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ATOMIC STRUCTURE · CHAPTER 1 OF 12
Inside the atom: particles and elements
Your phone, a silver ring and the air around you are all made from atoms. Atoms are tiny, but they contain even smaller particles. Their arrangement helps explain both matter and electricity.
By the end, you’ll be able to…
- describe the nucleus and the electrons surrounding it
- compare the locations, relative charges and relative masses of protons, neutrons and electrons
- explain why an atom has no overall electrical charge
- identify an element from its chemical symbol and atomic number
- interpret mass number and atomic number in nuclear notation
- describe the size of an atom and compare it with its nucleus.
A tiny centre, with electrons around it
An atom has a small, positively charged nucleus at its centre. The nucleus contains protons and, in most atoms, neutrons. Negatively charged electrons occupy the region around it.
Almost all the atom’s mass is concentrated in the nucleus. The nucleus takes up only a tiny part of the atom’s volume.
Explore inside a helium atom
Use the buttons to highlight each region and reveal what it contains.
Choose Whole atom, Nucleus or Electrons to explore the model.
* Each moving minus marker represents an electron. The circular path is a simplified GCSE model, not a literal track. Particle sizes, distances and speeds are not to scale. The nucleus is greatly enlarged.
Meet the three subatomic particles
“Subatomic” means smaller than an atom. Relative charge compares each particle’s charge with the charge of a proton. Relative mass compares its mass with that of a proton.
Compare charge and mass
Compare the three particles, then open each card’s explanation.
Proton
In the nucleus
Relative charge: +1
Relative mass: 1
Reveal its role
A proton gives the nucleus positive charge.
Neutron
In the nucleus
Relative charge: 0
Relative mass: 1
Reveal its role
A neutron adds mass but no electric charge.
Electron
Outside the nucleus
Relative charge: −1
Relative mass: About 1/1836
Reveal its role
An electron has the same magnitude of charge as a proton, but the opposite sign.
* The electron’s relative mass is sometimes rounded to 1/2000. It is very small, rather than zero. Equal-sized symbols here do not represent actual particle sizes. The 0 marks a neutron’s charge.
What makes one element different from another?
Every element has its own chemical symbol. For example, N represents nitrogen, while O represents oxygen.
The number of protons determines which element an atom belongs to. An atom with 7 protons is nitrogen. An atom with 8 protons is oxygen.
How do we read an element's symbol?
Use the labels beside nitrogen-14 to see what each part of the notation means.
The mass number is the total number of protons and neutrons. Nitrogen-14 has 7 protons and 7 neutrons. A neutral nitrogen atom also has 7 electrons.
Explore four different elements
Each element has a different atomic number. Look at the notation, then flip each card to discover how many particles the atom contains.
Click or tap each card to reveal its protons, neutrons and electrons. Click again to return to the symbol.
* These examples show neutral atoms of particular isotopes. The number of protons identifies the element. The number of neutrons can vary within an element.
Neutral does not mean “contains no charges”. An atom contains charged particles. Its total positive charge balances its total negative charge, so its overall charge is zero.
Why is an atom neutral?
In a neutral atom, the number of electrons equals the number of protons. Each proton has relative charge +1 and each electron has relative charge −1, so their charges cancel overall. Neutrons do not affect the charge.
Do these charges balance?
A carbon atom contains six protons, six neutrons and six electrons.
Predict its overall charge, then select an answer.
6 electrons
* Pink + circles: protons. White circles: neutrons. Blue − circles: electrons. Simplified shell model, not to scale; the nucleus is enlarged.
What if it had eight neutrons instead?
6 electrons
* Pink + circles: protons. White circles: neutrons. Blue − circles: electrons. Simplified shell model, not to scale; the nucleus is enlarged.
This version is carbon-14. It still has six protons and six electrons, but now has eight neutrons.
Its overall charge would still be zero. Neutrons have no electric charge, so changing their number does not change the balance between protons and electrons. We will explore these different versions of an element in Chapter 2.
How small is an atom?
A typical atom has a radius of about 1 × 10−10 m. Written as an ordinary decimal, that is 0.0000000001 m. Small molecules are also extremely small; their sizes are typically of the order of 10−10 m, though size varies between molecules.
The radius is the distance from the centre to the edge. The diameter is twice the radius: the full distance across.
The nucleus has a radius less than 1/10,000 of the atom’s radius. A typical nuclear radius is of the order of 10−15 m. These are approximate scales, not one exact size for every atom.
If an atom were the size of a stadium…
Imagine enlarging a typical atom until it is 200 m across. How big would its nucleus be?
Choose a basketball, a pea or a pinhead. Use the full distance across for both sizes.
* A size analogy using typical values; atoms, nuclei and pinheads vary in size. The stadium represents the atom’s overall width. The object choices and pinhead inset are enlarged illustrations, not drawn to a shared scale.
Where is the mass?
Protons and neutrons each have a relative mass of about 1. An electron has only about 1/1836 of that mass. Almost all the mass is therefore in the nucleus, even though the electrons occupy a much larger region around it.
At GCSE, we describe most of an atom as empty space. This means the nucleus occupies a tiny fraction of its volume; it does not mean the atom has no electrons or electrical interactions outside the nucleus.
Use the model carefully
Decide on your answer before opening each explanation.
1 · Which is heavier: a proton or an electron?
A proton. It has about 1836 times the mass of an electron. A neutron has approximately the same mass as a proton.
2 · A neutral atom has eight protons. How many electrons does it have?
Eight electrons. Equal numbers of protons and electrons give equal and opposite total charges.
3 · The nucleus is tiny. Does it contain very little of the atom’s mass?
No. Size and mass are different quantities. The tiny nucleus contains almost all the atom’s mass because it contains the protons and neutrons.
4 · An atom has a radius of 1 × 10⁻¹⁰ m. What is its diameter?
The diameter is twice the radius: 2 × 1 × 10−10 m = 2 × 10−10 m.
Choose one answer for each question. Feedback appears immediately. You can change your selection.
A student says, “An atom is neutral, so none of its particles can be charged. Also, the nucleus is so small that most of the mass must be outside it.” Explain both mistakes.
Use the charges and relative masses of the three subatomic particles in your explanation.
Click here to reveal the answer
An atom contains positively charged protons and negatively charged electrons. It is neutral because their numbers are equal and their charges balance overall. Neutrons have no electric charge.
The nucleus contains the protons and neutrons. Each has a relative mass of about 1, whereas an electron has a relative mass of only about 1/1836. Almost all the atom’s mass is therefore in its tiny nucleus.
CHAPTER 1
Chapter summary
- An atom has a small, positively charged nucleus with negatively charged electrons around it.
- Protons and usually neutrons are in the nucleus. Their relative masses are about 1; their relative charges are +1 and 0.
- Electrons have relative charge −1 and a relative mass of about 1/1836.
- A neutral atom has equal numbers of protons and electrons.
- The atomic number is the number of protons and identifies the element. The mass number is the total number of protons and neutrons. Chemical symbols identify elements, such as N for nitrogen and Na for sodium.
- A typical atomic radius is about 1 × 10−10 m. The nucleus’s radius is less than 1/10,000 of the atom’s radius.
- Almost all the mass is concentrated in the nucleus.
Next chapter →
02 · What are isotopes and ions?
Coming next