AS & A-Level Chemistry 01 — Atomic structure and electron arrangement

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Independent Deckloop AS Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 1 of 18: Atomic structure and electron arrangement. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level

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Mass and charge distribution and electric field deflection

The mass of an atom is overwhelmingly concentrated in its tiny nucleus, which contains the heavy protons and neutrons. The charge of an atom is distributed between the positive nucleus and the negative electrons residing in the surrounding electron shells. Because isolated atoms are neutral overall, the number of protons always equals the number of electrons. In ions, electrons are lost (forming positive cations) or gained (forming negative anions).

When beams of subatomic particles pass through a uniform electric field at the same velocity, their paths are affected by their charge and mass. Neutrons, being uncharged, remain undeflected. Protons, bearing a positive charge, are deflected towards the negative plate. Electrons, bearing a negative charge, are deflected towards the positive plate.

Because an electron has a much smaller mass than a proton

(\frac{1}{1836})

, it possesses a far greater charge-to

- \text{mass}\,(\frac{\mathrm{e}}{m})\,\text{ratio}

. Consequently, an electron is deflected significantly more than a proton entering the field at the same initial velocity. Evaluating the number of subatomic particles requires combining the proton number, nucleon number, and net charge.

Key points

Mass is almost entirely concentrated in the central dense nucleus.
Positive charge is localised in the nucleus; negative charge is dispersed in the outer electron shells.
In an electric field, neutrons are completely undeflected.
Protons deflect towards the negatively charged plate; electrons deflect towards the positively charged plate.
The angle of deflection depends on the charge-to $- \text{mass}\,\text{ratio}$ ; electrons are deflected vastly more than protons.

Worked example

Question

Determine the number of protons, neutrons and electrons in a

{}^{37}\mathrm{Cl}^{-}

ion. The atomic number of chlorine is 17.

Solution

1. The proton number is 17, so there are

17\,\text{protons}

.

2. The mass number is 37, so the number of neutrons is

37 - 17 = 20

.

3. The overall charge is -1, indicating there is one more electron than protons.

\text{Electrons} = 17 + 1 = 18

17\,\text{protons}

20\,\text{neutrons}

, and

18\,\text{electrons}

Common pitfalls

Assuming protons and electrons deflect by the same angle in opposite directions in an electric field. Correct reasoning: Although their charges are equal in magnitude, electrons have much less mass, giving a much larger charge-to $- \text{mass}\,\text{ratio}$ and thus a much greater angle of deflection.
Thinking the number of protons changes when an ion forms. Correct reasoning: Only electrons are lost or gained when a chemical ion forms; the nucleus itself remains strictly intact.

Prerequisites

Recall basic atoms, ions, chemical formulas, ratios and rearranging equations.