IB Chemistry HLTopic 1 — The Nuclear AtomPaper 1 & 2Core skill~12 min read
Protons, Neutrons and Electrons
Two numbers next to a chemical symbol tell you everything about what is inside an atom. Reading them takes a minute to learn and then works forever — provided you remember the one thing a chemical change can never do.
📚 What you need to know
Atomic number (Z) is the number of protons. It defines the element.
Mass number (A) is the total number of protons and neutrons, the nucleons.
Number of neutrons = A − Z.
In a neutral atom, electrons = protons = Z.
A positive ion has lost electrons; a negative ion has gained them.
Charge = protons − electrons, so electrons = Z − charge.
The number of protons never changes in any chemical reaction.
Reading the symbol
Everything is packed around the element symbol: the mass number above the atomic number on the left, and the charge, if there is one, on the upper right.
The atomic number is often left out, since the symbol already tells you the element. The mass number is the one you cannot deduce, which is why it is always written.
The three you need
protons = Z
neutrons = A − Z
electrons = Z − charge
That third one catches people because of the minus sign. For Mg2+ the charge is +2, so electrons = 12 − 2 = 10. For Cl− the charge is −1, so electrons = 17 − (−1) = 18. Subtracting a negative adds. If you would rather not deal with that, use the plain-English version: positive means fewer electrons, negative means more.
Ions: what changes and what does not
An ion forms when an atom gains or loses electrons. The nucleus is untouched, because chemistry only ever moves the outer electrons around — nothing in a beaker has the energy to disturb a nucleus.
The trap in exams is a question about an ion where a student changes the atomic number to “make” the charge. The charge comes from the electrons, always.
Isoelectronic species
Different species can end up with exactly the same number of electrons. When they do they are called isoelectronic, and the idea comes back repeatedly — in bonding, in shapes, and in the trends across a period.
Isoelectronic species have identical electron arrangements but different nuclear charges. That single difference explains most of what happens to their size.
🧩 Counting particles from any symbol
Find Z from the periodic table or from the bottom-left number. That is your proton count.
Find A, the top-left number. Subtract to get neutrons: A − Z.
Check for a charge on the upper right. No charge means electrons = protons.
Adjust for the charge: positive means remove that many electrons, negative means add them.
Sanity check: does protons − electrons give you back the charge you started with?
WORKED EXAMPLE
Determine the number of protons, neutrons and electrons in an 56Fe3+ ion.
ProtonsIron has an atomic number of 26, and forming an ion does not change it.26 protonsNeutronsA − Z = 56 − 26 = 3030 neutronsElectronsThe 3+ charge means three electrons have been lost from the neutral atom’s 26.26 − 3 = 2323 electronsCheckprotons − electrons = 26 − 23 = +3, which matches the charge given.
WORKED EXAMPLE
A species contains 16 protons, 18 neutrons and 18 electrons. Identify the element and write its full symbol including mass number and charge.
Identify the element from the protonsZ = 16, and the element with 16 protons is sulfur. Nothing else can be, since Z defines the element.sulfurFind the mass numberA = protons + neutrons = 16 + 18 = 34Find the chargecharge = protons − electrons = 16 − 18 = −2More electrons than protons, so the species is negative.34S2−, the sulfide ionThe electron count is what you must not use to identify the element. Eighteen electrons might tempt you towards argon, and that would be wrong.
WORKED EXAMPLE
Which of the following are isoelectronic? N3−, O2−, Na+, Mg2+, Al3+, Cl−
Count electrons for eachN3−: 7 + 3 = 10O2−: 8 + 2 = 10Na+: 11 − 1 = 10Mg2+: 12 − 2 = 10Al3+: 13 − 3 = 10Cl−: 17 + 1 = 18Group themthe first five are isoelectronic; Cl− is notWhat they have in commonAll five have the same electron arrangement, matching neon. Chloride has 18 and matches argon instead, so it belongs to a different isoelectronic set.Notice that isoelectronic species are not identical. They have very different nuclear charges, so they pull on those ten electrons with different strengths.
💡 Exam tip
Write down Z first, every time. Everything else follows from it.
Use neutrons = A − Z rather than trying to recall neutron counts.
Identify elements from the proton count only, never from electrons.
State explicitly that the proton number is unchanged when discussing ions.
Check your answer with protons − electrons = charge.
Know the term isoelectronic and be ready to spot a set.
⚠️ Common mix-up
Changing the atomic number to create an ion. The charge comes from electrons.
Adding electrons for a positive ion and removing them for a negative one.
Using the relative atomic mass from the periodic table as A — it is an average and rarely a whole number.
Naming an element from its electron count, so a sulfide ion becomes “argon”.
Forgetting that neutrons carry no charge, and letting them affect the ion’s charge.
Assuming isoelectronic means identical. The nuclei are completely different.
Up next: Isotopes — you have been treating A as fixed for an element. It is not. The next page is about atoms of the same element that carry different numbers of neutrons, and what that does to the periodic table’s masses.
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