The atom

1. Overview

The atom is the fundamental building block of all matter. Understanding its structure is essential for explaining how electricity flows, how chemical reactions occur, and how radioactive decay works. This topic explores the internal components of the atom and the historical evidence that led to our current nuclear model.

Key Definitions

• Atom: The smallest particle of an element, consisting of a central nucleus surrounded by orbiting electrons.
• Nucleus: The small, dense, positively charged center of an atom containing protons and neutrons.
• Electron: A negatively charged subatomic particle that orbits the nucleus in specific energy levels or shells.
• Proton: A positively charged subatomic particle found within the nucleus.
• Neutron: An uncharged (neutral) subatomic particle found within the nucleus.
• Ion: An atom that has become electrically charged by gaining or losing one or more electrons.

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Core Content

The Structure of the Atom

An atom consists of two main regions:

• The Nucleus: Located at the center. It contains protons (positive charge) and neutrons (no charge). Because it contains protons, the nucleus itself is always positively charged.
• The Electrons: These are much smaller than protons and neutrons. They carry a negative charge and orbit the nucleus at high speeds in "shells" or orbits.

Formation of Ions

In a neutral atom, the number of negative electrons equals the number of positive protons, so the charges cancel out. However, atoms can become charged by moving electrons:

• Positive Ions: Formed when an atom loses one or more electrons. It now has more positive protons than negative electrons.
• Negative Ions: Formed when an atom gains one or more electrons. It now has more negative electrons than positive protons.

Worked Example: A neutral Lithium atom has 3 protons and 3 electrons.

1. If it loses 1 electron, it has 3(+) and 2(-). Total charge = +1 (Positive Ion).
2. If it gains 1 electron, it has 3(+) and 4(-). Total charge = -1 (Negative Ion).

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Extended Content (Extended Curriculum Only)

The Alpha (α) Particle Scattering Experiment

In the early 1900s, Ernest Rutherford fired alpha particles (which are positively charged) at a very thin sheet of gold foil. The results provided evidence for the Nuclear Model:

1. Observation: Most alpha particles passed straight through the foil without changing direction.
   • Conclusion: The atom is mostly empty space.
2. Observation: Some alpha particles were deflected at large angles.
   • Conclusion: The nucleus is positively charged, causing it to repel the positive alpha particles.
3. Observation: A very small number of alpha particles bounced straight back.
   • Conclusion: The nucleus is very small and contains most of the mass of the atom (it is extremely dense).

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Key Equations & Data

Particle	Relative Charge	Relative Mass	Location
Proton	+1	1	Inside Nucleus
Neutron	0	1	Inside Nucleus
Electron	-1	1/1840 (negligible)	Orbiting Nucleus

• Net Charge of an Ion = (Number of Protons) – (Number of Electrons)

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Common Mistakes to Avoid

• ❌ Wrong: The nucleus is negatively charged and electrons are positive.
• ✓ Right: The nucleus is positive (due to protons) and electrons are negative.
• ❌ Wrong: Neutrons have a positive or negative charge.
• ✓ Right: Neutrons are neutral (zero charge).
• ❌ Wrong: Atoms become ions by changing the number of protons in the nucleus.
• ✓ Right: Atoms only gain or lose electrons to become ions; the nucleus never changes during ion formation.
• ❌ Wrong: Mistaking isotopes for ions.
• ✓ Right: Isotopes have different numbers of neutrons; ions have different numbers of electrons compared to the neutral atom.

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Exam Tips

• Be precise with alpha scattering: If a question asks what evidence shows the nucleus is small, specifically mention that "only a very small minority of alpha particles were deflected back."
• Check the charge: When calculating the charge of an ion, always compare the total number of negative electrons to positive protons. Do not add the two numbers together; find the difference between them.
• The "Empty Space" point: This is the most common answer for why most alpha particles pass through. Always use the phrase "mostly empty space" in your description.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0625 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Describe the structure of an atom in terms of its constituents and their arrangement. [3]

(b) State the charge of an electron, a proton, and a neutron. [2]

Worked Solution:

(a)

1. The atom consists of a central nucleus. Correct identification of the nucleus is necessary.
2. The nucleus contains protons and neutrons. Mention of both protons and neutrons is required.
3. Electrons orbit the nucleus. Correct placement of electrons around the nucleus is crucial.

How to earn full marks:

• Mention that the atom has a nucleus.
• State that protons and neutrons are in the nucleus.
• State that electrons orbit the nucleus.

(b)

1. Electron: Negative. State negative.
2. Proton: Positive. State positive.
3. Neutron: Neutral (or no charge). State neutral or no charge.

How to earn full marks:

• State the correct charge for each particle. Award 1 mark for each correct charge.

Common Pitfall: Many students confuse the charges of protons and electrons. Remember that protons are positive and electrons are negative. Also, be sure to mention that neutrons have no charge (neutral).

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Define the term isotope. [2]

(b) Chlorine has two common isotopes: chlorine-35 ($^{35}\text{Cl}$) and chlorine-37 ($^{37}\text{Cl}$). State one similarity and one difference between these two isotopes. [2]

(c) An atom of chlorine-35 loses an electron. State the charge of the resulting ion and explain how this ion is formed. [2]

Worked Solution:

(a)

1. Isotopes are atoms of the same element. Correctly identify that isotopes belong to the same element.
2. Isotopes have the same number of protons but a different number of neutrons. Mention both the same number of protons AND a different number of neutrons is crucial.

How to earn full marks:

• Mention same element.
• Mention same number of protons.
• Mention different number of neutrons.

(b)

1. Similarity: They have the same number of protons (or same chemical properties). Correctly state a similarity.
2. Difference: They have a different number of neutrons (or different mass number). Correctly state a difference.

How to earn full marks:

• Provide a correct similarity.
• Provide a correct difference.

(c)

1. Charge: Positive (or +1, or 1+). State positive charge.
2. Explanation: The atom loses an electron, resulting in more protons than electrons. Explain that loss of negative charge results in a net positive charge.

How to earn full marks:

• State that the ion has a positive charge.
• Explain that the atom loses an electron, creating a net positive charge.

Common Pitfall: It's easy to mix up isotopes and ions. Isotopes are atoms of the same element with different numbers of neutrons, while ions are atoms that have gained or lost electrons, resulting in a charge.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Rutherford's alpha scattering experiment provided evidence for the nuclear model of the atom.

(a) Describe the setup of the alpha scattering experiment. [3]

(b) State three observations made during the alpha scattering experiment. [3]

(c) Explain how each of these observations led to the development of the nuclear model of the atom. [2]

Worked Solution:

(a)

1. Alpha particles are fired at a thin gold foil. Mention of alpha particles and gold foil is necessary.
2. A detector (e.g., a fluorescent screen) is placed around the foil to detect the scattered alpha particles. Correctly identify a detector.
3. The detector can be moved to different angles to measure the number of alpha particles scattered at each angle. Mention of measuring scattering angles.

How to earn full marks:

• Mention alpha particles being fired at gold foil.
• Mention a detector.
• Mention measuring scattering angles.

(b)

1. Most alpha particles passed straight through the gold foil. State most pass straight through.
2. A small number of alpha particles were deflected through small angles. State some are deflected at small angles.
3. A very small number of alpha particles were deflected through large angles (or backscattered). State some are deflected at large angles or backscattered.

How to earn full marks:

• State that most passed straight through.
• State that some were deflected at small angles.
• State that a very few were deflected at large angles.

(c)

1. Most alpha particles passing straight through suggested that the atom is mostly empty space. Relate straight-through to empty space.
2. The deflection of some alpha particles suggested that there is a positively charged, dense nucleus. Relate deflection to a positive, dense nucleus.

How to earn full marks:

• Relate straight-through to empty space.
• Relate deflection to a positive, dense nucleus.

Common Pitfall: When describing the observations, be sure to include the relative proportions of alpha particles that were undeflected, deflected at small angles, and deflected at large angles. The fact that most went straight through is key to the conclusion that atoms are mostly empty space.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Two isotopes of hydrogen are hydrogen-1 ($^1\text{H}$) and hydrogen-3 ($^3\text{H}$), also known as tritium.

(a) State the number of protons, neutrons, and electrons in a neutral atom of hydrogen-1 and a neutral atom of hydrogen-3. [3]

(b) Hydrogen-3 is radioactive. Describe what happens during radioactive decay. [3]

(c) A sample initially contains $4.0 \times 10^{20}$ atoms of hydrogen-3. The half-life of hydrogen-3 is 12.3 years. Calculate the number of hydrogen-3 atoms remaining after 24.6 years. [3]

Worked Solution:

(a)

1. Hydrogen-1: 1 proton, 0 neutrons, 1 electron. Correctly state the number of each particle in hydrogen-1.
2. Hydrogen-3: 1 proton, 2 neutrons, 1 electron. Correctly state the number of each particle in hydrogen-3.
3. All numbers correct.

How to earn full marks:

• Correct number of protons, neutrons, and electrons for hydrogen-1.
• Correct number of protons, neutrons, and electrons for hydrogen-3.
• All numbers must be correct for full marks.

(b)

1. The nucleus is unstable (and decays). Mention an unstable nucleus.
2. The nucleus emits particles (e.g., alpha, beta, or gamma radiation). Mention emission of radiation.
3. The nucleus changes into a different nucleus (or a different element). Mention transformation into a different nucleus/element.

How to earn full marks:

• Mention an unstable nucleus.
• Mention emission of radiation.
• Mention transformation into a different nucleus/element.

(c)

1. Number of half-lives = 24.6 years / 12.3 years = 2. Calculate the number of half-lives.
2. Number of atoms remaining = $4.0 \times 10^{20}$ / $2^2$. Divide by $2^2$ or 4.
3. Number of atoms remaining = $\boxed{1.0 \times 10^{20} \text{ atoms}}$. State the final answer with correct units.

How to earn full marks:

• Correctly calculate the number of half-lives.
• Divide the initial number of atoms by $2^2$ (or 4).
• State the final answer with correct units.

Common Pitfall: In part (a), remember that a neutral atom has the same number of protons and electrons. In part (c), make sure you understand the concept of half-life and how to apply it to calculate the remaining amount of a radioactive substance after a certain time.