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1.3Principles of Chemistry

Atomic Structure — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 7 parts Lesson 11 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Atoms, Subatomic Particles, Isotopes and Relative Atomic Mass — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Define atom and molecule, describe the subatomic particles, calculate protons, neutrons and electrons, and calculate relative atomic mass from isotope abundance.An atom is the s…

  • Atomic number
  • Mass number
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Section 1 of 7Atoms and molecules

An atom is the smallest particle of an element, with electrons surrounding a nucleus that contains protons and neutrons.

Molecule
A molecule is a group of two or more atoms chemically combined to form an identifiable unit which keeps the properties and composition of the substance, for example H2O or O2.
All substances are made of atoms, which are the building blocks of matter.

Atoms of different elements differ in the number of protons, which is why each element has a unique atomic number. Ions form when atoms gain or lose electrons, and the number of protons never changes in a chemical reaction.

Section 2 of 7Subatomic particles

Protons and neutrons are in the nucleus (positively charged); electrons move very fast around it in shells. The mass of an atom is almost all in the nucleus, because electrons are about 2000 times lighter. Relative masses and charges are used because real values are too small to be practical:

ParticleRelative massCharge
Proton1+1
Neutron10
Electron1/1840-1

In an exam, the mass of an electron can be given as negligible or very small; you do not need to learn the value.

The nucleus has a positive charge because it contains protons, so the electrons are attracted to it.

Section 3 of 7Atomic number, mass number and isotopes

Atomic number = number of protons = number of electrons in a neutral atom. Mass number = protons + neutrons. Neutrons = mass number - atomic number. Think "mass = massive": the mass number is the larger number.

Atomic number, mass number and isotopes
Lithium (atomic number 3, mass number 7) has 3 protons, 3 electrons and 4 neutrons.
Element X with atomic number 29 and mass number 63 has 29 protons, 29 electrons and 34 neutrons.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same symbol but different mass numbers, such as 63Cu and 65Cu.

Section 4 of 7Relative atomic mass

The relative atomic mass Ar is the average mass of one atom of an element taking account of the abundance of all its isotopes, relative to 1/12 of the mass of a carbon-12 atom. It is calculated as:

Ar = (% of isotope A x mass of A + % of isotope B x mass of B) / 100

For three isotopes the numerator has a third term. Worked example: rubidium is 72% 85Rb and 28% 87Rb. Ar = (72 x 85 + 28 x 87) / 100 = (6120 + 2436) / 100 = 85.56, so 85.6 to 1 d.p. The Ar is not a whole number because it is an average over isotopes.

Common calculation tippercentages must add up to 100, so for two isotopes only one percentage may be given.

Relative atomic mass
If an element is 90% of one isotope, the other must be 10%.
The average must lie between the two mass numbers and closer to the more abundant isotope; if it does not, you have made an error.
For example neon is mainly neon-20 with some neon-22, so its relative atomic mass is just above 20 (20.2).

Section 5 of 7Worked examples

  1. An atom has atomic number 29 and mass number 63. How many neutrons? [1]1 mark

  2. Define isotopes. [2]2 marks

  3. Calculate Ar of an element with isotopes 10 (20%) and 11 (80%). [2]2 marks

  4. Why is the mass of an atom concentrated in the nucleus? [1]1 mark

Section 6 of 7Key terms

TermMeaning
AtomSmallest particle of an element.
MoleculeTwo or more atoms chemically combined.
Atomic numberNumber of protons in an atom.
Mass numberProtons plus neutrons.
IsotopeAtoms of an element with different numbers of neutrons.
A(r)Average mass of the isotopes compared with 1/12 of carbon-12.

Section 7 of 7Exam technique

1
Writing electrons as the heavy particles.
2
Mixing up atomic and mass numbers.
3
Saying isotopes have different numbers of protons.
4
Rounding the average Ar to a whole number too early.

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