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1.2 The nuclear atom

Syllabus
First assessment 2025
Topic
1.2
Level
SL

Reading Atomic Structure

An atom has a dense, positively charged nucleus containing protons and neutrons. Negatively charged electrons occupy the space outside the nucleus. Protons and neutrons are nucleons.

Quantity Meaning Rule
Atomic number, Z Number of protons p = Z
Mass number, A Protons plus neutrons n = A − Z
Ion charge Proton charge compared with electron charge charge = p − e

For a neutral atom, electrons equal protons. For an ion, use the stated charge to determine the electron count.

Read Z first to obtain protons, subtract Z from A to obtain neutrons, then use the ion charge to check or calculate electrons.

Apply the symbols in a fixed order. For ³⁵₁₇Cl⁻, Z = 17 gives 17 protons, A − Z gives 18 neutrons, and the 1− charge means one more electron than protons, so there are 18 electrons. The ion charge changes electron count, never the element identity.

Particle Location Relative charge Approximate relative mass
Proton nucleus +1 1
Neutron nucleus 0 1
Electron outside nucleus −1 about 1/1836

Most atomic mass is concentrated in the nucleus. Atomic number identifies the element through proton count; ion formation changes electron count, not proton count.

Atomic and Ionic Particle Counts

Assessment in practice

1–2 marks in the selected structured examples marks
How it is assessed

Questions ask learners to deduce proton, neutron, and electron counts from nuclear notation and ion charge, or construct a nuclear symbol from particle information.

Command terms

deduce / determine

What earns marks

Use atomic number for protons, mass number minus atomic number for neutrons, and the charge relationship to determine electrons; include all requested particle counts or the complete nuclear symbol.

Watch for

Using mass number as the neutron count, or treating a positive ion as having gained rather than lost electrons.

Representative question

Question 1

[Maximum number: 2]

Calculate the number of protons, neutrons and electrons in the 26Mg+{ }^{26} \mathrm{Mg}^{+}ion.

Protons:
Neutrons:
Electrons:

Isotopes and Relative Atomic Mass

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same electron arrangement and therefore the same chemical properties, while their different masses can give different physical properties.

Ar=∑(isotopemass×fractionalabundance)A_r = ∑(isotope mass × fractional abundance)

Convert percentage abundances to fractions, multiply each isotope mass by its fractional abundance, then add the contributions. The result is a weighted mean, not an unweighted average.

For a two-isotope sample containing 75% mass 35 and 25% mass 37, the weighted mean is (35 × 0.75) + (37 × 0.25) = 35.5. An answer between the isotope masses is a useful check; simply averaging 35 and 37 would ignore abundance.

Read isotope identity from the pair of nuclear symbols: ³⁵₁₇Cl and ³⁷₁₇Cl are both chlorine because Z = 17 in each, but they contain 18 and 20 neutrons respectively because A differs. Mass number belongs to one nuclide; relative atomic mass is the abundance-weighted mean for a sample.

Applying Isotope Abundance

Assessment in practice

2 marks in each selected structured example marks
How it is assessed

Questions ask learners to explain why isotopes share chemical properties but can differ in physical properties, or calculate relative atomic mass from isotope masses and abundances.

Command terms

explain / calculate

What earns marks

State the same electron arrangement for the chemical-property explanation, name a valid physical-property difference other than mass when required, and show the abundance-weighted sum rather than substituting a data-booklet value.

Watch for

Calling isotopes different elements, or replacing the abundance-weighted calculation with an unweighted mean or a memorized data-booklet value.

Representative question

Question 1

[Maximum number: 2]

Calculate the relative atomic mass of bromine from the sample, giving your answer to two decimal places.

The Nuclear Atom Summary

Retrieve the sequence: use nuclear notation to count particles, distinguish isotopes by neutron number, then read mass-spectrum positions and heights as isotope mass and abundance evidence.

When checking an answer, ask: Did I separate A, Z, and charge? Did I explain isotope properties through electron arrangement? Did I weight each isotope by its abundance?

Objective notes

2 learning objectives
1.2.1Atomic structure• Nucleus: protons + neutrons (nucleons), positively charged, dense• Electrons: negatively charged, occupy space outside nucleus• Use nuclear symbols to deduce protons, neutrons, and electrons in atoms and ionsView1.2.2Isotopes• Same element, different numbers of neutrons• Nuclear symbol: Z^A X (Z = protons, A = mass number)• Calculate relative atomic masses from isotopic abundance• Understand isotope physical property differences; specific isotope examples are not requiredView