13.4 Periodic Trends in Atomic Size Remember that, according to the quantum mechanical model, an atom does not have a specifically defined boundary that sets the limit of its size. Remember that, according to the quantum mechanical model, an atom does not have a specifically defined boundary that sets the limit of its size.

13.4 Periodic Trends in Atomic Size However, there are ways to estimate the relative sizes of atoms. However, there are ways to estimate the relative sizes of atoms. X-ray diffraction – estimates the size of atoms in crystalline solids X-ray diffraction – estimates the size of atoms in crystalline solids The distance between the nuclei of diatomic molecules (examples: O 2 or Br 2 ) can be used to estimate the atomic radius of an atom. The distance between the nuclei of diatomic molecules (examples: O 2 or Br 2 ) can be used to estimate the atomic radius of an atom. atomic radius – half the distance between the nuclei of two like atoms

13.4 Atomic Size – Group Trends Atomic size generally increases as you move down a group of the periodic table Atomic size generally increases as you move down a group of the periodic table The size increases because electrons are added to higher principle energy levels The size increases because electrons are added to higher principle energy levels The added charge of nucleus pulls electrons inward, but the net effect is an increase in size because electrons are further from nucleus The added charge of nucleus pulls electrons inward, but the net effect is an increase in size because electrons are further from nucleus

13.4 Atomic Size – Periodic Trends Atomic size generally decreases as you move from left to right across a period Atomic size generally decreases as you move from left to right across a period The size decreases because electrons are added to the same principle energy level, but the added charge of nucleus pulls electrons inward; the net effect is a decrease in size The size decreases because electrons are added to the same principle energy level, but the added charge of nucleus pulls electrons inward; the net effect is a decrease in size This trend is less pronounced in periods where there are more electrons in the occupied principle energy levels between the nucleus and the outermost electrons; this is referred to as the shielding effect This trend is less pronounced in periods where there are more electrons in the occupied principle energy levels between the nucleus and the outermost electrons; this is referred to as the shielding effect

13.5 Periodic Trends in Ionization Energy When an atom gains or loses an electron it forms an ion. When an atom gains or loses an electron it forms an ion. The energy that is required to overcome the attraction of the nuclear charge and remove an electron from a gaseous atom is called the ionization energy The energy that is required to overcome the attraction of the nuclear charge and remove an electron from a gaseous atom is called the ionization energy The first ionization energy is the amount needed to remove the first outermost electron The first ionization energy is the amount needed to remove the first outermost electron The second ionization energy is the amount needed to remove the next outermost electron The second ionization energy is the amount needed to remove the next outermost electron The third ionization energy is the amount to remove the third and so on (Table 13.1 page 362) The third ionization energy is the amount to remove the third and so on (Table 13.1 page 362)

13.5 Periodic Trends in Ionization Energy Ionization energies can be used to predict how many electrons an atom will gain or lose in a chemical reaction Ionization energies can be used to predict how many electrons an atom will gain or lose in a chemical reaction 1A vs. 2A – Table 13.1 page 362 1A vs. 2A – Table 13.1 page 362 Two factors affect ionization energy: nuclear charge and distance from the nucleus Two factors affect ionization energy: nuclear charge and distance from the nucleus

13.5 Ionization Energy – Group Trends In general, the first ionization energy decreases as you move down a group on the periodic table. In general, the first ionization energy decreases as you move down a group on the periodic table. The size of the atoms increases as you move down; thus the outermost electron is farther from the nucleus and more easily removed The size of the atoms increases as you move down; thus the outermost electron is farther from the nucleus and more easily removed This results in a lower ionization energy This results in a lower ionization energy

13.5 Ionization Energy – Periodic Trends For the representative elements, the first ionization energy generally increases as you move from left to right across a period. For the representative elements, the first ionization energy generally increases as you move from left to right across a period. The nuclear charge is increasing and the atomic size is decreasing, therefore there is more of an attraction between the nucleus and the outermost electron The nuclear charge is increasing and the atomic size is decreasing, therefore there is more of an attraction between the nucleus and the outermost electron This results in a higher ionization energy This results in a higher ionization energy