negative overall charge (anion)

Atoms that lose e- have a positive overall charge (cation)

Electrostatic repulsion Remember like charges repel each other…

electrons in orbitals repel/push each other away

Ionization energy (IE) is the energy required to remove an electron from a gaseous atom (kJ/mol)

Energy is needed to overcome the attraction between the positive protons and negative electrons

IE indicates how strongly a nucleus can hold onto its valence e-

High IE= strong hold on e-; less likely to make positive ions

Low IE= atom can lose valence e- easily; likely to make positive ions

Ionization Energy

Octet Rule

What is the electron configuration for sodium?

What is the new configuration when a sodium atom becomes a +1 sodium atom?

What noble gas does this configuration look like?

OCTET RULE: Atoms tend to gain, lose, or share electrons in an

order to acquire a full set of 8 valence electrons First period elements are en exception to the rule

(2Ve-) This rule helps to determine what kind of ions will

form

More about ionization energy…

After removing the 1st e- it is still possible to remove additional electrons

Second ionization energy: The IE needed to remove a

second electron from a +1 ionFor sodium IE will be high since it does not want to lose anymore e-, reached 8 valence e-

Third ionization energy: The IE needed to remove a

third electron from a +2 ion Sodium

2e-

8e-

1e-

11p+ 12n

1st Ionization Energy (1st e-)

Ionization Energy

What type of IE does group 1A have? High or low?

Low IE Likely to form + cations

What type of IE does group 8A have? High IE Unlikely to form cations

From left to right, the IE to remove successive electrons always increases

Does not happen smoothly…requires large jump of energy

IE continued….

Trends in periods: First IE generally increases as you move left-

to-right The nuclear charge increases (greater

attraction) with each successive element… What does this have to do valence electrons?

Increase the nuclear charge=stronger hold on valence electrons

IE continued…

Trends within groups… First IE generally decrease as you move

down a group What happens to the size of an atom as you

move down a group?Increases

This cause decrease in IE

Valence electrons are farther from the nucleus

Easier to remove

Electronegativity

The ability of atoms of an element to attract electrons in a chemical bond

Fluorine is the most electronegative element Value: 3.98

Cesium and Francium are the least electronegative Values: 0.79 and 0.7

Atom with greater electronegativity attracts electrons in a chemical bond the strongest

Leave out noble gases (they really don’t react)

Electronegativity (EN)

Trends in periods and groups: EN decreases as you move down a group Increases as you move left to right on table

Where are the lowest electronegatives found? Lower left side of the periodic table

Where are the highest electronegatives found? Upper right side of the periodic table

Electron Affinity

The energy change that occurs when an electron is acquired by a neutral atom

Measure in kJ/mol High Eaffinity = more negative the nrg, easier to gain

e- Low Eaffinity = more positive the nrg, easier to lose e- Noble gases have “zero” Eaffinity

Electron Affinity

Many atoms release energy when they gain an electron

A + e- A- + energy

A + e- + energy A- Some atoms need energy to be “forced” to gain an

electron Produces unstable ions that lose electron quickly

Food for thought….

Do atoms on the right side of the periodic table tend to gain or lose electrons?

Gain electrons

What type of ions are they likely to form? Negative ions

Do atoms on the left side of the periodic table tend to gain or lose electrons?

lose e-

What type of ions do they form? Positive ions

Atomic Radius

Electron cloud Spherical surface in which there is a 90%

probability of finding an e- Not physical

Atomic size is defined by how closely an atom lies to its neighboring atom Each atom has different properties so there

are going to be different sizes… Different sizes in different blocks…

Atomic Radius of a Metal

Metals Atomic radius

half the distance between adjacent nuclei in a crystal of that element

Atomic Radius of a Nonmetal

Usually occur as molecules

Atomic radius: Half the distance between nuclei of identical

atoms that are chemically bonded together Diatomic molecules

Hydrogen

Bromine

Iodine

Oxygen

Atomic Radius: Trends in PeriodsDECREASE in atomic radii left-to-rightIncreasing positive charge in the nucleusPrinciple energy level (n) remains the same throughout the period

Each successive element the atomic # Increase Add proton and electron Electron gets added to same Prin. E level (n) No additional electrons come across valence

electrons and nucleusVal. E- are not shielded from increased nuclear chargeTherefore the increased nuclear charge brings the outermost electrons closer to the nucleus …which means…Smaller atomic radii going

Atomic Radius: Trends within Groups

INCREASE as you move down a groupNuclear charge increasesWhat happens to your principle energy level as you move down a group?

Therefore…Electrons added to higher principle energy levels

Although nuclear charge increased, other factors in play to overpower increased nuclear charge...

Outermost orbital increases in sizeElectrons are farther from the nucleus b/c of bigger orbitalMore resistant to higher nuclear charge b/c of increased distance

Principle energy level increasedPuts more orbital with electrons between the nucleus and the outermost electronsThese electrons in between shield the outermost electrons from the pull of the nucleus

Solve…

Which has the largest radius? Magnesium (Mg) Silicon (Si) Sulfur (S) Sodium (Na)

Which has the smallest radius?

And the answer is…

Largest: Na (sodium)

Smallest: S (sulfur)

Ionic Radius

Losing/gaining electrons affects size of an atomGain an electron atom becomes negative atom becomes larger

Electrostatic repulsion between atom’s outer electrons increase

Forces outer electrons to move further apartMakes radius bigger

Lose an electron atom becomes positive atom becomes smaller

Electron lost is valence electronCan lead to completely empty orbital=smaller radius

Electrostatic repulsion between atoms decrease so they can be pulled closer to nucleus

Ionic Radius: Trends in Period

What type of ions do you think will be formed on the left side of the table?

Smaller positive ions

What type of ions do you think will be formed on the right side of the table?

Larger negative ions

General Rule: Left-to-right across a period, the size of positive ions

gradually decreases Around 5A and 6A, the size of much larger negative ions

gradually decreases

Ionic Radius: Trends in Group

What happens to the principle energy level as we move down a group? Increases

Ion’s outer electrons are in a higher principle level This means an increase in ionic size

Ionic radii increases as we move down a group for both positive and negative ions