Subatomic particles and the nuclear atomIB MYP Chemistry: Revision notes
Section 1
The three subatomic particles
Every atom is made from three types of subatomic particle.
- Proton: relative mass 1, relative charge +1, found in the nucleus
- Neutron: relative mass 1, relative charge 0, found in the nucleus
- Electron: relative mass very small (about 1/1840), relative charge -1, found in shells around the nucleus
In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges cancel and the atom has no overall charge.
Learn the table as three rows: proton (1, +1, nucleus), neutron (1, 0, nucleus), electron (about 1/1840, -1, shells).
Section 2
The nuclear atom
The nucleus is tiny compared with the whole atom, yet it holds almost all of the atom's mass because protons and neutrons are about 1840 times heavier than an electron. The electrons move around the nucleus in shells, and the space between the nucleus and the electrons is empty. This means an atom is mostly empty space.
The nucleus is positively charged overall, because it contains protons and the neutrons have no charge.
Do not say that the electron has a mass of 0. It is very small, but not zero.
Section 3
How ideas about the atom developed
Scientists changed their model of the atom as new evidence appeared.
- Dalton (early 1800s): atoms are tiny, solid spheres that cannot be divided
- Thomson (1897): the plum pudding model, a ball of positive charge with electrons embedded in it
- Rutherford (1911): the nuclear model, a very small, dense, positive nucleus with electrons around it
- Bohr (1913): electrons move in fixed shells at set distances from the nucleus
Later work found that the nucleus contains neutrons as well as protons.
Section 4
Rutherford's gold foil experiment
Alpha particles (positively charged) were fired at very thin gold foil.
- Most passed straight through: so the atom is mostly empty space
- A few were deflected through large angles: so something small and positive repelled them
- A very few bounced almost straight back: so the positive charge is concentrated in a tiny, dense nucleus
These results could not be explained by the plum pudding model, so the model was replaced. This shows that scientific models change when new evidence is found.
Alpha particles were repelled by the positive nucleus. They did not bounce off electrons, which are far too light.
Must know
- Proton (1, +1), neutron (1, 0), electron (about 1/1840, -1)
- Protons and neutrons are in the nucleus; electrons are in shells
- A neutral atom has equal numbers of protons and electrons
- The atom is mostly empty space with a tiny, dense, positive nucleus
- Dalton, Thomson, Rutherford, Bohr: how the model developed
- Rutherford's results: most straight through, a few deflected, a very few bounced back
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Subatomic particles and the nuclear atom
- A school technician is making a poster about the particles inside a carbon atom. The poster must show students the three types of subatomic particle, where each one is found in the atom, and how their masses and charges compare.Describe the relative mass and relative charge of a proton and of an electron.2 marks
- In 1911 a team of scientists fired a beam of positively charged alpha particles at a very thin sheet of gold foil and used a detector to record where the particles ended up. Before this, many scientists pictured the atom as a ball of positive charge with tiny electrons scattered through it. The team found that nearly all the alpha particles passed straight through the foil, a small number were deflected through large angles, and a very few bounced almost straight back.Explain why most alpha particles passed straight through the foil but a very few bounced back.2 marks
- A science teacher is preparing a lesson on a lithium atom, which contains 3 protons, 4 neutrons and 3 electrons. She also wants to show how scientists' ideas about the atom changed between the early 1800s and the early 1900s. Students may use the fact that protons and neutrons have a relative mass of 1 and that an electron has a negligible relative mass.Explain why a lithium atom has no overall charge and why almost all of its mass is in the nucleus.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).