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Atomic structure and the Periodic Table

Paper 1Paper 2Paper 3Paper 4

This section is examined in Paper 1, Paper 2, Paper 3 and Paper 4.

The Structure of the Atom
An atom consists of a central nucleus surrounded by electrons moving in shells. The nucleus contains two types of particles: protons and neutrons. This structure is fundamental because the number of protons determines the identity of the element, while the arrangement of electrons determines its chemical properties.
To understand the atom, we must define the properties of its constituent particles. Note that relative mass and charge are compared to a standard (proton = 1, proton = +1).
NucleusProton1+1
NucleusNeutron10
Shells (orbiting nucleus)Electron1/1840-1
Proton Number and Mass Number
Proton number (atomic number): The number of protons in the nucleus of an atom. This defines the element's identity.
Mass number (nucleon number): The total number of protons and neutrons in the nucleus of an atom.
Calculating Particle Numbers
Example: An atom has 11 protons and 12 neutrons. What is its proton number, mass number, and electron count (assuming it is neutral)?
  1. Proton number = 11 (by definition).
  2. Mass number = Protons + Neutrons = 11 + 12 = 23.
  3. Electron count: In a neutral atom, the number of electrons equals the number of protons to balance charge. Therefore, electrons = 11.
⚠︎ Confusing Mass Number with Proton Number
Error: Students often state that the mass number is the same as the proton number, or they forget to add neutrons.

Correct Understanding: The mass number includes both protons and neutrons. For example, Carbon-12 has 6 protons and 6 neutrons; its mass number is 12, not 6.

Describing Atomic Structure Accurately
When to use: When asked to 'describe the structure of an atom' in Paper 3 or 4.

Why examiners accept this: Examiners look for specific keywords: nucleus, protons, neutrons, electrons, and shells/orbiting. Vague terms like 'parts' or 'center' are insufficient.

Correct Usage: 'The atom has a central nucleus containing protons and neutrons, surrounded by electrons in shells.'

Identifying Particles from Data
Q:
An atom has 15 protons, 16 neutrons, and 15 electrons. What is its proton number?
A:
15
Q:
What is the mass number of this atom?
A:
31
Electronic Configuration
Electrons occupy shells at specific energy levels. Lower shells are filled first because they have lower energy. The maximum number of electrons in the first few shells is:

  • 1st shell: 2 electrons
  • 2nd shell: 8 electrons
  • 3rd shell: 8 electrons (for elements with proton number up to 20)

Why this matters: The arrangement of electrons determines how an element reacts chemically.

Determining Configuration for Ions: To find the configuration of an ion, follow these steps:

  1. Write the configuration of the neutral atom.
  2. Identify the charge (e.g., 2+ means lose 2 electrons; 2- means gain 2 electrons).
  3. Add or remove electrons from the outermost shell first.
Noble Gases and Group/Period Rules
Group VIII (Noble Gases): These elements have a full outer electron shell. This makes them stable and unreactive. Helium has 2 electrons in its outer shell (full first shell); others have 8.
Group Number Rule: For Groups I to VII, the number of outer shell electrons is equal to the group number.

Period Number Rule: The number of occupied electron shells is equal to the period number.

Determining Ion Configuration

Example 1: Magnesium (Mg) has proton number 12.

  • Neutral Mg configuration: 2, 8, 2.
  • Mg ion is Mg^{2+} (loses 2 electrons).
  • Remove 2 from outer shell: 2, 8.

Example 2: Oxygen (O) has proton number 8.

  • Neutral O configuration: 2, 6.
  • Oxide ion is O^{2-} (gains 2 electrons).
  • Add 2 to outer shell: 2, 8.
⚠︎ Confusing Group Number with Shell Number

Error: Students often confuse the group number (outer electrons) with the period number (occupied shells).

Correct Understanding:

  • Group I-VII tells you how many electrons are in the outer shell.
  • Period I-VIII tells you how many shells are occupied.
Explaining Noble Gas Stability
When to use: When asked why noble gases are unreactive.

Why examiners accept this: You must link the full outer shell to stability. Simply saying 'they have 8 electrons' is incomplete because Helium only has 2.

Correct Usage: 'Noble gases have a full outer electron shell, which makes them stable and unreactive.'

Applying Periodic Table Rules
Q:
State the connection between the number of occupied electron shells in an atom and its period number.
A:
They are the same.
Q:
An element has 4 occupied electron shells. Which period is it in?
A:
Period 4
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