Atoms & Isotopes (AQA GCSE Physics Combined Science): Revision Notes
The structure of the atom
Basic components of atoms
All atoms are made up of three main particles that work together to form the building blocks of matter. These particles are protons, neutrons, and electrons, each with different properties and locations within the atom.
The nucleus sits at the centre of every atom. This tiny but incredibly important part contains both protons and neutrons packed tightly together. Think of the nucleus as the atom's control centre - it's where most of the atom's mass is concentrated.
Electrons behave very differently from the particles in the nucleus. Instead of being fixed in one place, they move around the nucleus in areas called shells or energy levels. These shells are like invisible tracks that electrons follow as they orbit the nucleus.
Properties of atomic particles
Each type of particle in an atom has its own unique characteristics that determine how atoms behave and interact with each other.
Protons carry a positive electrical charge (written as +1) and have a mass of 1 atomic mass unit. These positively charged particles are found only in the nucleus and never move around the atom. The number of protons in an atom determines what element it is.
Neutrons are electrically neutral, meaning they have no charge at all (written as 0). Like protons, they have a mass of 1 atomic mass unit and stay fixed in the nucleus. Neutrons help hold the nucleus together and add to the atom's overall mass.
Electrons are negatively charged particles (written as -1) that have virtually no mass compared to protons and neutrons. They continuously move around the nucleus in electron shells, and their movement creates the atom's overall size.
Key Properties of Atomic Particles:
- Protons: Positive charge (+1), mass = 1 amu, located in nucleus
- Neutrons: No charge (0), mass = 1 amu, located in nucleus
- Electrons: Negative charge (-1), virtually no mass, orbit in shells
Why atoms have no overall charge
Even though atoms contain charged particles, the atom as a whole is electrically neutral. This happens because atoms always contain equal numbers of protons and electrons. The positive charges from the protons exactly balance out the negative charges from the electrons, leaving the atom with no net charge.
Worked Example: Charge Balance in Carbon
A carbon atom has 6 protons (giving +6 charge) and 6 electrons (giving -6 charge).
Total charge calculation: (+6) + (-6) = 0
Therefore, the carbon atom has no overall charge and is electrically neutral.
This balance is crucial for atoms to be stable and form the matter we see around us.
Comparing the size of atoms and nuclei
The scale of atomic structures is mind-boggling when you think about it. A typical atom has a diameter of about 0.0000000001 metres (written as m in scientific notation). That's incredibly tiny - you could fit millions of atoms across the width of a human hair.
The nucleus is even more amazingly small. Its diameter is roughly 0.000000000000001 metres ( m), making it about 100,000 times smaller than the whole atom. To put this in perspective, if an atom were the size of a football stadium, the nucleus would be like a small marble sitting in the centre.
This means that atoms are mostly empty space, with electrons moving around in the vast area surrounding the tiny nucleus. Despite being so small, the nucleus contains over 99% of the atom's total mass because protons and neutrons are much heavier than electrons.
Electron energy levels and movement
Electrons don't just randomly float around the nucleus - they occupy specific energy levels or shells at different distances from the nucleus. Think of these like the floors of a building, where each floor represents a different energy level.
Electrons can move between these energy levels, but only under certain conditions. This movement is fundamental to understanding how atoms interact with light and energy.
When an electron absorbs electromagnetic radiation (like light or X-rays), it gains energy and can jump from a lower energy level to a higher one, moving further from the nucleus.
The opposite can also happen - when an electron emits electromagnetic radiation, it loses energy and drops from a higher energy level to a lower one, moving closer to the nucleus. This process of absorption and emission is fundamental to how atoms interact with light and other forms of electromagnetic radiation.
Working with atomic measurements
Because atomic dimensions are so incredibly small, scientists use standard form (also called scientific notation) to write these numbers more easily. This system uses powers of 10 to express very large or very small numbers.
Worked Example: Dividing Numbers in Standard Form
When dividing numbers in standard form, use this method: divide the number parts normally, then subtract the powers of 10.
Calculate:
Step 1: Divide the numbers
Step 2: Subtract the powers
Step 3: Combine the results
This mathematical approach helps scientists compare atomic and nuclear sizes, calculate how many nuclei could fit across an atom, and solve other problems involving these extremely small measurements.
Key Points to Remember:
- Atoms contain three particles: protons and neutrons (in the nucleus) plus electrons (in shells around the nucleus)
- Charge balance makes atoms neutral: equal numbers of positive protons and negative electrons cancel each other out
- The nucleus is tiny but heavy: it's 100,000 times smaller than the atom but contains over 99% of the mass
- Electrons can change energy levels: they absorb radiation to move up levels or emit radiation to move down levels
- Standard form helps with calculations: essential for working with the incredibly small measurements in atomic physics