Trends in the Periodic Table PDF

R. Gallagher www.theconicalflask.ie Trends in the Periodic Table Trends in Atomic Radii The atomic radius of an atom is half the distance between the nuclei of two atoms of the same element tha...

R. Gallagher www.theconicalflask.ie Trends in the Periodic Table Trends in Atomic Radii The atomic radius of an atom is half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond. Atomic radii: Decreases across a period: - Reason: Increase in nuclear charge. - Nuclear charge is the attraction between electrons and the protons. Atomic radii: Increases down a group: - Reason: New energy level and screening effect (alkali metals only). - The screening effect is the outer electron being shielded from the pull of the positive protons due to the full inner orbits. Thus counteracting the increase in nuclear charge. Note: Nobel gases (group 8) don’t form atomic radii as they are stable, so intramolecular bonds don’t exist. Trends in Ionisation Energy The first ionisation energy of an atom is the minimum energy required to completely remove the most loosely bound electron from a neutral gaseous state. 1st ionisation energy: Increases across a period: - Reason: Increasing nuclear charge and decreasing atomic radius. 1st ionisation energy: Decreases down a group: - Reason: Increasing atomic radius and screening effect (alkali metals only). Exceptions to ionisation trend: - Any half full P sub-level or full D sub-level has extra stability thus slightly higher ionisation values. 10 R. Gallagher www.theconicalflask.ie 2nd Ionisation energy: The energy required to remove an electron from an ion with one positive charge. - For every electron removed, there is an increase in ionisation energy. - Reason: For each electron removed, the effect of the positive charge is increased meaning a greater attraction between nucleus and remaining electrons. - Note: - Small peaks in ionisation energy correspond to removing an electron from a full or half filled p or d sub-levels. - Large jumps in ionisation energy correspond to removing an electron from a change in energy level. Trends in Electronegativity Electronegativity is relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond. Electronegativity: Increases across a period: - Reason: Increasing nuclear charge and decreasing atomic radius. Electronegativity: Decreases down a group: - Reason: Increasing atomic radius and screening effect (alkali metals). Trends within groups Group 1: The Alkali Metals - All very reactive metals as they have a low first ionisation energy. - Reactivity increases down the group because atomic radius increases. Group 7: Halogens - Very reactive non-metals due to them needing only one more electron to be stable. - Reactivity decreases down the group due to electronegativity decreasing. 11