Atomic structure and isotopes (AQA GCSE Chemistry Combined Science): Revision Notes
Atomic structure and isotopes
What are atoms made of?
Atoms are made up of three main particles called subatomic particles:
- Protons - have a positive charge (+1) and a mass of 1
- Neutrons - have no charge (0) and a mass of 1
- Electrons - have a negative charge (-1) and a very small mass
The protons and neutrons cluster together in the centre of the atom. This central part is called the nucleus. The electrons move around the nucleus in shells or orbits.
The arrangement of particles in an atom is often compared to a solar system - with the nucleus as the sun and electrons as planets orbiting around it. However, unlike planets, electrons don't follow fixed circular paths but exist in probability clouds called orbitals.
How atoms are represented
Scientists use a special way to show atoms using symbols. For example, sodium looks like this:
- Na = chemical symbol for sodium
- 23 = mass number (top number)
- 11 = atomic number (bottom number)
Key facts to remember:
- Mass number = number of protons + number of neutrons
- Atomic number = number of protons only
The atomic number is what defines an element - it tells us how many protons are in the nucleus. This never changes for a given element.
Size of atoms
Atoms are incredibly tiny! Here are some facts about their size:
- Atoms have a radius of about m (0.1 nm)
- The nucleus is even smaller - less than 1/10000th of the atom's radius
- Almost all the atom's mass is in the nucleus
- To put this in perspective: if an atom was the size of a tennis ball, a bacterium would be 1000 nm long!
The scale difference between an atom and its nucleus is enormous. If we could scale up an atom to the size of a football stadium, the nucleus would be about the size of a marble at the centre!
What are isotopes?
Isotopes are different versions of the same element. They have:
- The same number of protons (same atomic number)
- Different numbers of neutrons (different mass numbers)
This means isotopes are the same element but with different masses.
Worked Example: Chlorine Isotopes
Chlorine has two main isotopes:
- Chlorine-35: 17 protons, 18 neutrons (Mass number = 17 + 18 = 35)
- Chlorine-37: 17 protons, 20 neutrons (Mass number = 17 + 20 = 37)
Both are chlorine because they have 17 protons, but they have different masses because of the different numbers of neutrons.
Relative atomic mass
The relative atomic mass is the average mass of all the isotopes of an element. It takes into account how common each isotope is (their abundance).
Worked Example: Calculating Relative Atomic Mass
If chlorine is 75% chlorine-35 and 25% chlorine-37:
Step 1: Convert percentages to decimals
- 75% = 0.75
- 25% = 0.25
Step 2: Apply the formula
Step 3: Calculate
This is why the relative atomic mass isn't always a whole number! It's a weighted average based on the natural abundance of each isotope.
Why isotopes behave the same way
Even though isotopes have different masses, they react in exactly the same way chemically. This is because:
- They have the same number of electrons
- Chemical reactions depend on electrons, not neutrons
- The electronic structure is identical
Remember: It's the electrons that determine how atoms bond and react with other atoms. Since isotopes have the same number of electrons, they have identical chemical properties.
Key Points to Remember:
- Atoms contain protons, neutrons and electrons
- Protons and neutrons are in the nucleus, electrons orbit around it
- Isotopes are the same element with different numbers of neutrons
- Relative atomic mass is the average mass of all isotopes
- Isotopes react the same way because they have the same electron structure