4.2 Periodic Table and Element Discovery

4.2.1 The Patterns Emerge

• Historical Context:
    - Two thousand years ago, only ten elements were identified.
    - By the early eighteenth century, a few more elements were discovered.
    - Classifying these elements was a significant challenge, similar to solving a jigsaw puzzle with many missing pieces.
    - Progress in science and chemistry during the seventeenth and eighteenth centuries led to many new discoveries, necessitating an organization system.

• Dmitri Mendeleev:
    - Russian chemist who presented an arrangement of known elements in 1869.
    - Arrangement was based on properties and atomic weights.
    - Mendeleev left gaps for undiscovered elements, predicting properties of germanium 15 years prior to its discovery.
    - Modern scientists similarly predict properties of new elements before discovery.

4.2.2 The Periodic Table

• Mendeleev’s Legacy:
    - Mendeleev's work led to the creation of the periodic table as we know it today.
    - Elements are organized in a grid with rows (periods) and columns (groups).
    - Rows (Periods): Contain elements arranged in order of increasing mass or atomic weight.
    - Columns (Groups): Contain families of elements with similar properties.

• Current Discoveries:
    - Elements with atomic numbers up to 118 have been identified.
    - Confirmation of four elements (113, 115, 117, 118) by IUPAC in 2016.

4.2.3 An Educated Guess

• Mendeleev’s Predictions:
    - Confidence in periodic law allowed Mendeleev to leave gaps.
    - Example: Predicted properties of eka-silicon (germanium), later confirmed with remarkable accuracy.

• Differences in Modern Table:
    - More elements exist today, including transition metals, lanthanoids, and actinoids.
    - Modern table has 18 columns, with groups labeled 1-2 and 13-18.

4.2.4 Counting Sub-Atomic Particles

• Atomic Number & Mass:
    - Atomic Number (Z): Number of protons in an atom determines the element's identity.
    - Atoms with the same atomic number have identical chemical properties.
    - Each atom is neutral, having equal numbers of protons and electrons.

• Mass Number (A):
    - Mass number is the sum of protons and neutrons in the nucleus.
    - The number of neutrons can be calculated using the formula:
      $\text{Number of neutrons} = A - Z$
    - Example: For carbon with atomic number 6 and mass number 12, it has 6 neutrons.

4.2.5 How Heavy Are Atoms?

• Measuring Atomic Mass:
    - Difficult due to atomic size; chemists compare equal numbers of atoms.

• Historical Context:
    - Explores implications of lead poisoning in Roman times and its societal impacts.

• Lead and History:
    - Lead was used in plumbing and utensils, causing health issues.
    - Introduction of unleaded petrol in 1986 in Australia to reduce exposure.

4.2.6 Families of Elements

• Groups and Families:
    - The table contains nine groups or families.
    - Examples of family names include:
      - Group 1: Alkali metals (react with water)
      - Group 2: Alkaline earth metals
      - Group 15: Pnictogens (nitrogen family)
      - Group 16: Chalcogens (oxygen family)
      - Group 17: Halogens (reactive, colorful)
      - Group 18: Noble gases (inert)

• Transition Metals: Found in the middle section of the periodic table.

4.2.7 Metals, Non-Metals and Metalloids

• Classification in the Periodic Table:
    - Metals (3/4 of elements) are on the left-hand side; non-metals are on the upper right.
    - Metalloids are elements that border the line between metals and non-metals.

• Properties of Metals:
    - Solid at room temperature, except mercury.
    - Good conductors of heat and electricity, malleable, ductile.

4.2.8 Following a Trend

• Periodic Trends:
    - Atomic radius changes down groups and across periods.
    - Characteristic patterns include:
      - Atomic number/mass increases down a group and across a period.
      - Atomic radius increases down a group and decreases across a period.
      - Melting points and reactivity vary across and down the groups.

4.2.9 Is the Periodic Table Finished?

• Current Discoveries: The periodic table continues to grow; last four elements were added in 2016.

• Element Production:
    - Elements beyond atomic number 92 are generally created artificially through nuclear reactions.

• Naming New Elements:
    - New elements undergo extensive testing before naming; claims may lead to disputes.
    - Temporary names assigned until verified; for example, tennessine (number 117) was previously known as ununseptium.

Conclusion

• Ongoing research continues to explore new elements and their potential applications, highlighting the dynamic nature of the scientific endeavor.