Periodic Trends

Discovery/ies: the form of the periodic table is evolving due to knowledge-challenging discoveries thus enhancing its function of showing trends in the physical and chemical properties of the element.

GLOBAL CONTEXT: Scientific and technical innovation periodic table, Orientation in Space & Time

Atomic Size

The atomic size of an atom, also called the atomic radius, refers to the distance between an atom's nucleus and its valence electrons. Remember, the closer an electron is to the nucleus, the lower its energy and the more tightly it is held.

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. True, electrons are also added as the elements move from left to right across a period, but these electrons reside in the same energy shell and do not offer increased shielding.

Moving down a group

The atomic radius increases moving down a group. Once again protons are added moving down a group, but so are new energy shells of electrons. The new energy shells provide shielding, allowing the valence electrons to experience only a minimal amount of the protons' positive charge.

Further Facts

Halogens: Non-metals, is a conductor of electricity, group 7, Cl, BR, I, Di atomic element means 2 atoms, 7e-, reactivity increases when going down in the group

Alkalis: Metals, good conductors of electricity, higher boiling point =, elements of group 1, Examples; Na, Li, K, 1e-, reactivity increases when going down in the group.

Why does group 7 exist as diatomic elements?
Halogens are very reactive. They only need one electron to achieve a full shell (known as an octet or nobel gas configuration). Now because they only need one electron, they can bond covalently with another atom of the same electrons so that they both have a full octet. This is why halogens are diatomic.