Atomic Radius

• Size of an atom’s electron cloud
• Distance to an atom’s valence (outermost) electrons
• Can also be the distance to a neighboring atom’s outer electrons.
• Elements with more occupied electron shells are larger.
  • Elements further down the Periodic Table have more occupied electron shells, so elements get larger as you move down the Periodic Table.
• Elements with more protons (but same number of occupied electron shells) are smaller, because more protons increases Coulombic attraction force. More force pulling on the electrons causes those electrons to stay closer to the nucleus.
  • Elements have more protons moving across the table from left to right, because the elements are in order of increasing number of protons. Therefore, elements get smaller as you go left to right on the Periodic Table.

Ionization Energy

• Energy required to remove a valence electron from an atom
• It takes energy to separate things that have a force holding them together, such as the force between protons’ charge (+) and an electron’s charge (−).
• If the Coulombic Attraction between protons and electron is stronger, the Ionization energy is higher. (It takes more energy to overcome a stronger force)
• Elements further down the Periodic Table have lower ionization energies. This is because they have more occupied electron shells. this means the outer electrons farther away from the nucleus, which weakens the Coulombic attraction force.
• Elements further to the right on the Periodic Table, within the same row, usually have higher ionization energy.  This is because the elements to the right have more protons in their nucleus. More protons results in stronger Coulombic attractions to the outer electrons. (Elements in the same horizontal row have the same number of electron shells, so the increased ionization energy is not explained by number of electron shells)
• An atom that loses an electron becomes a positively charged ion. That’s why this property is called ionization energy.
• Ionization energy is higher for an atom that has already lost one or more electrons, especially if the next electron is in an electron shell that’s closer to the nucleus.

Electronegativity

• Ability to attract electrons from a neighboring atom
• Affected by distance between the atom’s nucleus and its neighbor’s electrons (closer = stronger Coulombic attraction)
• Affected by number of protons in nucleus (more protons = stronger Coulombic attraction)
• Elements further down the Periodic Table generally have lower electronegativity, because they have more occupied electron shells. The larger an atom’s radius, the farther away its neighbor’s electrons are, so the Coulombic attraction forces to those neighboring atom’s electrons are weaker.
• Atoms further to the right on the Periodic Table, within the same row, generally have higher electronegativity. This is because they have more protons, which give them a stronger Coulombic attraction to the electrons in a neighboring atom. (Elements in the same row have the same number of electron shells, so this increased electronegativity is not explained by number of electron shells)
• Atoms with stronger Coulombic attraction have higher electronegativity; having more protons (more charge) or having fewer electron shells (closer distance to nucleus) can each cause an atom to have a stronger Coulombic attraction.
• Atoms with higher electronegativity are better at “stealing” electrons from other elements. They tend to gain electrons, which gives them a negative overall charge.
• Fluorine has the highest electronegativity of any element. This is for three reasons: 1) it has a short distance to its neighbor – only two occupied electron shells between its nucleus and the other atom; 2) It has the most protons (9) of any element that has room to add an electron to its second shell.
• Bigger elements with lots of electron shells have weaker electronegativity because of the long distance to neighboring atoms (too many electron shells), and also because of the shielding effects: their inner electrons repel the outer electrons, making it harder to attract more electrons.
• Noble gases don’t have  electronegativity in the usual sense, because they hardly ever form compounds. This is because they have full valence shells: there’s no room in their valence shells to add electrons from a neighboring atom, and they don’t have enough protons to attract an electron into a higher energy (further from nucleus) electron shell.
• Nonmetals are more electronegative than metals in general

Ionization Energy and Electronegativity Trends

Although these are different properties, they have similar trends, because they both increase as the Coulombic Attractions between nucleus and electrons near the edge of the electron cloud.

As you move down the Periodic Table, the elements’ ionization energy and electronegativity decrease. This is because the elements have more occupied electron shells, so the distance to the electrons at the edge of the atom is increasing. Farther distance causes weaker Coulombic attraction, and weaker Coulombic attraction makes electronegativity weaker and ionization energy smaller.

As you move across the Periodic Table from left to right, ionization energy and electronegativity both increase. This is due to elements having more protons. More protons causes stronger Coulombic attraction between nucleus and atoms near the outside of the atom’s radius. This stronger Coulombic attraction causes stronger electronegativity toward neighboring electrons and makes it require more energy to remove an atom’s own outer electrons.