Atomic structure
Lajoy Tucker
Teacher
Contents
Definition
The atomic model has evolved over time as scientific discoveries have improved our understanding of atomic structure. Modern atomic theory describes atoms as consisting of a dense, positively charged nucleus surrounded by negatively charged electrons in specific energy levels.
Subatomic Particles
Protons determine the atomic number and the identity of the element.
Neutrons contribute to the mass of the atom but do not affect chemical properties.
Electrons are responsible for chemical bonding and reactivity.
Particle | Relative Charge | Relative Mass | Location |
Proton | +1 | 1 | Nucleus |
Neutron | 0 | 1 | Nucleus |
Electron | -1 | 1/1840 | Orbitals around the nucleus |
Atomic Number and Mass Number
Atomic Number (Z) = Number of protons in the nucleus.
Mass Number (A) = Total number of protons and neutrons in the nucleus.
Number of Neutrons = Mass Number – Atomic Number.
Example 1
Number of protons + |
| 3 |
Number of electrons - (in a neutralatom) |
| 3 |
Number of neutrons |
| 4 |
Example 2
Determine the number of protons, neutrons, and electrons in
Number of protons = Atomic number = 11
Number of neutrons = Mass number - Atomic number = 23 - 11 = 12
Number of electrons = Number of protons (in a neutral) atom = 11
Understanding the size of an Atom
Atoms are extremely small - their size is around 0.1 nonemetres (nm) in diameter.
1 nanometer =
Therefore, the diameter of an atom ~
Comparing Sizes
Object | Approximate size (metres) |
Football | 0.2 m |
Human hair | 1 × 10⁻⁴ m |
Red blood cell | 7 × 10⁻⁶ m |
Bacterium | 1 × 10⁻⁶ m |
Virus | 1 × 10⁻⁸ m |
Atom | 1 × 10⁻¹⁰ m |
Nucleus of the atom | 1 × 10⁻¹⁴ m |
The nucleus is about 10,000 times smaller than the atom.
Scale models of atoms
To imagine something so small, scientists use scale models.
If an atom were enlarged so you could see it:
A hydrogen atom (0.1 nm) scaled up by 10¹⁰ times would be 1 metre across.
The nucleus would then be about the size of a small marble inside a football stadium.
Practice Calculation Questions
Question 1
An atom has a diameter of 1 × 10⁻¹⁰ m.
If a model of the atom is made 10 cm wide, what is the scale factor of the model?
Solution:
Convert both to the same units:
10 cm = 10/100 = 0.1 m
Scale factor = model size ÷ actual size
= 0.1m ÷ (1 × 10⁻¹⁰ m)
= 1 × 10⁹
Question 2
A nucleus is 1 × 10⁻¹⁴ m across.
Using the same scale factor calculated in a) how large would the nucleus appear in the model?
Solution:
Model size = actual size × scale factor
= (1 × 10⁻¹⁴) × (1 × 10⁹)
= 1 × 10⁻⁵ m
= 0.01 mm
The nucleus would be 0.01 mm across – smaller than a speck of dust
Exam Tips and Reminders
Atoms are tiny, roughly 0.1 nm (1x10-10 m) wide.
The nucleus is around 1/10,000 the size of the atom.
Scale models help visualise atoms.
Always check units and use scientific notation when calculating.
No answer provided.