Periodic Trends SCH 3U SECTION 1.3

Atomic Size (Atomic Radius)  The atomic size or radius of an refers to the distance between an atom's nucleus and its valence electrons.  Measured in Å, angstroms

Moving Across a Period  Moving from left to right across a period, the atomic radius decreases.  The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons.

Moving Down a Group  The atomic radius increases moving down a group.  Moving down a group, new energy shells of electrons are added. The new energy shells provide shielding, allowing the valence electrons to experience only a minimal amount of the protons' positive charge.

Cations and Anions  The atomic radius of a cation is smaller than in the normal neutral atom  the positive charge of the nucleus is distributed over a smaller number of electrons and electron-electron repulsion is decreased, meaning that the electrons are held more tightly.  The atomic radius of an anion is greater than the neutral atom  in atoms that have gained electrons, electron- electron repulsion increases and the positive charge of the nucleus is distributed over a larger number of electrons, meaning the electrons are held less tightly.

Trend in Atomic Radii Which atom is the largest? Which atom is the smallest?

Ionization Energy and Electron Affinity  The process of gaining or losing an electron requires energy. There are two common ways to measure this energy change: ionization energy and electron affinity.  Remember, the closer an electron is to the nucleus, the lower its energy and the more tightly it is held.

Ionization Energy  The ionization energy is the energy it takes to fully remove an electron from the atom.  When several electrons are removed from an atom, the energy that it takes to remove the electrons are called successive ionization energies:  first electron is called the first ionization energy, the energy it takes to remove the second electron is the second ionization energy, and so on.

Ionization Energy First Ionization Energy Vs. Atomic Number

Across a Period  Ionization energy increases moving across the periodic table from left to right.  From left to right, as the number of protons are increasing, the electrons therefore become more tightly held meaning it takes more energy to pry them loose.

Down a Group  Ionization energy decreases moving down a group.  The atomic radius increases, as new energy shells are added. This means it takes less energy to remove an electron, as they become more shielded from the attraction of the nucleus by core electrons.