Periodic Table: Groups, Periods, and Isotopes

Periodic Table Organization

Elements are organized in columns (groups) and rows (periods).
Group: up-down (vertical).
Period: left-right (horizontal).
The term periodic comes from how elements repeat properties across periods.

Groups and Periods

Group = vertical direction; Period = horizontal direction.
Example coordinate idea: period $3$ , group $16$ identifies the element at that location.
Carbon: group $14$ , period $2$ .
Some older periodic tables use numbers $1-18$ or different labeling; the middle block contains transition metals.
Roman numerals and letters like $1A, 2A, 3A$ are part of older group labeling.

Transition Metals and Old Notation

The middle section of the table contains transition metals.
Older conventions use labels such as $1A, 2A, 3A, 4A, 5A, 6A, 7A$ for groups, sometimes with Roman numerals.

Atomic Weight and Isotopes

Atomic weight is typically a fractional value, not a whole number.
Atomic number Z = number of protons in the nucleus.
Protons and neutrons each weigh about $1$ amu.
The fractional atomic weights arise because elements have multiple isotopes with different masses.
Changing the atomic number would change the element; isotopes differ in mass, not in Z.

Isotopes: Definition and Consequences

Isotopes have the same atomic number $Z$ but different mass number $A$ .
Abundances of isotopes determine the average atomic weight.

Hydrogen Isotopes

Protium: $^{1}H$ (mass ~1 amu).
Deuterium (D): $^{2}H$ (mass ~2 amu).
Tritium: $^{3}H$ (mass ~3 amu).
These isotopes are present in very small, varying amounts on Earth.

Carbon Dating

Fossils are dated using carbon-14 dating: $^{14}C$ dating.
Carbon-12 and carbon-13 isotopes also exist and vary in natural abundance.

Quick Takeaways

Groups = columns (vertical), Periods = rows (horizontal).
Atomic number Z = protons; mass varies due to isotopes.
Isotopes share Z but have different A.
Hydrogen has three main isotopes: $^{1}H$ , $^{2}H$ , $^{3}H$ .
Carbon dating relies on $^{14}C$ .