Atomic structure and isotopesEdexcel A-Level Chemistry: Revision notes
Section 1
Inside the atom
An atom has a tiny, dense nucleus containing protons and neutrons, surrounded by electrons in shells. The three particles are compared using relative mass and relative charge:
- Proton: relative mass 1, relative charge +1
- Neutron: relative mass 1, relative charge 0
- Electron: relative mass about 1/1840 (1/1836 is also accepted), relative charge −1
The mass of an atom is therefore almost entirely in its nucleus, and electron mass is ignored when finding the mass of an atom or ion. An atom is electrically neutral because the number of protons equals the number of electrons.
Section 2
Atomic number and mass number
The atomic (proton) number, Z, is the number of protons in the nucleus of an atom. It defines the element and its position in the periodic table. The mass number, A, is the total number of protons plus neutrons in the nucleus.
An atom is written with its mass number as a superscript and its atomic number as a subscript before the symbol, for example ²³₁₁Na (mass number 23, atomic number 11). Number of neutrons = A − Z.
Mass number is a whole number for one particular atom. It is not the same as relative atomic mass, which is an average.
Section 3
Counting particles in atoms, ions and molecules
Atom: protons = Z, electrons = Z, neutrons = A − Z.
Positive ion (cation): electrons = Z − charge. Negative ion (anion): electrons = Z + charge. The number of protons and neutrons never changes when an ion forms.
Worked example: ³⁴S²⁻ (Z = 16). Protons = 16; neutrons = 34 − 16 = 18; electrons = 16 + 2 = 18.
Molecules: add up the particles of every atom. For a ¹H₂¹⁶O molecule (¹H has Z = 1, ¹⁶O has Z = 8): protons = 1 + 1 + 8 = 10; neutrons = 0 + 0 + 8 = 8; electrons = 10.
Changing the number of protons when working out an ion. Only the number of electrons changes.
Section 4
Isotopes
Isotopes are atoms of the same element with the same number of protons (the same atomic number) but different numbers of neutrons, so they have different mass numbers. For example, ³⁵Cl has 18 neutrons and ³⁷Cl has 20 neutrons, but both have 17 protons and 17 electrons.
Isotopes of an element have the same chemical properties, because chemical reactions depend on the number and arrangement of electrons. They have slightly different physical properties that depend on mass, such as density and rate of diffusion.
Saying isotopes have a different number of protons or electrons. They differ only in neutrons.
Must Know
- Proton: mass 1, charge +1; neutron: mass 1, charge 0; electron: mass about 1/1840, charge −1
- Atomic number = protons; mass number = protons + neutrons
- Neutrons = A − Z; electrons = Z for an atom, adjusted for the charge for an ion
- Isotopes: same protons, different neutrons, same chemical properties
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Atomic structure and isotopes
- Sulfur has several naturally occurring isotopes, including ³²S and ³⁴S (atomic number 16). A technician is studying a sample of sodium sulfide, which contains the sulfide ion ³⁴S²⁻.Explain why ³²S and ³⁴S have the same chemical properties but different masses.2 marks
- A physics teacher describes the sodium ion ²³Na⁺ (atomic number 11) to a class using the relative mass and relative charge of the three sub-atomic particles.Deduce the total relative charge of the ²³Na⁺ ion and its approximate relative mass. Explain why the electrons can be ignored when finding the relative mass.2 marks
- Chlorine exists naturally as two isotopes, ³⁵Cl and ³⁷Cl (atomic number 17). A student studies the chloride ion ³⁷Cl⁻ and a chlorine molecule in which one atom is ³⁵Cl and the other is ³⁷Cl.State the number of protons, neutrons and electrons in one ³⁷Cl⁻ ion.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).