Atomic Structure and the Periodic Table — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 11Parts of an atom

• Atoms
  • all substances are made of atoms, the building blocks of matter.
• Nucleus
  • protons and neutrons are at the centre.
  • Electrons
    • they move very fast around the nucleus in shells.
  • Mass
    • electron mass is negligible, so nearly all the atom mass is in the nucleus.

Section 2 of 11Relative mass and charge

Particle	Relative mass	Charge
Proton	1	1+
Neutron	1	0
Electron	1/1840 (negligible)	1-

• Relative atomic mass unit
  • atoms are too small for grams, so masses are compared with 1/12 of a carbon-12 atom.
• No units
  • relative atomic mass is a ratio.
  • Example
    • hydrogen is 1, so 12 hydrogen atoms have the same mass as one carbon atom.

Section 3 of 11Proton number and nucleon number

• Proton (atomic) number Z
  • the number of protons in the nucleus.
• Neutral atom
  • it equals the number of electrons.
  • Identity
    • it fixes which element the atom is and its place in the Periodic Table.
• Nucleon (mass) number A
  • the total number of protons plus neutrons.
• Neutrons
  • neutrons = nucleon number – proton number.
  • Nucleons
    • protons and neutrons together are called nucleons.
• Atomic notation
  • written with the mass number at the top left and the atomic number at the bottom left of the symbol.
• Periodic Table use
  • the larger number is the mass number and the smaller is the proton number, though some tables reverse them.
  • Relative atomic mass
    • for exam purposes it can be used as the mass number, except for chlorine (35.5).

Section 4 of 11Deducing particle numbers

1. Protons = atomic number (the same for every atom and ion of that element).
2. Electrons = protons in a neutral atom; for an ion add or subtract the charge.
3. Neutrons = mass number – protons.

• Example
  • element X has atomic number 29 and mass number 63.
• Protons and electrons
  • 29 each.
  • Neutrons
    • 63 – 29 = 34.

Section 5 of 11Electron shells and configuration

• Shells
  • electrons occupy energy levels, and the further from the nucleus the higher the energy.
• Filling order
  • the shell nearest the nucleus fills first, then the next.
  • Capacity
    • the first shell holds 2, the second 8, and in this course the third holds 8 for the first 20 elements before the fourth begins.
  • Valence shell
    • the outermost shell, and a full one makes the atom stable.
• Electronic configuration
  • the number of electrons in each shell written with commas.
• Carbon
  • 6 electrons, so 2,4.
  • Sodium
    • 11 electrons, so 2,8,1.
  • Notation
    • full stops or plus signs instead of commas are accepted.

Section 6 of 11First twenty elements

Element	Z	Configuration	Element	Z	Configuration
hydrogen	1	1	sodium	11	2,8,1
helium	2	2	magnesium	12	2,8,2
lithium	3	2,1	aluminium	13	2,8,3
beryllium	4	2,2	silicon	14	2,8,4
boron	5	2,3	phosphorus	15	2,8,5
carbon	6	2,4	sulfur	16	2,8,6
nitrogen	7	2,5	chlorine	17	2,8,7
oxygen	8	2,6	argon	18	2,8,8
fluorine	9	2,7	potassium	19	2,8,8,1
neon	10	2,8	calcium	20	2,8,8,2

• Potassium and calcium
  • their extra electrons go into the fourth shell before the third fills beyond 8, for stability.

Section 7 of 11Electronic configuration of ions

• Positive ions
  • atoms lose electrons, and the charge size shows how many.
• Negative ions
  • atoms gain electrons, again as many as the charge.
• Method
  • write the atom configuration, then remove or add electrons in the outer shell.
  • Mg²⁺
    • magnesium 2,8,2 loses two electrons to become 2,8.
  • Cl⁻
    • chlorine 2,8,7 gains one electron to become 2,8,8.

Section 8 of 11Configuration and the Periodic Table

• Period
  • the number of occupied shells gives the period number.
• Chlorine
  • 2,8,7 has three numbers so is in Period 3.
• Group
  • the last number gives the outer electrons and so the group for Groups I to VII.
• Same group
  • elements in the same group have the same number of outer electrons.
  • Valency electrons
    • another name for outer shell electrons.
• Noble gases
  • Group VIII (0) atoms have full outer shells, so they are very stable and unreactive.
• Other atoms
  • they react by losing, gaining or sharing electrons to reach a full outer shell.

Section 9 of 11Worked examples

1. Calculate the numbers of protons, neutrons and electrons in a neutral atom of sodium-23 (atomic number 11). [3]3 marks

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2. Write the electronic configuration of the Al³⁺ ion. Aluminium has atomic number 13. [2]2 marks

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3. An element has configuration 2,8,6. State its group and period and name it. [3]3 marks

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4. Explain why the noble gases are unreactive. [2]2 marks

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Section 10 of 11Key terms

Term	Meaning
Proton number	Number of protons in the nucleus, also the electrons in a neutral atom.
Nucleon number	Total number of protons and neutrons in the nucleus.
Nucleon	A proton or a neutron in the nucleus.
Electronic configuration	The number of electrons in each shell, written in order from the nucleus.
Valence shell	The outermost occupied electron shell.
Noble gas	A Group VIII element with a full outer shell and very low reactivity.

Section 11 of 11Exam technique

1. Using mass number as the proton number — the smaller Periodic Table number is usually the proton number.
2. Removing electrons from the inner shell when forming a positive ion — they come from the outermost shell.
3. Writing 2,8,18 for calcium — for this course the third shell takes 8 and calcium is 2,8,8,2.
4. Giving the electron a mass of 1 — it is about 1/1840.