Modern periodic law and the long form table: groups and periods
Moseley showed that atomic number, not mass, is basic. The modern long form table has 18 groups and 7 periods, and its order follows the electronic configuration.
From atomic mass to atomic number
Mendeleev's table put some heavier elements before lighter ones, and had no places for isotopes. In 1913, Henry Moseley showed that the atomic number (Z), the number of protons, is the most basic property of an element. So the law was changed. The modern periodic law says that the physical and chemical properties of elements are a periodic function of their atomic numbers. Mendeleev's table needed little change, because he had trusted properties. Using atomic number removed the anomalous pairs and the problem of isotopes.
Why properties repeat: the group 1 elements
| Element (Z) | Electronic configuration |
|---|---|
| Lithium (3) | 2, 1 |
| Sodium (11) | 2, 8, 1 |
| Potassium (19) | 2, 8, 8, 1 |
| Rubidium (37) | 2, 8, 18, 8, 1 |
| Caesium (55) | 2, 8, 18, 18, 8, 1 |
The cause of periodicity
- Each of these alkali metals (group 1, apart from hydrogen) has one valence electron. So all are soft, very reactive metals that form ions with a charge of +1.
- All react with water to give a hydroxide and hydrogen gas, for example 2Na + 2H₂O → 2NaOH + H₂. All form basic oxides.
- The same outer arrangement comes back at regular intervals of atomic number: 3, 11, 19, 37, 55.
- So the repeating of similar outer electronic configurations is the cause of periodicity.
The periods of the long form table
| Period | Number of elements | Name | New shell that starts filling |
|---|---|---|---|
| 1 | 2 (H, He) | Very short | K |
| 2 | 8 (Li to Ne) | Short | L |
| 3 | 8 (Na to Ar) | Short | M |
| 4 | 18 (K to Kr) | Long | N |
| 5 | 18 (Rb to Xe) | Long | O |
| 6 | 32 (Cs to Rn) | Very long | P |
| 7 | 32 (Fr to Og) | Very long | Q |
Groups and periods in the long form
- Groups: 18 vertical columns, numbered 1 to 18. Elements in a group have the same number of valence electrons, so they have the same valency and similar properties.
- Going down a group, one more shell is added at each step: F 2, 7; Cl 2, 8, 7; Br 2, 8, 18, 7. (Your NIOS lesson's table prints bromine as 2, 8, 8, 7, a misprint: bromine has 35 electrons.)
- Periods: 7 horizontal rows. Elements in a period have consecutive atomic numbers and the same number of shells.
- The period number is the number of the shell being filled. In period 3, the M shell fills from sodium (2, 8, 1) to argon (2, 8, 8).
- The lanthanoids and actinoids belong to periods 6 and 7, but are printed in two rows below the table for convenience.
Merits of the modern table
- Isotopes: all isotopes of an element have the same Z, so they share one place.
- Anomalous pairs disappear: cobalt (Z = 27) comes before nickel (Z = 28) naturally.
- It follows electronic configuration, so it links each element's properties to its electrons.
- Metals, non-metals and metalloids sit in clear zones, and the transition elements have a clear place.
- It shows the trends in properties, and it is simple to learn and use.
Engineers making solar panels and phone chips use the groups of this table to pick elements that behave alike. Silicon and germanium, both in group 14, are one such pair.
2 marks: “Why do all the elements of group 1 show similar properties? Explain with the configurations of lithium and sodium.” They have the same outer configuration: one valence electron (Li 2, 1; Na 2, 8, 1). Similar outer configurations give similar properties, which is the cause of periodicity.
1 marks: “How many elements are there in the fourth period?” 18.
2 marks: “How did the modern periodic table remove the problems of isotopes and anomalous pairs?” It arranges elements by atomic number. Isotopes have the same Z, so they share one place. Cobalt (27) comes before nickel (28) by atomic number, so the order is natural even though cobalt is heavier.
18 groups go up and down, 7 periods go across. Period = number of shells; same group = same number of valence electrons.