Periodic Table Position and Electron Configuration

The Periodic Table is an organised arrangement of all known elements, sequenced by increasing atomic number. This structure is not random; it is broken into regular intervals to create a grid of horizontal rows called Periods and vertical columns called Groups. Elements in the same Group share similar chemical properties because of the arrangement of electrons in their outermost shell.

The Relationship between Periods and Shells

The horizontal rows, or Periods, indicate the highest occupied energy level or shell for the atoms of that element. For example, any element in Period 2 has its outermost electrons in the second shell. Any element in Period 3 has its outermost electrons in the third shell.

The Relationship between Groups and Electrons

The vertical columns, or Groups, are labelled 1 to 18 according to IUPAC conventions. For the first 20 elements, the Group number tells us about the number of electrons in the outermost shell:

1. Group 1 (Alkali Metals): These atoms have 1 electron in their outermost shell. Examples include Lithium (2,1), Sodium (2,8,1), and Potassium (2,8,8,1).
2. Group 2 (Alkaline Earth Metals): These atoms have 2 electrons in their outermost shell. Examples include Beryllium (2,2), Magnesium (2,8,2), and Calcium (2,8,8,2).
3. Group 16: These atoms have 6 electrons in their outermost shell, such as Oxygen (2,6) and Sulfur (2,8,6).
4. Group 17 (Halogens): These atoms have 7 electrons in their outermost shell, such as Fluorine (2,7) and Chlorine (2,8,7).
5. Group 18 (Noble Gases): These atoms have a complete outermost shell, making them very stable. Examples are Helium (2), Neon (2,8), and Argon (2,8,8).

Between Groups 2 and 13, you will find the transition metals.

Trends in Chemical Reactivity

Because elements in the same Group have the same number of outer shell electrons, they behave similarly in chemical reactions. However, the reactivity changes as you move down a Group:

• In Metal Groups (e.g. Groups 1 and 2): Reactivity increases as you move down the Group. This is because the outer electron is further from the nucleus and more easily lost.
• In Non-metal Groups (e.g. Groups 16 and 17): Reactivity decreases as you move down the Group. This is because it becomes harder for the nucleus to attract an incoming electron into the outer shell.

Case Study: Group 1 Metals and Water

The trend of increasing reactivity down Group 1 is clearly seen when these metals react with cold water to form an alkaline solution and hydrogen gas:

• Lithium (Li): Reacts slowly, floats, and gives off hydrogen which burns with a crimson red flame if ignited.
• Sodium (Na): Reacts more vigorously, melts into a ball, moves on the surface, and gives off hydrogen which burns with a yellow-orange flame.
• Potassium (K): Reacts very vigorously, moves rapidly, and the hydrogen produced self-ignites, burning with a lilac flame.

Worked Example: Identifying Position from Electrons

Question: An ion $X^{-}$ has 18 electrons. Work out the position of element $X$ in the Periodic Table.

Step 1: Determine the number of electrons in the neutral atom. Since the ion has a $1-$ charge and 18 electrons, the neutral atom must have $18 - 1 = 17$ electrons. Step 2: Write the electron configuration. Following the 2,8,8 rule, 17 electrons are arranged as 2,8,7. Step 3: Identify the Period. There are three occupied shells, so $X$ is in Period 3. Step 4: Identify the Group. There are 7 electrons in the outermost shell, so $X$ is in Group 17.

Worked Example: Identifying Position from Reactivity

Question: Atom $Y$ has two electrons in its outermost shell and is the least reactive in its Group. Where is $Y$ located?

Step 1: Use the outer electron count. Two outer electrons place $Y$ in Group 2. Step 2: Use the reactivity trend. Group 2 consists of metals. Reactivity in metal groups increases as you move down. Therefore, the least reactive element must be at the top of the Group. Step 3: Identify the position. The top element in Group 2 is in Period 2. Thus, $Y$ is in Period 2, Group 2.