Atomic Structure and Elements
Explore atoms, mass numbers, electron arrangement, and classification of elements.
1.1.1 Structure of the Atom
All matter is made up of extremely tiny particles called atoms.
These atoms may exist:
- Alone (as in noble gases)
- Combined with other atoms to form molecules
- Combined chemically to form compounds
Atoms are so small that millions of them can fit on the tip of a pin.
What is an Atom?
An atom is the smallest particle of an element that can participate in a chemical reaction while still retaining the properties of that element.
For example:
- A gold atom still behaves like gold.
- An oxygen atom still behaves like oxygen.
If the atom is broken down further, it loses the properties of that element.
Characteristics of Atoms
Atoms have several important characteristics:
- Atoms are extremely small and cannot be seen with the naked eye.
- Atoms retain their chemical properties even after reaction.
- Atoms combine with other atoms to form molecules or compounds.
- Atoms usually cannot be broken down by chemical reactions but can be split in nuclear reactions which release large amounts of energy.
Parts of an Atom
An atom has two main regions:
- The nucleus
- Electron energy levels (shells)
The nucleus is located at the centre, while electrons move around it.
Figure: Structure of an Atom
The Nucleus
The nucleus is the central core of the atom. Although it is very small compared to the whole atom, it contains almost all the mass of the atom.
Characteristics of the Nucleus
- It is extremely small.
- It is very dense.
- It contains most of the mass of the atom.
- It is positively charged because of the presence of protons.
The nucleus holds the atom together.
Particles in the Nucleus
The nucleus contains two types of particles:
- Protons
- Neutrons
These particles are collectively called nucleons.
Electron Energy Levels (Shells)
Electrons move around the nucleus in regions called energy levels, electron shells, or orbitals.
These shells are arranged in layers around the nucleus.
Characteristics of Electron Shells
- Electrons move around the nucleus in energy levels.
- Each shell has a maximum number of electrons it can hold.
- Shells nearer the nucleus have lower energy.
- Shells further away from the nucleus have higher energy.
Electrons normally fill the lower energy levels first before moving to higher ones.
Sub-Atomic Particles
Atoms are made up of three main sub-atomic particles.
| Particle | Symbol | Charge | Location |
| Proton | p | +1 | Nucleus |
| Neutron | n | 0 (neutral) | Nucleus |
| Electron | e | −1 | Energy levels |
Each particle has a different role and function in the atom.
Protons
Protons are positively charged particles found in the nucleus.
Characteristics of Protons
- Carry a positive charge (+1).
- Located in the nucleus.
- Determine the atomic number of an element.
- Determine the identity of the element.
Examples:
- Hydrogen has 1 proton
- Carbon has 6 protons
- Oxygen has 8 protons
Changing the number of protons changes the element itself.
Neutrons
Neutrons are neutral particles found in the nucleus.
Characteristics of Neutrons
- Have no electrical charge.
- Found in the nucleus.
- Help increase the mass of the atom.
- Help stabilize the nucleus.
Electrons
Electrons are negatively charged particles that move around the nucleus.
Characteristics of Electrons
- Carry a negative charge (-1).
- Move around the nucleus in energy levels or shells.
- Have very small mass compared to protons and neutrons.
- Participate in chemical reactions and bonding.
When atoms react chemically, it is the electrons that are gained, lost or shared.
Important Relationship in Atoms
In a neutral atom, the number of positive charges must equal the number of negative charges.
Therefore:
Number of protons = Number of electrons
Example
| Element | Protons | Electrons |
| Hydrogen | 1 | 1 |
| Carbon | 6 | 6 |
| Oxygen | 8 | 8 |
Because the charges cancel each other, the atom has no overall charge.
This is why atoms are electrically neutral.
Watch: Structure of the Atom Explained
Video: Understanding the Structure of the Atom
1.1.2 The Mass Number of Elements
Every element is made of atoms. Although atoms are extremely small, scientists can describe them using important numerical values.
Two of the most important numbers used to identify atoms are:
- Atomic number
- Mass number
These numbers help scientists understand the structure of atoms, determine the type of element, and calculate the number of particles in the atom.
Atomic Number
The atomic number is the number of protons in the nucleus of an atom.
It is represented by the symbol: Z
Relationship Between Atomic Number, Protons and Electrons
In a neutral atom:
Atomic number = Number of protons = Number of electrons
This happens because:
- Protons carry positive charge (+1)
- Electrons carry negative charge (−1)
When the numbers are equal, the charges cancel out, making the atom electrically neutral.
Examples of Atomic Numbers
| Element | Number of Protons | Atomic Number |
| Hydrogen | 1 | 1 |
| Carbon | 6 | 6 |
| Nitrogen | 7 | 7 |
| Sodium | 11 | 11 |
| Aluminium | 13 | 13 |
For example:
- Carbon always has 6 protons
- Oxygen always has 8 protons
If the number of protons changes, the atom becomes a different element.
Mass Number
The mass number is the total number of protons and neutrons in the nucleus of an atom.
It is represented by the symbol: A
Mass number helps determine the total mass of the nucleus because protons and neutrons account for almost all the mass of the atom.
Formula for Mass Number
Mass number = Number of protons + Number of neutrons
Electrons are not included because their mass is extremely small compared to protons and neutrons.
Examples of Mass Numbers
| Element | Protons | Neutrons | Mass Number |
| Lithium | 3 | 4 | 7 |
| Nitrogen | 7 | 7 | 14 |
| Aluminium | 13 | 14 | 27 |
| Sodium | 11 | 12 | 23 |
Example Explanation
For aluminium:
Protons = 13
Neutrons = 14
Mass number = Number of protons + Number of neutrons
Mass Number = 13 + 14 = 27
Therefore, the mass number of aluminium is 27.
Finding the Number of Neutrons
Sometimes the number of neutrons is not given directly. It can be calculated using the formula:
Neutrons = Mass number − Atomic number
Example Calculation
For sodium (Na):
Mass number = 23
Atomic number = 11
Neutrons = Mass number − Atomic number
Neutrons = 23 − 11 = 12
Therefore, the number of neutrons in sodium is 12.
Another Example
For nitrogen (N):
Mass number = 14
Atomic number = 7
Neutrons = Mass number − Atomic number
Neutrons = 14 − 7 = 7
Therefore, the number of neutrons in nitrogen is 7.
Writing Atomic and Mass Numbers
The atomic number and mass number are usually written beside the chemical symbol of an element.
They are written as:
- Mass number at the top left
- Atomic number at the bottom left
Example for Lithium:
⁷₃Li
Where:
- 7 = Mass number
- 3 = Atomic number
- Li = Chemical symbol
More Examples of Atomic Notation
| Element | Symbol Representation |
| Hydrogen | ¹₁H |
| Helium | ⁴₂He |
| Carbon | ¹²₆C |
| Nitrogen | ¹⁴₇N |
| Oxygen | ¹⁶₈O |
| Sodium | ²³₁₁Na |
| Aluminium | ²⁷₁₃Al |
Isotopes
Atoms of the same element can sometimes have different mass numbers. These atoms are called isotopes.
Definition of Isotopes
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.
This means they have:
- The same number of protons
- The same number of electrons
- Different numbers of neutrons
Example: Carbon Isotopes
| Isotope | Protons | Neutrons | Mass Number |
| Carbon-12 | 6 | 6 | 12 |
| Carbon-13 | 6 | 7 | 13 |
| Carbon-14 | 6 | 8 | 14 |
All these atoms are carbon because they have 6 protons, but they have different numbers of neutrons.
Importance of Atomic and Mass Numbers
These numbers help scientists to:
- Identify elements
- Calculate the number of sub-atomic particles
- Understand atomic structure
- Study chemical reactions
- Classify elements in the periodic table