Periodic Trends

Periodic Trends In the Periodic Table, chemical and physical properties appear “periodically” So, what is a trend? A trend is the general direction in which something tends to happen.

Periodic Trends The elements on the Periodic Table of Elements show many trends in their physical and chemical properties. Across the rows (periods or series) Down the columns (groups or families)

Atomic Radius ½ the distance between the nuclei of 2 like atoms in a diatomic molecule The atoms of the 8 main groups are shown here.

Atomic Radius - Groups Atomic radius increases as you move down a group Why? More electrons in more Principal Energy Levels More Electron shells Atomic size increases

Atomic Radius - Periods Atomic radius decreases as you move across a period Why? (-) electrons increase, but so do (+) protons !!! Increased (+) nuclear charge pulls the (-) electrons closer to the nucleus Atomic size decreases

Atomic Radius - Periods The size trend in periods is less pronounced than in groups because of the electron shielding effect.

Shielding Effect Reduction in effective nuclear charge on an electron that is caused by the repulsive forces of other electrons between it and the nucleus In an atom with one electron, that electron experiences the full charge of the positive nucleus. However, in an atom with many electrons, the outer electrons are simultaneously attracted to the positive nucleus and repelled by the negatively charged electrons.

Atomic Radius - Graph

Atomic Radius

Quick Check Hold up 1 or 2 fingers, Which atom is bigger? K or 2. Li 1. Al or 2. Cl

Ionic Radius What are ions? Ions are charged atoms, either + or - Cations are positive ions Cations form when atoms lose electrons Anions are negative ions Anions form when atoms gain electrons

The Octet Rule – Atoms will tend to gain, lose or share electrons so that they have a set of 8 Valence electrons. Cation Anion

What kind of Ion will it make? Calcium Oxygen

Ionic Radius

Ionic Radius – Cations Group Cations are smaller than their parent atoms. Why? By losing their valence electrons, they lose their entire valence shell Cations are formed by the metals on the left side of the Periodic Table

Ionic Radius – Cations Groups Ionic size increases as you move down a group for the same reason atomic size increases Number of principal energy levels increases

Ionic Radius – Anions Groups Anions are larger than their parent ions Why? When extra (-) electrons are added, extra (+) protons are NOT added to the nucleus Effective nuclear attraction is less for the increases number of electrons

Ionic Radius – Anions Group Ionic size increases as you move down a group for the same reason atomic size increases Number of principal energy levels increases Anions are formed by nonmetals on the right side of the Periodic Table

Ionic Radius - Periods Just like their parent atoms… Cations get smaller as you move from left to right Anions get smaller as you move from left to right Increased (+) nuclear charge pulls the (-) electrons closer to the nucleus

Ionic Radius

Quick Check Which one is bigger? 1. Li+ or 2. Na+ 1. Cl- or 2. Ar

Ionization Energy Energy is needed to remove an electron from an atom The energy needed to overcome the attraction of the nuclear charge and remove an electron (from a gaseous atom) is called the Ionization Energy

1st Ionization Energy The energy needed to remove the 1st electron from an atom is the 1st Ionization Energy

Factors Affecting Ionization Energy Atomic Radius Smaller atoms hang on to valence electrons more tightly, and so have higher ionization energy

Factors Affecting Ionization Energy Charge The higher the positive charge becomes, the harder it is to pull away additional electrons Second ionization energy is always higher than the first

Factors Affecting Ionization Energy Orbital Type It's easier to remove electrons from p orbitals than from s orbitals, which are “deeper”

Factors Affecting Ionization Energy Electron Pairing Within a subshell, paired electrons are easier to remove than unpaired ones Reason: repulsion between electrons in the same orbital is higher than repulsion between electrons in different orbitals

Factors Affecting Ionization Energy Electron Pairing – Example On the basis of gross periodic trends, one might expect O to have a higher ionization energy than N. However, the ionization energy of N is 1402 kJ/mol and the ionization energy of O is only 1314 kJ/mol. Taking away an electron from O is much easier, because the O contains a paired electron in its valence shell which is repelled by its partner.

1st Ionization Energy

2nd, 3rd, 4th Ionization Energies, etc. Subsequent electrons require more energy to remove than the first electron How much more energy is needed depends on what energy levels and orbitals the electrons are in

2nd, 3rd, 4th Ionization Energies, etc. Source: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/ie_ea.html

Electronegativity Ability of an atom to attract electrons toward itself in a chemical bond

Electronegativity The difference between the electronegativities of two atoms will determine what kind of bond they form Linus Pauling used an element's ionization energy and electron affinity to predict how it will behave in a bond. The more energy it takes to pull off the outer electron of an atom, the less likely it is to allow another atom to take those electrons. The more energy the atom releases when it gains an electron, the more likely it is to take electrons from another atom in bonding. These two energies were used to compute a numerical score.

Electronegativity - Periods Electronegativity increases going left to right across the periodic table. Fluorine's high nuclear charge coupled with its small size make it hold onto bonding electrons more tightly than any other element. Lithium has a lower nuclear charge and is actually larger than fluorine. Its valence electron is not tightly held and it tends to surrender it in chemical bonds. Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Electronegativity - Groups Electronegativity decreases going down a group The bonding electrons are increasingly distant from the attraction of the nucleus H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7

Electronegativity

Electronegativity

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Credits PowerPoint: Adela J. Dziekanowski