The Nuclear Model Of The Atom

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Notes

Atomic Structure

Atoms consist of a small, dense, **positively charged nucleus** surrounded by **negatively charged electrons** in orbit.
Atom radius ≈ 1 × 10⁻¹⁰ m; nucleus radius is over 10,000 times smaller but contains almost all the mass.
A neutral atom has equal numbers of **protons** and **electrons**.
**Ions** are formed when an atom loses or gains electrons: losing electrons → **positive ion**; gaining electrons → **negative ion**.

Rutherford's Alpha Scattering Experiment (Extended Tier)

In 1909, Rutherford, Geiger and Marsden fired **α-particles** at thin gold foil.
Most α-particles passed straight through → atom is mostly **empty space**.
Some were deflected → repelled by a small, **positively charged nucleus**.
A very few bounced back → nucleus is **tiny** and contains most of the mass.
This disproved the Plum Pudding model and provided evidence for the **nuclear model**.

The Nucleus

The nucleus contains **protons** (positive charge) and **neutrons** (no charge); overall positive charge.
**Proton number (Z)** = number of protons = atomic number; determines the element.
**Nucleon number (A)** = total number of protons and neutrons (mass number).
Number of neutrons = **nucleon number − proton number**.

Nuclide Notation

Nuclide notation: \ce{^A_ZX} where X = element symbol, A = nucleon number, Z = proton number.
Example: \ce{¹⁹⁷₇₉Au} has 79 protons, 118 neutrons (197−79), and 79 electrons.
A **nuclide** is a group of atoms with the same number of protons and neutrons.

Isotopes

**Isotopes** are atoms of the same element with the same number of protons but different numbers of neutrons.
They have the same Z but different A.
Example: \ce{²³⁵₉₂U} and \ce{²³⁸₉₂U} are isotopes of uranium.
Isotopes tend to be more unstable due to imbalance of protons and neutrons.

Protons, Neutrons & Electrons

Relative charges: proton = **+1**, neutron = **0**, electron = **−1**.
Relative masses: proton = **1**, neutron = **1**, electron = **negligible** (≈1/2000).
**Nuclear charge** = number of protons × (+1).
**Relative mass of atom** ≈ nucleon number (mass of electrons negligible).

Nuclear Fission (Extended Tier)

**Nuclear fission** is the splitting of a large, unstable nucleus into two smaller nuclei.
A neutron collides with the nucleus, which splits into **daughter nuclei** and 2–3 neutrons, releasing energy and gamma rays.
Example: \ce{²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3¹₀n + energy}.
The total nucleon and proton numbers are conserved; mass decreases slightly as mass is converted to energy.
Used in nuclear power stations with fuels like uranium-235 and plutonium.

Nuclear Fusion (Extended Tier)

**Nuclear fusion** is the joining of two light nuclei to form a heavier nucleus.
Occurs in stars (e.g., hydrogen fusing to helium) and requires extremely high temperature and pressure.
Example: \ce{²₁H + ¹₁H → ³₂He + energy}.
Mass of product is slightly less than reactants; mass converted to energy releases huge amounts (1 kg hydrogen ≈ 10 million kg coal).

Helium atom: nucleus with 2 protons and 2 neutrons, surrounded by 2 electrons in the first shell.

Lithium atom: 3 protons, 4 neutrons, electron configuration 2,1.

Practice questions

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1.Which statement about the three isotopes of hydrogen is correct?
Easy
- AThey must have different numbers of electrons orbiting their nucleus.
- BThey must all have one proton in their nuclei.
- CThey must all have one neutron in their nuclei.
- DThey must all have the same number of nucleons in their nucleus.
2.An atom consists of electrons orbiting a nucleus. The nucleus contains protons and neutrons. For the atom to be neutral, which statement must be correct?
Easy
- AThe number of electrons is equal to the number of protons.
- BThe number of protons is equal to the number of neutrons.
- CThe number of electrons, protons and neutrons are all different.
- DThe number of electrons is equal to the number of neutrons.
3.The charge on an electron is -e. Which answer correctly describes the charge of a proton and the charge on a neutron?
Easy
- AProton: 0, Neutron: +e
- BProton: +e, Neutron: -e
- CProton: +e, Neutron: 0
- DProton: -e, Neutron: +e
4.An isotope of uranium is represented by the symbol 92²³⁵U. Which of the answers below could be a different isotope of uranium?
Medium
- A91²³⁵U
- B92²³⁸U
- C235⁹²U
- D90²³⁵U
5.Atom X loses an electron to form an ion. Atom Y gains an electron to form an ion. Which row in the table is correct?
Easy
- Acharge on ion of X: positive, charge on ion of Y: positive
- Bcharge on ion of X: positive, charge on ion of Y: negative
- Ccharge on ion of X: negative, charge on ion of Y: positive
- Dcharge on ion of X: negative, charge on ion of Y: negative
6.Where is the positive charge in an atom?
Easy
- Aon the orbiting particles
- Bin the nucleus
- Cin the space between the nucleus and the orbiting particles
- Dspread throughout the atom
7.Which row correctly identifies the relative charges of the three types of particle in an atom?
Easy
- Aproton: +1, neutron: -1, electron: 0
- Bproton: +1, neutron: 0, electron: -1
- Cproton: -1, neutron: +1, electron: 0
- Dproton: -1, neutron: 0, electron: +1
8.How many neutrons are in a nucleus of aluminium? The chemical symbol is 13²⁷Al.
Medium
- A13
- B27
- C14
- D40

Slides

The Nuclear Model of the Atom

Atoms are the building blocks of all matter.
They consist of a small, dense, positively charged nucleus surrounded by negatively charged electrons in orbit.
The nucleus contains protons (positive) and neutrons (neutral).
The radius of the nucleus is over 10,000 times smaller than the atom, but it contains almost all of the mass.

Where is the positive charge in an atom?

on the orbiting particles
in the nucleus
in the space between the nucleus and the orbiting particles
spread throughout the atom

Rutherford's Alpha Scattering Experiment

In 1909, Rutherford, Geiger and Marsden fired α particles at thin gold foil.
Most α particles passed straight through – the atom is mostly empty space.
Some were deflected – they came close to the positive nucleus and were repelled.
A few bounced back – the nucleus is tiny, dense and positive.

Which of these are true about Rutherford's experiment?

Most α particles bounced straight back.
The atom is mostly empty space.
The nucleus is positively charged.
The experiment supported the Plum Pudding model.

Protons, Neutrons and Electrons

Protons have a relative charge of +1 and relative mass of 1.
Neutrons have a relative charge of 0 and relative mass of 1.
Electrons have a relative charge of -1 and negligible relative mass (1/2000).
In a neutral atom, number of protons = number of electrons.

Which row correctly identifies the relative charges of proton, neutron and electron?

proton +1, neutron -1, electron 0
proton +1, neutron 0, electron -1
proton -1, neutron +1, electron 0
proton -1, neutron 0, electron +1

Nuclide Notation

Nuclide notation: ^A_Z X, where X is the element symbol.
A (top) = nucleon number = number of protons + neutrons.
Z (bottom) = proton number = number of protons.
Number of neutrons = A - Z.

In nuclide notation, the top number is the ____ number, and the bottom number is the ____ number.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
They have the same proton number (Z) but different nucleon numbers (A).
Isotopes tend to be more unstable due to imbalance of protons and neutrons.

Which two nuclides are isotopes of the same element?

³⁵₁₇Cl and ³⁵₁₈Cl
²³⁸₉₂U and ²³⁵₉₂U
¹²₆C and ¹⁴₈C
¹⁶₈O and ¹⁴₇N

Ions

An ion is an atom that has lost or gained electrons, giving it a net charge.
Losing electrons → more protons than electrons → positive ion.
Gaining electrons → more electrons than protons → negative ion.
The number of protons in the nucleus does not change.

Atom X loses an electron. Atom Y gains an electron. What are the charges on the ions?

X positive, Y positive
X positive, Y negative
X negative, Y positive
X negative, Y negative

Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
It is triggered by a neutron colliding with the nucleus.
Products: two daughter nuclei, 2-3 neutrons, and energy (gamma rays).
The total nucleon and proton numbers are conserved in the reaction.

In nuclear fission, a large nucleus splits into two ____ nuclei, releasing ____ and neutrons.

Nuclear Fusion

Nuclear fusion is when two light nuclei join to form a heavier nucleus.
It requires extremely high temperature and pressure (e.g., in stars).
The mass of the product is slightly less than the sum of the reactants; the 'lost' mass is converted into energy.
Fusion releases huge amounts of energy – e.g., hydrogen fusion in the Sun.

Which nuclear reaction takes place in the Sun and which in a nuclear power station?

Sun: fusion, power station: fusion
Sun: fusion, power station: fission
Sun: fission, power station: fission
Sun: fission, power station: fusion

Conservation in Nuclear Reactions

In both fission and fusion, nucleon number (A) and proton number (Z) are conserved.
The sum of top numbers on left = sum of top numbers on right.
The sum of bottom numbers on left = sum of bottom numbers on right.
Mass is not conserved; a small amount of mass is converted into energy (E=mc²).

In a fission equation: ²³⁵___₉₂U + ¹___₀n → ¹⁴¹___₅₆Ba + ⁹²___₃₆Kr + ____ ¹___₀n. The number of neutrons emitted is ____.

Which statements about nuclear fusion are true?

It involves two light nuclei joining to form a heavier nucleus.
It occurs in nuclear power stations.
The mass of the product is greater than the sum of the reactants.
It releases energy from the conversion of mass.

Summary

Atoms: nucleus (protons + neutrons) + orbiting electrons.
Isotopes: same protons, different neutrons.
Ions: atoms with unequal protons and electrons.
Fission: splitting large nuclei; Fusion: joining light nuclei; both release energy.

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