Modern periodic law and the modern periodic table
Properties of elements repeat in order of their atomic number. The long form table arranges them in 18 groups and 7 periods, split into s, p, d and f blocks.
From atomic mass to atomic number
Mendeleev arranged elements by atomic mass. A few pairs did not fit: argon (39.9) had to come before potassium (39.1), and cobalt before nickel. Henry Moseley studied the X-rays given out by elements. He showed that the atomic number (number of protons) is the basic property of an element. So the law was restated as the modern periodic law: properties of elements are periodic functions of their atomic numbers. Periodic means they repeat at regular intervals.
The seven periods
| Period | Elements | Name | From and to (atomic number) |
|---|---|---|---|
| 1 | 2 | Shortest period | H to He (1 to 2) |
| 2 | 8 | Short period | Li to Ne (3 to 10) |
| 3 | 8 | Short period | Na to Ar (11 to 18) |
| 4 | 18 | Long period | K to Kr (19 to 36) |
| 5 | 18 | Long period | Rb to Xe (37 to 54) |
| 6 | 32 | Longest period, with the lanthanides (57 to 71) | Cs to Rn (55 to 86) |
| 7 | 32 | Also 32 elements, like period 6, with the actinides (89 to 103) | Fr to Og (87 to 118) |
Groups and blocks
| Block | Groups | Family names and examples |
|---|---|---|
| s-block | 1 and 2 | Group 1: alkali metals (Li, Na, K). Group 2: alkaline earth metals (Mg, Ca) |
| p-block | 13 to 18 | 13 boron family, 14 carbon family, 15 nitrogen family, 16 oxygen family (chalcogens), 17 halogens (F, Cl), 18 noble gases (He, Ne, Ar) |
| d-block | 3 to 12 | Transition elements (Fe, Cu, Zn, Ag) |
| f-block | Two rows at the bottom | Inner transition elements: lanthanides (57 to 71) and actinides (89 to 103) |
Features, merits and limits
- Elements in a group have the same number of valence electrons (electrons in the outermost shell), so they show similar chemical properties.
- The period number equals the number of shells. Sodium (2, 8, 1) has three shells, so it is in period 3.
- Metals are on the left and in the middle, non-metals on the upper right, and metalloids (B, Si, Ge, As, Sb, Te) lie along a zigzag line between them.
- Merits: it is based on atomic number, so isotopes need no separate places and the argon and potassium problem disappears.
- Limits: the position of hydrogen is still not settled, the lanthanides and actinides sit outside the main body, and helium (2 electrons) sits in group 18 with p-block elements.
Worked example: placing an element
An element has atomic number 12. Find its period, group and block.
- Write the electronic configuration: 12 electrons = 2, 8, 2
- Number of shells = 3, so period = 3
- Valence electrons = 2, filling an s subshell, so it is in the s-block, group 2
Period 3, group 2, s-block. The element is magnesium (Mg).
Elements in one group behave alike, and our bodies need several of them. Sodium comes from table salt and potassium from tender coconut water and bananas (both group 1). Calcium and magnesium (group 2) come from ragi, milk and green leafy vegetables.
1 marks: “Elements with atomic numbers 57 to 71 are called ______.” Lanthanides (f-block inner transition elements).
2 marks: “State the modern periodic law.” The physical and chemical properties of elements are periodic functions of their atomic numbers.
7 marks: “Explain the main features of the modern periodic table.” Law; 18 groups with families; 7 periods with element counts; s, p, d, f blocks; metals, non-metals, metalloids; an outline sketch; one merit and one limit.
Periods: 2, 8, 8, 18, 18, 32, 32. Groups: 1 to 18. Blocks from left to right: s, d, p, with f parked below.