كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

Theme E — Nuclear and Quantum Physics

E.3.11 Nuclear Stability — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 28 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

E.3.11 Nuclear Stability — IB PHYSICS HL notes: Use the N–Z graph, the strong force and the neutron–proton ratio to explain nuclear stability and predict decay modes.Common elements — most have N and Z below 20, plus iron with Z = 26 and N = 30Reason — lighter nuclei with fewer p…

Light nuclei
Heavy nuclei
Electromagnetic repulsion
Extra neutrons
Above the band (too many neutrons)
Below the band (too many protons)

Part 1 of 7Stable nuclei

Common elements

Common elements most have N and Z below 20, plus iron with Z = 26 and N = 30

Reason

Reason lighter nuclei with fewer protons are much more stable than heavy ones

N–Z graph

N–Z graph plotting N against Z shows a band of stable nuclei called the valley of stability

Light nuclei

Light nuclei for Z below about 20 the stable nuclei follow N = Z

Heavy nuclei

Heavy nuclei the band bends upward because stable nuclei need N greater than Z

Ratio

N/Z rises from 1 to about 1.5 for lead

Limit

Limit no nuclide above Z = 83 is stable

Part 2 of 7Why nuclei become unstable

Too many neutrons

Too many protons

Too many nucleons, so the nucleus is too heavy

Too much energy

Goal

Goal an unstable nucleus decays to become more stable

Part 3 of 7Strong force and neutron–proton ratio

1Strong force

Strong force at 1–3 fm nucleons are bound
Below about 1 fm it is repulsive and prevents collapse

2Electromagnetic repulsion

Electromagnetic repulsion it acts between protons at longer range, so more protons mean more instability

3Extra neutrons

Extra neutrons they space the protons apart to cut repulsion and add strong-force binding

4Why

Why electrostatic repulsion grows roughly as Z² while the strong force reaches only nearest neighbours

Part 4 of 7Predicting the decay mode

Predicting the decay mode

Above the band (too many neutrons)

β⁻ decay turns a neutron into a proton
N falls by 1 and Z rises by 1

Below the band (too many protons)

β⁺ decay or electron capture turns a proton into a neutron
N rises by 1 and Z falls by 1

Large nuclei (A above about 200)

α decay removes 2 protons and 2 neutrons
N and Z each fall by 2

Excited nucleus

Excited nucleus gamma emission, with N and Z unchanged

N/Z

N/Z a nuclide with N/Z too high for its Z is a β⁻ emitter and one with N/Z too low is a β⁺ emitter or electron capturer

Part 5 of 7Decay chains on the N–Z grid

α decay

α decay two squares down and two left

β− decay

β− decay one square down and one right

β+ decay

β+ decay one square up and one left

Uranium-238 series

Uranium-238 series each alpha step changes A by 4 so the members have A = 238 − 4n
It ends at lead-206 after eight alpha and six beta decays

Energy

Energy a nuclide is unstable if some decay leads to a lower total mass

Part 6 of 7Magic numbers and pairing

Magic numbers

Magic numbers nuclei with 2, 8, 20, 28, 50, 82 or 126 protons or neutrons are unusually stable

Shell model

Shell model nucleons occupy discrete energy levels with filled shells giving extra stability

Pairing

Pairing nuclei with even numbers of both protons and neutrons are more stable

Part 7 of 7More from the specification

Key terms

Term	Meaning
Valley of stability	The band of stable nuclei on a graph of N against Z.
Neutron–proton ratio	N/Z, which rises above 1 for heavier stable nuclei.
Strong nuclear force	The short-range attraction between nucleons that holds the nucleus together.
Magic number	A number of protons or neutrons that gives unusually stable nuclei.
Electron capture	The capture of an orbital electron by a proton, forming a neutron and a neutrino.

Avoid these mistakes

Reversing the sides of the band — too many neutrons gives β⁻ decay.

Believing N = Z for all stable nuclei — heavier nuclei need N greater than Z.

Thinking alpha decay changes N/Z dramatically — it removes equal numbers of both.

Forgetting that no nuclide above Z = 83 is stable.

Assuming more neutrons is always more stable.

Last updated: 18 October 2026