Chemical Periodicitypre-ap 08

Trends in Atomic Radius • Since atoms are so small, we are unable to measure an atom directly. Therefore, we can use the atomic radius of elements to calculate their relative size. • Atomic radius – ½ the distance between the nuclei of two like atoms in a diatomic molecule. • Group Trends: – Atomic size generally increases as you move down a group. – Atomic size generally decreases as you move from left to right across a period.

• Why? – As you move across a period to the right, the nucleus is becoming more positive. Therefore it can hold electrons more tightly.

Atomic Radius

Trends in Ionization Energy • When an atom gains or loses an electron, it becomes an ion. • The energy required to overcome the attraction of the nuclear charge and remove an electron from a gaseous atom is called the ionization energy. • Group Trends – – Ionization energy generally decreases as you move down a group of the periodic table. (Bigger atom = less ionization energy) – Ionization energy generally increases as you move from left to right across the periodic table.

More on Ionization Energy • Can more than one electron be removed? • Yes—these are the 2nd ionization energies, 3rd ionization energies, etc. (IE2, IE3, etc.) • Usually it takes more energy to take away the 2nd and 3rd electrons as the remaining + charge in the nucleus “holds” the remaining electrons tightly. • Some atoms will lose electrons easily until they have the same electron configurations as Noble Gases.

Ionization Energy

Electron Affinity • Electron Affinity – how much an atom wants to gain electrons. • Most atoms release energy when they gain an electron which is often written as a negative energy. • Electron affinities are largest at the top right of the periodic table, but there is some mixing in the trends. • Halogens gain an electron most easily because then they will have full shells (Noble Gas electron configuration). ENERGY CHANGE FROM A NEUTRAL ATOM GETTING AN ELECTRON

Ionic Radius • When an atom forms an ion, the size of the atom is changed. • Positive ions (cations) are always smaller than the neutral atoms from which they form. (loss of electrons) • Negative ions (anions) are always larger than the neutral atoms from which they form. (Gain of electrons) • Metals tend to form cations and nonmetals tend to form anions.  Cationic radii decrease across a period to Group 14.  Anionic radii decrease across a period from Group 15-18. Down a group, ionic radii

Ionic Radius

Trends in Electronegativity • Electronegativity – the tendency for the atoms of the element to attract electrons when they are chemically combined with atoms of another element (form bonds).  Representative elements have low electronegativities. (lowest = cesium)  Non-metallic elements have high electronegativities. (highest = fluorine)

• Periodic Trends:  Electronegativity generally decreases as you move down a group.  Electronegativity generally increases as you move left to right on the periodic table (excluding the noble gases)

Electronegativity

Valence Electrons & Oxidation Numbers

• Valence electrons are the electrons in the outside shell that are available for chemical reactions & bonding. • Normally, only the outside/outermost s & p orbital are considered valence electrons. • This means that the Transition metals are skipped & the pattern goes 1,2,3,4,5,6,7,8 valence electrons for the “main group” families. • Oxidation numbers: When atoms form ions, they tend to gain or lose a set amount of electrons based on their

Questions • Of the following elements, which has the greatest atomic radii? Ca, Cl, or Br – Calcium • List the following elements in order of increasing electronegativity (from least to most). Ca, Mg, Rb – Rubidium, Calcium, Magnesium • Of the following elements, which has the greatest electron affinity? Si, Se, S – Sulfur • List the following elements in order of decreasing ionization energy (from most to least) Al, Ar, P – Ar, P, Al • Of the following elements or ions, which has the greatest radii? Cl, Cl-1, Br, Br-1 – Br-1

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