Atomic Structure
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Structure of the Atom
- All matter is composed of atoms - the smallest particles of an element that can take part in chemical reactions
- Atoms are made up of even smaller particles called sub-atomic particles
- Atom: the smallest particle of an element that can exist; all substances are made of atoms
Components of an Atom
The structure of an atom, showing the nucleus and surrounding electron shells
- Atoms consist of three main components:
- the nucleus is the small, dense centre of the atom containing protons and neutrons
- protons are positively charged particles located in the nucleus
- neutrons are neutral particles (no charge) located in the nucleus
- electrons are negatively charged particles that orbit the nucleus in shells (energy levels)
Sub-atomic Particles
| Particle | Location | Electrical Charge | Relative Mass |
|---|---|---|---|
| Proton | Nucleus | Positive (+1) | 1 |
| Neutron | Nucleus | Neutral (0) | 1 |
| Electron | Shells (orbiting) | Negative (−1) | Very small (1/1840) |
- Protons and neutrons have approximately the same mass
- Electrons have negligible mass compared to protons and neutrons
- The nucleus is very small but contains almost all the atom's mass
- Most of an atom's volume is empty space
How the Model of the Atom Has Changed
- Ideas about the structure of the atom are scientific models - simplified representations that scientists use to explain what they observe
- Scientific models change as new evidence emerges: a model that fits all currently known evidence may later be revised or replaced once new evidence is discovered from further experiments
The Plum Pudding Model (1904)
- An early model of the atom, proposed before the nucleus had been discovered
- Describes the atom as a sphere of positive charge with negatively charged electrons distributed (embedded) throughout it
- Does not include a nucleus - the positive charge is spread throughout the whole atom
The Nuclear Model (1914)
- Developed after experimental evidence showed that most of an atom's mass and positive charge is concentrated in a tiny central nucleus
- Describes the atom as a small, dense, positively charged nucleus with electrons orbiting around it
- This is essentially the modern model of the atom still used today
Comparing the Two Models
| Plum Pudding Model (1904) | Nuclear Model (1914) | |
|---|---|---|
| Nucleus | No nucleus - positive charge spread throughout | Small, dense nucleus at the centre |
| Electrons | Embedded/spread throughout the positive sphere | Orbit around the nucleus |
- Similarities: both models contain electrons; both show a positive charge; both are drawn as spherical/circular; both can represent the same number of electrons for a given atom
- Differences: the nuclear model has a nucleus (the plum pudding model does not); electrons are arranged in shells/orbits in the nuclear model but spread throughout in the plum pudding model
Strengths and Limitations of the Modern (Nuclear) Model
- Strength: it is simple to understand and visualise, and shows the key features of an atom (a nucleus and electrons)
- Limitation: electrons do not actually orbit in fixed circular paths - this is an oversimplification of how electrons really behave
Exam Tip
- If asked why a model of the atom "may change in the future", the answer is that new evidence may be discovered, or new experiments might show different results - scientific understanding develops as evidence changes
Atomic Number and Mass Number
- Atomic number: the number of protons in an atom - this defines what element it is
- Mass number: the total number of protons and neutrons in an atom
- Mass Number = Number of Protons + Number of Neutrons
Nuclide notation: mass number above, atomic number below, to the left of the element symbol
Neutral Atoms
- In a neutral atom (an atom with no overall electrical charge):
- number of protons = number of electrons
- the positive charges (protons) balance the negative charges (electrons)
- example: a neutral carbon atom has 6 protons and 6 electrons
Calculating Number of Neutrons
- Number of Neutrons = Mass Number − Atomic Number
Worked Example: Neutrons in Carbon-12
Muumbe is asked to find the number of neutrons in Carbon-12, which has an atomic number of 6 and a mass number of 12.
- Step 1: Number of Neutrons = Mass Number − Atomic Number
- Step 2: Number of Neutrons = 12 − 6 = 6 neutrons
Electron Shells
- Electrons orbit the nucleus in shells (energy levels); each shell can hold a maximum number of electrons:
- first shell (closest to nucleus): maximum 2 electrons
- second shell: maximum 8 electrons
- third shell: maximum 8 electrons (at this level)
Electron shell arrangement showing maximum capacities for the first three shells
Electron Configuration
- Electrons fill the shells starting from the innermost shell (closest to the nucleus) outward
- Shells must be filled in order - the first shell must be completely filled (2 electrons) before electrons occupy the second shell
Worked Example: Electron Configuration of Oxygen
Museli works out the electron configuration of an oxygen atom, which has 8 electrons.
- Step 1: fill the first shell (maximum 2 electrons) → 2 electrons
- Step 2: remaining electrons = 8 − 2 = 6 electrons
- Step 3: fill the second shell with the remaining 6 electrons
- Electron configuration = 2, 6 (first shell full with 2, second shell has 6)
Worked Example: Electron Configuration of Potassium (Beyond 18 Electrons)
Bwalya works out the electron configuration of a potassium atom, which has 19 electrons.
- Step 1: fill the first shell (maximum 2 electrons) → 2 electrons used, 17 remaining
- Step 2: fill the second shell (maximum 8 electrons) → 8 electrons used, 9 remaining
- Step 3: fill the third shell up to its maximum of 8 electrons → 8 electrons used, 1 remaining
- Step 4: once the third shell reaches 8 electrons, any further electrons start a new, fourth shell → the remaining 1 electron goes into the fourth shell
- Electron configuration of potassium = 2.8.8.1
- This shows that once a shell reaches its maximum of 8 electrons (at this level), the next electron starts a new shell further out, rather than continuing to add to the existing shell
Isotopes
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