The Nuclear Model of the Atom — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

Section 1 of 25Structure and size of the atom

Structure and size of the atom

Nuclear model

• Nuclear model an atom has a small, dense, positively charged nucleus with negatively charged electrons orbiting it.

Size

• Size an atom has a radius of about 1 × 10⁻¹⁰ m, so about a hundred million atoms fit side by side across a thumbnail.

Mass

• Mass the nucleus has a radius over 10 000 times smaller than the atom, but it contains almost all of its mass.

Section 2 of 25Rutherford’s alpha scattering experiment (Extended)

1Set-up

• Set-up in 1909 Geiger and Marsden, directed by Rutherford, fired a beam of alpha particles (He2+ ions) at thin gold foil.
• They were testing the plum pudding model.

2Most pass straight through

• Most pass straight through so the atom is mostly empty space.

3Some are deflected

• Some are deflected they come close to the positively charged nucleus, which repels the positive alpha particles.

4A few bounce back

• A few bounce back the nucleus is tiny, dense and carries most of the mass.
• This could not be explained by the plum pudding model, so it gave the first evidence for the nuclear atom.

Section 3 of 25Atoms and ions

Neutral atom

• Neutral atom the number of protons equals the number of electrons.

Positive ion

• Positive ion an atom that loses electrons has more protons than electrons.

Negative ion

• Negative ion an atom that gains electrons has more electrons than protons.

Net charge

• Net charge the overall charge; an atom with 5 protons, 5 neutrons and 6 electrons has a net negative charge.

Section 4 of 25Composition of the nucleus

Composition of the nucleus

Particles

• Particles protons are positive and neutrons are neutral, so the nucleus is positively charged overall.

Terminology

• Terminology the atom is the nucleus plus its electrons; the nucleus is just the protons and neutrons.

Proton number Z

• Proton number Z the number of protons (atomic number);
  • it decides which element the atom is and equals the number of electrons in a neutral atom.
• Hydrogen has Z = 1, sodium 11 and uranium 92.

Nucleon number A

• Nucleon number A the total number of protons and neutrons (mass number); electron mass is negligible.

Neutrons

• Neutrons number of neutrons = A − Z.

Section 5 of 25Nuclide notation

Form

• ^A_ZX, with the nucleon number on top and the proton number below the element symbol X.

Nuclide

• Nuclide a type of atom with a particular number of protons and neutrons.

Section 6 of 25Comparing the plum pudding and nuclear models

Feature	Plum pudding model	Nuclear model
Positive charge	Spread through the whole atom	Concentrated in a tiny nucleus
Electrons	Embedded in the positive material	Orbit the nucleus in mostly empty space
Alpha scattering result	Predicts only small deflections	Explains the large deflections and rebounds

Section 7 of 25More from the specification

More from the specification

Subatomic structure deduction

• Rutherford alpha scattering experiments provided a revolutionary means to discover the small, dense, positively charged atomic nucleus.

Rutherford alpha scattering experiment

• Alpha particles fired at thin gold foil mostly passed straight through, with a few deflected at large angles, proving atoms consist of a tiny, dense positive nucleus surrounded by empty space.

Nuclide notation and composition

• A nuclide is denoted by atomic number (Z, protons) and mass number : mass number = protons + neutrons.
  • (A, total nucleons)

Nuclear fission vs nuclear fusion

• Fission splits heavy unstable nuclei (uranium-235) into smaller fragments releasing energy;
  • fusion combines light nuclei (hydrogen) under extreme temperature and pressure.

Nuclear fission and fusion energy

• Nuclear fission and fusion energy nuclear fission splits heavy parent nuclei into lighter products,
  • while fusion joins light nuclei, releasing massive energy.

Isotopes and nuclide notation

• Isotopes and nuclide notation isotopes possess identical atomic numbers but different mass numbers, written in standard AZX nuclide notation.

Subatomic particle relative properties

• Subatomic particle relative properties atomic structure consists of positively charged protons, neutral neutrons in the nucleus,
• and orbiting negative electrons of negligible mass.

Relative charge

• Charge compared with the fundamental charge 1.6 × 10⁻¹⁹ C.

Nuclide notation

• ^A_ZX.

Nuclide equation

• An equation showing nuclei and particles in a nuclear reaction.

Fission

• Splitting of a large nucleus releasing energy.

Fusion

• Joining of light nuclei into a heavier one.

Isotopes

• Nuclei with the same proton number but different neutron numbers.

Section 8 of 25Isotopes

Definition

• Definition isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Properties

• Properties an element can have several isotopes, and isotopes tend to be less stable and more likely to decay.

Hydrogen isotopes

• Hydrogen isotopes hydrogen, deuterium (1 neutron) and tritium (2 neutrons).
• About two in 10 000 hydrogen atoms are deuterium and tritium is far rarer.

Identifying isotopes

• Identifying isotopes look for the same element symbol and proton number but different nucleon numbers.
• ²³⁸₉₂U and ²³⁵₉₂U are isotopes of each other.

Section 9 of 25Relative charge

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/2000 (negligible)

Fundamental charge

• Fundamental charge the charge of a proton, 1.6 × 10⁻¹⁹ C, is the unit for relative charge;
  • the electron has the same size with opposite sign.

Unitless

• Unitless relative charge is a ratio, so it has no unit.

Section 10 of 25Nuclear charge and nuclear mass (Extended)

Nuclear charge

• Nuclear charge equals the proton number times +1; carbon (Z = 6) has a nuclear charge of +6.

Nuclear mass

• Nuclear mass equals the nucleon number times 1.
• A relative mass of 1 is about 1.67 × 10⁻²⁷ kg.

Atom mass

• Atom mass essentially the nuclear mass, because electrons are negligible.

Section 11 of 25Nuclear fission

1Definition

• Definition fission is the splitting of a large unstable nucleus into two smaller nuclei.

2Process

• Process a neutron hits an unstable nucleus, which splits into two daughter nuclei and two or three neutrons, with gamma rays emitted.

3Energy

• Energy energy is transferred from the nuclear store of the parent nucleus to the kinetic store of the fast-moving products.

Section 12 of 25Fission equation

Equation

• ²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3 ¹₀n + energy.

Balancing

• Balancing nucleon numbers balance: 235 + 1 = 92 + 141 + 3; proton numbers balance: 92 + 0 = 36 + 56 + 0.

Fuels

• Fuels large nuclei such as uranium and plutonium undergo fission and fuel nuclear power stations.

Section 13 of 25Fission in a power station

Chain reaction

• Chain reaction the neutrons released can hit other uranium nuclei and cause more fissions.

Control

• Control in a reactor the number of neutrons is controlled so that the reaction releases energy steadily.

Energy use

• Energy use the thermal energy produced heats water to make steam, which turns turbines connected to generators.

Section 14 of 25Isotope example: carbon

Carbon-12

• 6 protons and 6 neutrons, stable.

Carbon-14

• 6 protons and 8 neutrons, unstable and radioactive.

Same element

• Same element both have proton number 6.

Section 15 of 25More from the specification

More from the specification

Atomic structure and nuclide notation

• An atom consists of a dense positive nucleus surrounded by negative orbital electrons; nuclide notation shows atomic number and mass number (A, nucleons).
  • (Z, proton count)

Nuclear fusion reactions

• Nuclear fusion is a process where light atomic nuclei join together to form a heavier nucleus, releasing vast amounts of energy when the mass of reactants exceeds the mass of products.

Alpha particle emission and nuclide transformation

• Radioactive decay by alpha particle emission means the parent nucleus loses 2 protons and 2 neutrons, transforming into a new daughter nuclide with atomic number reduced by 2.
  • (helium-4 nucleus)

Isotopes and nuclear composition

• Isotopes are atoms of the same chemical element having identical atomic numbers but different neutron numbers,
• forming distinct nuclides of that element.

Atomic structure and nuclide notation

• An atom consists of a dense positive nucleus surrounded by negative orbital electrons; nuclide notation shows atomic number and mass number (A, nucleons).
  • (Z, proton count)

Nuclear fusion reactions

• Nuclear fusion is a process where light atomic nuclei join together to form a heavier nucleus, releasing vast amounts of energy when the mass of reactants exceeds the mass of products.

Alpha particle emission and nuclide transformation

• Radioactive decay by alpha particle emission means the parent nucleus loses 2 protons and 2 neutrons, transforming into a new daughter nuclide with atomic number reduced by 2.
  • (helium-4 nucleus)

Isotopes and nuclear composition

• Isotopes are atoms of the same chemical element having identical atomic numbers but different neutron numbers,
• forming distinct nuclides of that element.

Alpha scattering

• Experiment firing alpha particles at gold foil that showed the nuclear atom.

Fusion

• Joining of light nuclei into a heavier one.

Nuclide

• Nucleus with a specific number of protons and neutrons.

Nuclear fusion

• Joining of light nuclei to form a heavier nucleus, releasing energy.

Alpha particle

• Helium nucleus, charge +2.

Nuclide notation

• ^A_ZX.

Section 16 of 25Energy and mass in fission

Energy source

• Energy source energy is released from the nuclear store of the parent nucleus into the kinetic store of the products.

Mass change

• Mass change the products have fractionally less mass than the original nucleus;
  • the missing mass is converted into the released energy.

Fuels

• Fuels large nuclei such as uranium and plutonium are used in nuclear power stations.

Section 17 of 25Nuclear fusion

Definition

• Definition fusion is when two light nuclei join to form a heavier nucleus.

Where it happens

• Where it happens in stars, hydrogen nuclei fuse to form helium and release huge amounts of energy.

Conditions

• Conditions fusion needs extremely high temperature and pressure, so it is very hard to reproduce on Earth.

Equation

• ²₁H + ¹₁H → ³₂He + energy.
• Here deuterium has 1 proton and 1 neutron, hydrogen has one proton, and helium-3 has two protons and one neutron.

Section 18 of 25Energy and mass in fusion

Mass change

• Mass change the fused nucleus has slightly less mass than the two original nuclei; that mass becomes energy.

Scale

• Scale the energy from 1 kg of hydrogen undergoing fusion equals that from burning about 10 million kg of coal.

Section 19 of 25Fission compared with fusion

Feature	Fission	Fusion
Change	Large nucleus splits into two smaller nuclei	Two light nuclei join into a heavier nucleus
Example fuel	Uranium, plutonium	Hydrogen isotopes
Conditions	Started by a neutron	Very high temperature and pressure
Where used	Nuclear power stations	Stars; not yet practical on Earth
Output	Energy, daughter nuclei and neutrons	Energy and a heavier nucleus

Section 20 of 25Balancing nuclide equations

1

Add the nucleon numbers (top numbers) of the reactants.

2

Add the nucleon numbers of the products, including the unknown.

3

Set the two sums equal and solve for the missing value.

4

Repeat for the proton numbers (bottom numbers) to check.

Section 21 of 25Fusion in stars and on Earth

Stars

• Stars gravity gives the huge pressure and temperature needed for hydrogen nuclei to overcome their repulsion and join.

Repulsion

• Repulsion nuclei are positively charged, so they repel each other;
  • very high speeds are needed to bring them close enough to fuse.

On Earth

• On Earth scientists are trying to build fusion reactors, but sustaining the conditions is very difficult.

Advantage

• Advantage fusion produces a very large amount of energy per kilogram of fuel and hydrogen is abundant.

Section 22 of 25Comparing the energy released

Fission of uranium

• Fission of uranium a kilogram of uranium-235 releases several million times more energy than a kilogram of coal.

Fusion of hydrogen

• Fusion of hydrogen releases even more energy per kilogram than fission and produces no long-lived radioactive waste.

Practical difference

• Practical difference fission power stations operate today, while controlled fusion remains experimental.

Check

• Check in both cases the products have slightly less mass than the starting nuclei, and the difference is released as energy.

Section 23 of 25More from the specification

More from the specification

Nuclear binding curve interpretation

• Nuclear binding curve interpretation plotting binding energy per nucleon against mass number provides a theoretical means to explain nuclear energy release.

Nuclear Model of the Atom

• In Rutherford's nuclear model, atoms consist of a tiny, positively charged central nucleus surrounded by orbiting negative electrons;
  • this structure was discovered by scattering alpha particles off gold foil.

Subatomic Particle Masses and Charges

• The relative charge of a proton is +1, an electron is -1,
• and a neutron is 0; protons and neutrons each have a relative atomic mass of approximately 1,
• while electrons have negligible relative mass (1/1840).

Nuclide Notation and Isotopes

• In nuclide notation, an element is written as ᴬ_Z X,
• where the atomic number Z represents the number of protons and mass number A represents total nucleons;
  • isotopes of an element have the same number of protons and electrons but differing numbers of neutrons.

Relative charge

• Charge compared with the fundamental charge 1.6 × 10⁻¹⁹ C.

Alpha particle

• Helium nucleus, charge +2.

Nuclide notation

• ^A_ZX.

Alpha scattering

• Experiment firing alpha particles at gold foil that showed the nuclear atom.

Electron

• Negatively charged particle in an atom.

Section 24 of 25Key terms

Term	Meaning
Nucleus	The small dense central part of an atom made of protons and neutrons.
Proton number (Z)	The number of protons in the nucleus.
Nucleon number (A)	The number of protons plus neutrons in the nucleus.
Ion	An atom or group of atoms with a net charge from losing or gaining electrons.
Nuclide notation	Writing an atom as AZX.
Alpha scattering	The experiment in which alpha particles are fired at gold foil, giving evidence for the nucleus.

Term	Meaning
Isotope	Atoms with the same proton number but different numbers of neutrons.
Relative charge	The charge of a particle compared with the fundamental charge of a proton.
Fundamental charge	1.6 × 10−19 C, the size of the charge on a proton or electron.
Nuclear fission	Splitting of a large unstable nucleus into smaller nuclei releasing energy.
Daughter nuclei	The smaller nuclei produced by fission.
Nuclide equation	An equation showing nuclei and particles in a nuclear reaction.

Term	Meaning
Nuclear fusion	Two light nuclei joining to form a heavier nucleus, releasing energy.
Nuclear fission	A large nucleus splitting into smaller nuclei, releasing energy.
Daughter nuclei	The smaller nuclei produced in fission.
Mass defect idea	The products have slightly less mass than the reactants; the missing mass becomes energy.
Deuterium	An isotope of hydrogen with one proton and one neutron.
Reactants and products	Substances before and after a nuclear reaction.

Section 25 of 25Exam technique

1

Saying electrons are in the nucleus — they orbit outside it.

2

Using the electron number for the nucleon number — nucleon number counts protons and neutrons only.

3

Mixing up A and Z in the notation — A is on top.

4

Saying the atom is mostly solid — it is mostly empty space.

1

Saying isotopes have different numbers of protons — they have the same number.

2

Giving electrons a relative mass of 1 — it is negligible (about 1/2000).

3

Forgetting the neutron that starts the fission reaction.

4

Not balancing both top and bottom numbers in nuclide equations.

1

Saying fusion splits nuclei — fusion joins them.

2

Believing energy is created from nothing — mass is converted to energy.

3

Forgetting that fusion powers stars.

4

Not checking both nucleon and proton numbers balance.