Atomic structure, nuclide notation and isotopes
Read atomic models and nuclide notation, calculate neutrons and identify isotopes.
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The core idea
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Learning objectives
- state the relative charges and approximate relative masses of a proton, a neutron and an electron
- describe, with the aid of diagrams, the structure of an atom as consisting of protons and neutrons (nucleons) in the nucleus and electrons arranged in shells (energy levels) (knowledge of s, p, d and f classification is not required; a copy of the Periodic Table will be available in the examination)
- define proton (atomic) number and nucleon (mass) number
- interpret and use nuclide notations such as ¹²₆C
- define the term isotopes
Your goal is to interpret atomic structure, nuclide notation and isotopes, including calculating neutrons with A-Z.
1. Retrieve what you know
What property stays fixed when an atom undergoes a physical change of state? Recall that element identity is not set by particle spacing or motion.
Check the retrieval
The atom’s proton number stays fixed. A physical state change does not alter its nucleus.
2. Atomic and nuclide models
Swipe or scroll sideways to inspect the complete overview.
Use this as a model, not a scale drawing: the nucleus is extremely small compared with the atom. Protons and neutrons are in the nucleus; electrons occupy shells around it. A proton has charge 1 + and relative mass 1, a neutron has charge 0 and relative mass 1, and an electron has charge 1- and a much smaller relative mass.
For ^A_ZX, read Z first. It is the proton number and fixes the element. A is the nucleon number, the total number of protons and neutrons. Therefore:
neutrons = A-Z
Isotopes are atoms of the same element with the same proton number but different neutron numbers.
3. Modelled example: decode a nuclide
Modelled example 1
Read the nucleus in a safe order
Problem
Find the proton and neutron counts in ²³₁₁Na.
Study the worked solution
Read Z
Method
Use the lower number as proton number.Reason
Proton number identifies the element.Working
Protons = 11.Calculate A − Z
Method
Subtract proton number from nucleon number.Reason
Nucleon number counts protons plus neutrons.Working
Neutrons = 23-11 = 12.
4. Guided attempt: identify isotopes
Guided practice 2
Decide whether two nuclides are isotopes
Problem
Compare ³⁵₁₇Cl and ³⁷₁₇Cl.
Try this before viewing the solution
Hints
Hint 1: identity first
Compare proton numbers before any other particle count.
Hint 2: neutron difference
Use A-Z for each nuclide, then compare the neutron counts.
View solution step by step
Fix identity
Method
Compare proton numbers first.Reason
Both have Z = 17, so both are chlorine.Working
Same element.Compare neutrons
Method
Calculate 18 and 20 neutrons.Reason
Same protons but different neutrons is the definition of isotopes.
Working
They are isotopes of chlorine.
5. Misconception contrast
Nucleon number is not neutron number: it counts protons and neutrons. Isotopes do not become different elements because their neutron counts differ; only a change in proton number would change element identity.
6. Changed-context transfer
An unfamiliar neutral nuclide has nucleon number 40 and proton number 18. Determine its proton, neutron and electron counts, then state what another isotope of that element must share.
Show transfer feedback
It has 18 protons, 40-18 = 22 neutrons and 18 electrons because it is neutral. Another isotope must also have 18 protons but would have a different neutron count.
7. Independent check
Decode ³¹₁₅P and compare it with ³²₁₅P. State the decisive evidence for their element identity and isotope relationship.