Moseley and the Modern Periodic Law ആധുനിക പീരിയോഡിക് നിയമം
Mendeleev's table was brilliant, but it was built on atomic mass – and that caused the problems you found on the last page. In 1913 a young English physicist, Henry Moseley, used X-rays to look at the nucleus of each element and found the number that really decides an element's place: the atomic number. On this page you repeat his idea in a virtual lab, read the modern periodic law, and see how it removes all three problems at once.
- Atomic number അറ്റോമിക നമ്പർ Z = number of protons in the nucleus = number of electrons in the atom (page 2).
- Mendeleev arranged elements by atomic mass. Its limitations: no place for isotopes; Ar (40) placed before K (39); Na, K grouped with Cu, Ag (page 3).
1. Henry Moseley and his X-ray experiments
ഹെൻറി മോസ്‌ലി (Henry Moseley) തന്റെ എക്സ്റേ (X-ray) ഡിഫ്രാക്ഷൻ പരീക്ഷണങ്ങളിലൂടെ മൂലകങ്ങളുടെ ഗുണങ്ങൾ പ്രധാനമായും ആശ്രയിച്ചിരിക്കുന്നത് അറ്റോമിക മാസിനെ അല്ല, മറിച്ച് അറ്റോമിക നമ്പറിനെയാണ് എന്ന് കണ്ടെത്തി.
Through his X-ray diffraction experiments എക്സ്റേ ഡിഫ്രാക്ഷൻ പരീക്ഷണങ്ങൾ, Henry Moseley found that the properties of elements depend mainly not on the atomic mass but on the atomic number. He then revised Mendeleev's periodic law; the revised law is known as the modern periodic law ആധുനിക പീരിയോഡിക് നിയമം.
What did he actually do? In simple words: he made each element give out X-rays and measured their frequency. Step through it:
The vertical axis is the square root of the frequency of each element's strongest X-ray line, in relative units (calculated from Moseley's rule, √f grows by the same step for each element; approx.). Atomic masses from this guide's element data.
2. The modern periodic law
മൂലകങ്ങളുടെ രാസഗുണങ്ങളും ഭൗതികഗുണങ്ങളും അവയുടെ അറ്റോമിക നമ്പറിന്റെ ആവർത്തനഫലങ്ങളാണ്.
The chemical properties രാസഗുണങ്ങൾ and physical properties ഭൗതികഗുണങ്ങൾ of elements are periodic functions ആവർത്തനഫലങ്ങൾ of their atomic numbers.
Compare it with Mendeleev's law: only one word changes – "atomic masses" becomes "atomic numbers". But that one word fixes the problems, because the atomic number is a whole number that goes up by exactly 1 from one element to the next, with no ties and no reversals.
"Periodic function" means the properties repeat as the atomic number grows. Here is one property you already know – the number of electrons in the outermost shell (page 2). Watch it repeat:
Electron configurations from this guide's element data (as in textbook Fig 2.1). For Z = 21–30 the outermost shell stays at 2 (sometimes 1) – you will see why on page 7.
3. The modern periodic table
ആധുനിക പീരിയോഡിക് നിയമത്തിന്റെ അടിസ്ഥാനത്തിൽ മോസ്‌ലി മൂലകങ്ങളെ അറ്റോമിക നമ്പറിന്റെ ആരോഹണക്രമത്തിൽ വിന്യസിക്കുകയും ആധുനിക പീരിയോഡിക് ടേബിളിന് (Modern periodic table) രൂപം നൽകുകയും ചെയ്തു.
On the basis of the modern periodic law, Moseley arranged the elements in the ascending order of atomic number ആരോഹണക്രമം and gave shape to the modern periodic table ആധുനിക പീരിയോഡിക് ടേബിൾ. Let us look at its merits മേന്മകൾ. Each one fixes a limitation of Mendeleev's table – switch between the two tables to see the difference.
The three merits (textbook page 25)
- Similar elements in one group. Mendeleev's table put soft, very reactive metals like sodium (Na) and potassium (K) in one group with the much less reactive copper (Cu) and silver (Ag). In the modern table Moseley made sure that elements with similar properties are in the same group – so if you know the properties of one element, you get an idea of the properties of the others in its group.
- Isotopes need no separate places. The elements are arranged in the ascending order of atomic number, and all isotopes of an element have the same atomic number – so the limitation that isotopes could not be given different places is solved.
- The atomic-mass disorder no longer matters. In Mendeleev's table argon (Ar, atomic mass 40) came before potassium (K, atomic mass 39), against the order of mass. In the modern table the order is by atomic number (Ar 18, K 19), so this kind of disorder in atomic mass is not relevant.
Think (page 25). Why did isotopes cause a problem for Mendeleev but not for Moseley?
4. Periods and groups
വിവിധ രൂപങ്ങളിലുള്ള പീരിയോഡിക് ടേബിളുകൾ കാലാകാലങ്ങളായി രൂപപ്പെട്ടിട്ടുണ്ട്. – Periodic tables of many different shapes have been made over the years (spirals, pyramids, round ones!). The one in use now contains 118 elements – the long form you will explore on the next page.
പീരിയോഡിക് ടേബിളിൽ വിലങ്ങനെയുള്ള നിരകളെ (horizontal rows) പീരിയഡുകൾ എന്നും കുത്തനെയുള്ള കോളങ്ങളെ (vertical columns) ഗ്രൂപ്പുകൾ എന്നും വിളിക്കുന്നു. ഒരേ ഗ്രൂപ്പിലുള്ള മൂലകങ്ങൾ രാസഭൗതിക സ്വഭാവങ്ങളിൽ സമാനത പ്രകടിപ്പിക്കുന്നു.
The horizontal rows of the periodic table are called periods പീരിയഡുകൾ (വിലങ്ങനെയുള്ള നിരകൾ) and the vertical columns are called groups ഗ്രൂപ്പുകൾ (കുത്തനെയുള്ള കോളങ്ങൾ). Elements in the same group show similarity in chemical and physical properties.
Memory trick: a period runs across like a line of text you read; a group stands up like a pillar of a temple. Where they cross there is exactly one element.
5. Mendeleev's table and the modern table side by side
| Mendeleev's periodic table (1869) | Modern periodic table (Moseley, 1913 onwards) | |
|---|---|---|
| Basis | atomic mass | atomic number |
| Law | properties are periodic functions of atomic masses | properties are periodic functions of atomic numbers |
| Isotopes | no place for them | share one place (same Z) |
| Ar and K | Ar (40) before K (39) – breaks the order | Ar (18) before K (19) – in order |
| Na, K and Cu, Ag | in the same group | in different groups (1 and 11) |
| Noble gases | not known in 1869; added later as a new group | group 18 |
| Number of elements | about 63 known | 118 |
Chemical properties depend on the electrons, especially those in the outermost shell (page 2). The number of electrons equals the atomic number. So arranging by atomic number is really arranging by electron number – and the repeating pattern of outermost electrons makes the properties repeat. Atomic mass also depends on the neutrons, which do not take part in chemical reactions.
- 📄 Dmitri Mendeleev Wikipedia
The life of the scientist who built the first periodic table in 1869. - 📄 Mendeleev's predicted elements Wikipedia
How the gaps in his table predicted gallium and germanium (eka-aluminium, eka-silicon). - 📄 Henry Moseley Wikipedia
The young physicist whose X-ray experiments showed that atomic number is the true order. - 📄 Moseley's law Wikipedia
Go further: the X-ray result behind the modern periodic law.
6. Check yourself
- Henry Moseley (1887–1915), through X-ray experiments, showed that the properties of elements depend mainly on the atomic number, not on the atomic mass.
- Modern periodic law: the chemical and physical properties of elements are periodic functions of their atomic numbers.
- The modern periodic table arranges elements in increasing atomic number (118 elements). Merits: similar elements are in one group; isotopes need no separate places; the Ar–K mass disorder no longer matters.
- Horizontal rows = periods; vertical columns = groups. Elements in one group show similar chemical and physical properties.