The Modern Periodic Table
NCERT Class 10 • Science • Chapter "Periodic Classification of Elements"
The Modern Periodic Table
Elements were classified so that their properties could be predicted rather than memorised one by one. Several early attempts failed on particular elements, and the problem was finally settled by arranging elements according to atomic number rather than atomic mass.
From Early Attempts to the Modern Law
- Dobereiner's triads grouped elements in threes where the middle element's atomic mass was about the average of the other two, but very few such triads existed.
- Newlands' law of octaves noted that every eighth element repeated properties, but it failed beyond calcium.
- Mendeleev's periodic table arranged elements by increasing atomic mass and left gaps for undiscovered elements, whose properties he predicted successfully.
- Its problems were the position of hydrogen, the placing of isotopes, and a few pairs that had to be reversed against mass order.
- The modern periodic law states that properties of elements are a periodic function of their atomic number.
- This solved the anomalies, because atomic number and not mass is the fundamental property of an element.
Structure and Trends
- The table has 18 vertical groups and 7 horizontal periods.
- Elements in the same group have the same number of valence electrons, so they show similar chemical properties.
- The period number tells how many shells the atom has.
- Group 1 are alkali metals, group 2 alkaline earth metals, group 17 halogens and group 18 noble gases.
- Going left to right across a period, atomic size decreases, metallic character decreases and non metallic character increases.
- Going down a group, atomic size increases, metallic character increases and the number of shells increases.
- Valency first increases and then decreases across a period, while it stays the same down a group.
- The table separates metals on the left, non metals on the right and metalloids along the dividing line.
| Trend | Across a Period | Down a Group |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Number of shells | Same | Increases |
| Valence electrons | Increases | Same |
Modern periodic law — the properties of elements are a periodic function of their atomic number.
Hindi me samjhein
एक ही समूह के तत्वों के संयोजी इलेक्ट्रॉन समान होते हैं, इसलिए उनके रासायनिक गुण मिलते-जुलते हैं। आवर्त संख्या बताती है कि परमाणु में कितने कोश हैं।
Exam me kaise aata hai
- The modern periodic table is arranged by — atomic number
- Group 18 elements are called — noble gases
- Atomic size across a period from left to right — decreases
- Who left gaps for undiscovered elements — Mendeleev
UPSC/State PSC ke liye
- Atomic size decreases across a period because the nuclear charge rises while electrons enter the same shell, so the pull on them increases.
- Mendeleev's greatest success was predicting properties of elements not yet discovered, which turned the table from a listing into a predictive tool.
Yahan confuse hote hain
✗ The modern table is arranged by atomic mass | It is arranged by atomic number | ✓
✗ Atomic size increases across a period | It decreases across a period | ✓
✗ Elements in the same period have similar properties | Similar properties belong to the same group | ✓
Ek nazar me
- Triads and octaves failed; Mendeleev used atomic mass and left gaps.
- The modern law uses atomic number, which removed the anomalies.
- 18 groups and 7 periods; group means same valence electrons.
- Across a period size and metallic character fall; down a group both rise.