Difficulty: Introductory | Prerequisites: none, this is foundational material
Every atom is built from protons, neutrons, and electrons. The number of protons defines the element; changing the neutron count gives you an isotope; gaining or losing electrons gives you an ion. Most of what this exam tests comes down to counting those three particles and knowing what happens when the count changes.
Proton
A positively charged subatomic particle found in the nucleus. The number of protons equals the atomic number and defines which element you are looking at.
In simple terms: protons are the identity card of an element. Change the proton count and you have a different element entirely.
Neutron
An electrically neutral subatomic particle found in the nucleus alongside protons. Neutrons contribute to mass but not to charge.
In simple terms: neutrons add weight without changing what element it is.
Electron
A negatively charged subatomic particle that orbits the nucleus in energy levels (shells). Electrons have negligible mass compared with protons and neutrons.
In simple terms: electrons are the tiny, light particles on the outside that determine how an atom bonds and reacts.
Atomic number (Z)
The number of protons in an atom's nucleus. Every atom of a given element has the same atomic number.
Think of it as the element's permanent ID number on the periodic table.
Mass number (A)
The total count of protons plus neutrons in the nucleus. Mass number = protons + neutrons.
In simple terms: add up everything heavy in the nucleus and that is the mass number.
Ion
An atom (or group of atoms) that carries a net electrical charge because it has gained or lost electrons.
Think of it as an atom that is no longer electrically balanced.
Cation
A positively charged ion, formed when an atom loses one or more electrons. Metals typically form cations.
In simple terms: fewer electrons than protons means a positive charge.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. Non-metals typically form anions.
In simple terms: more electrons than protons means a negative charge.
Isotope
Atoms of the same element (same number of protons) that differ in the number of neutrons. Isotopes have different mass numbers but identical chemical behaviour.
Think of it as the same element wearing a heavier or lighter coat.
Electron configuration
The distribution of an element's electrons across its energy levels and subshells (1s, 2s, 2p, 3s, etc.). Written in order of filling: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰, and so on.
In simple terms: it is the address system that tells you where every electron lives.
Valence electrons
The electrons in the outermost energy level (highest principal quantum number) of an atom. These electrons participate in bonding and chemical reactions.
In simple terms: the electrons on the outside that do all the interesting chemistry. For main-group elements, the group number (in the A system) tells you how many there are.
Every atom has a nucleus containing protons (+) and neutrons (neutral), surrounded by electrons (−)
Protons determine the element (atomic number, Z)
Mass number (A) = protons + neutrons. Electrons are too light to count toward mass number
A neutral atom has equal numbers of protons and electrons
Metals lose electrons to form cations (positive ions). Example: K loses 1 electron to become K⁺
Non-metals gain electrons to form anions (negative ions). Example: Cl gains 1 electron to become Cl⁻ (17 protons, 18 electrons)
The number of protons never changes when an ion forms; only the electron count changes
To find the charge: charge = protons − electrons. If electrons > protons, the ion is negative. If protons > electrons, the ion is positive
When given a table of protons and electrons, look for entries where the electron count exceeds the proton count. Those are anions (negatively charged). If electrons = protons, the species is neutral.
The superscript to the left of the element symbol is the mass number (A)
Example: ³⁴S has 16 protons (sulfur's atomic number), so neutrons = 34 − 16 = 18. A neutral sulfur atom also has 16 electrons
Different isotopes of the same element have different neutron counts but the same proton count
Fill orbitals in order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, and so on
Maximum electrons per subshell: s = 2, p = 6, d = 10, f = 14
Abbreviated (noble gas core) notation: [Ne]3s²3p⁴ means neon's 10 electrons plus 6 more in the 3s and 3p subshells, giving 16 total (sulfur)
When forming a positive ion, electrons are removed from the highest principal energy level first, not necessarily the last subshell filled. Calcium (1s²2s²2p⁶3s²3p⁶4s²) loses its two 4s electrons to become Ca²⁺ with configuration 1s²2s²2p⁶3s²3p⁶
For main-group (A-group) elements, the number of valence electrons equals the group number in the American (IA through VIIIA) system
Group VIA elements have 6 valence electrons
[Ne]3s²3p⁴ has 6 valence electrons (the 3s and 3p electrons together)
[Ne]3s²3p³ is phosphorus (P), with 5 valence electrons
Students often confuse mass number with atomic mass. Mass number is a whole-number count of protons + neutrons for a specific atom. Atomic mass on the periodic table is a weighted average of all isotopes and is usually not a whole number.
Students sometimes think forming an ion changes the number of protons. It does not. Only the electron count changes.
When calcium forms Ca²⁺, a common error is removing electrons from the 1s orbital (the first one filled). Electrons are actually removed from the outermost shell first (4s, not 1s).
Students confuse "valence electrons" with "total electrons." [Ne]3s²3p⁴ has 16 total electrons but only 6 valence electrons (those in the n = 3 shell).
Mass number (A) = number of protons + number of neutrons
Atomic number (Z) = number of protons = number of electrons (in a neutral atom)
Number of neutrons = A − Z
Ion charge = number of protons − number of electrons
For a cation: electrons = protons − charge
For an anion: electrons = protons + |charge|
⚠️ Expect questions that hand you a table of proton, neutron, and electron counts and ask which species are ions (positive or negative). Just compare protons to electrons.
⚠️ Isotope notation questions are common: given ³⁴S, you need to extract protons, neutrons, and electrons quickly using the atomic number from the periodic table.
⚠️ Electron configuration of ions is heavily tested. Remember: for cations, remove electrons from the highest n first (Ca²⁺ loses the 4s electrons, not the 1s electrons).
⚠️ Valence electron counting from abbreviated configurations appears frequently. Count only the electrons after the noble gas core that sit in the highest principal energy level.
⚠️ Name/symbol pairs: the exam tests whether you know that Ca = calcium, Na = sodium, K = potassium, Cl = chlorine, Mg = magnesium. Watch out for traps like So, Po, C, or Mn paired with the wrong element name.
True or false: the mass number of an atom with 6 protons and 8 neutrons is 14. (Answer: true)
Fill in the blank: a chloride ion (Cl⁻) has 17 protons and ___ electrons. (Answer: 18)
True or false: when calcium forms Ca²⁺, it gains two electrons. (Answer: false, it loses two electrons)
Fill in the blank: the element with abbreviated configuration [Ne]3s²3p³ is ___. (Answer: phosphorus, P)
True or false: isotopes of an element have different numbers of protons. (Answer: false, they have different numbers of neutrons)
Q: What is the mass number of an atom with 6 protons, 8 neutrons, and 6 electrons?
A: 14. Mass number = protons + neutrons = 6 + 8 = 14. Electrons do not contribute to mass number.
Q: A chloride ion has how many protons and electrons?
A: 17 protons and 18 electrons. Chlorine has atomic number 17. The chloride ion (Cl⁻) gains one electron, giving it 18 electrons total.
Q: How many protons, neutrons, and electrons are in ³⁴S?
A: 16 protons, 18 neutrons, 16 electrons. Sulfur has atomic number 16, so protons = 16. Neutrons = 34 − 16 = 18. A neutral atom has electrons equal to protons.
Q: Calcium has the electron configuration 1s²2s²2p⁶3s²3p⁶4s². How does the configuration change when it forms Ca²⁺?
A: It loses two electrons from the 4s subshell to become 1s²2s²2p⁶3s²3p⁶. Electrons are removed from the highest principal energy level first.
Q: How many valence electrons are in an atom with abbreviated configuration [Ne]3s²3p⁴?
A: 6. The 3s and 3p electrons (2 + 4) are the valence electrons. The neon core electrons are not valence electrons.
Q: Which element has the abbreviated configuration [Ne]3s²3p³?
A: Phosphorus (P). Neon has 10 electrons; adding 2 + 3 = 5 more gives 15 total, which is phosphorus.
Q: Which name/symbol pair is correct: calcium/Ca, sodium/So, potassium/Po, chlorine/C, or magnesium/Mn?
A: Calcium/Ca. Sodium is Na (not So), potassium is K (not Po), chlorine is Cl (not C), and magnesium is Mg (not Mn).
This material connects directly to chemical nomenclature and ionic bonding: you need to know ion charges (from electron configurations) to write correct chemical formulas. It also underpins periodic trends (atomic radius, ionic radius, ionisation energy), since those trends depend on electron configuration and effective nuclear charge.
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