The Periodic Table and IUPAC Conventions for the ESAT

Structure of the Periodic Table

The Periodic Table is a systematic arrangement of all known chemical elements. These elements are sequenced in order of increasing atomic number. This sequence is not a simple list but is broken at specific intervals to create a grid of columns and rows.

1. Periods are the horizontal rows. The number of a Period identifies the outermost occupied electron shell for the atoms of the elements within that row. For example, elements in Period 2 have their outer electrons in the second shell.
2. Groups are the vertical columns, labelled 1 to 18 according to IUPAC conventions. Elements within the same Group share a similar arrangement of electrons in their outermost shell, which results in similar chemical behaviour.

Classifying Elements by Group

The position of an element in the table allows us to classify it into specific families with shared characteristics:

• Alkali Metals (Group 1): These elements have one electron in their outermost shell. They are highly reactive metals.
• Alkaline Earth Metals (Group 2): These elements have two electrons in their outermost shell.
• Transition Metals: These are found in the central block of the table, between Groups 2 and 13.
• Group 16 Non-metals: These elements have six electrons in their outermost shell.
• Halogens (Group 17): These are highly reactive non-metals with seven electrons in their outermost shell.
• Noble Gases (Group 18): These elements have a complete outermost shell of electrons, making them exceptionally stable and unreactive.

Reactivity Trends within Groups

Because elements in the same Group have the same number of outer-shell electrons, they react in similar ways. However, the intensity of these reactions changes as you move up or down a Group:

• Metal Groups (e.g. Groups 1 and 2): Reactivity increases as you descend the Group.
• Non-metal Groups (e.g. Groups 16 and 17): Reactivity decreases as you descend the Group.

Case Study: Group 1 Reactions with Water

The increase in reactivity down Group 1 is clearly demonstrated by how these metals react with cold water:

1. Lithium: Reacts slowly. It floats and gradually dissolves to form an alkaline solution of lithium hydroxide. Hydrogen gas is released and burns with a crimson red flame.
2. Sodium: Reacts more vigorously. It melts into a ball, moves rapidly on the surface, and dissolves quickly. Hydrogen gas is released and burns with a yellow-orange flame.
3. Potassium: Reacts very vigorously. It moves rapidly and dissolves almost instantly. The hydrogen gas produced self-ignites, burning with a lilac flame.

Electron Configuration and Position

There is a direct relationship between an atom's electron configuration and its place in the table. We can determine an element's position if we know its electron count or ion state.

Worked Example 1: Finding an Element from an Ion An ion $X^-$ has 18 electrons. Work out the position of element $X$ in the Periodic Table.

1. If the ion $X^-$ has 18 electrons, it must have gained one electron. Therefore, the neutral atom $X$ has 17 electrons.
2. The electron configuration for 17 electrons is 2,8,7.
3. Because there are three shells, the element is in Period 3.
4. Because there are seven electrons in the outer shell, it is in Group 17.

Worked Example 2: Finding an Element from Reactivity An atom of element $Y$ has two electrons in its outermost shell and is the least reactive element in its Group. Work out its position.

1. Two electrons in the outer shell places the element in Group 2.
2. Group 2 is a metal group. Reactivity increases down a metal group, so the least reactive element must be at the top.
3. The top element of Group 2 is in Period 2. Therefore, $Y$ is in Period 2, Group 2.