HKDSE Physics – Atomic Model

HKDSE Physics
Chapter 2: Atomic Model | 原子模型

Billy Sir’s Smart Notes: Master Atomic Structure, Isotopes, Decay Equations, and Applications of Radioactivity.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握原子結構、同位素、衰變方程及放射性應用。

1. Basics of Atomic Model | 原子模型基礎

An atom consists of a dense, positively charged nucleus (containing protons and neutrons) surrounded by negatively charged electrons. 原子由一個緻密、帶正電的原子核（包含質子和中子）及圍繞其運行的帶負電電子組成。

⚛️ Nuclide Notation | 核素符號

$$ ^A_Z X $$

Atomic Number ($Z$) / 原子序數: The number of protons in the nucleus. It determines the chemical element. (原子核內的質子數目，決定了化學元素的種類。)
Mass Number ($A$) / 質量數: The total number of protons and neutrons (nucleons) in the nucleus. (原子核內質子和中子（核子）的總數。)
Neutron Number ($N$) / 中子數: Calculated by $ N = A – Z $. (計算方法為 $ N = A – Z $。)

2. Isotopes & Radioactive Isotopes | 同位素與放射性同位素

🔬 What are Isotopes? | 什麼是同位素？

Isotopes are atoms of the same element that have the same atomic number ($Z$) but a different mass number ($A$). This means they have the same number of protons but a different number of neutrons. (同位素是具有相同原子序數（質子數）但質量數（中子數）不同的同種元素原子。)

Radioactive Isotopes (Radioisotopes): Isotopes with unstable nuclei that spontaneously emit radiation (alpha, beta, or gamma) to become more stable. (原子核不穩定的同位素，會自發釋放輻射（阿爾法、貝他或伽馬）以變得更穩定。)

3. Radioactive Decay Equations | 放射性衰變方程

When an unstable nucleus decays, its atomic composition changes according to conservation laws (conservation of mass number and atomic number). 當不穩定原子核衰變時，其原子成分會根據守恆定律（質量數和原子序數守恆）發生變化。

Decay Type (衰變類型)	Equation (衰變方程)	Change in Nucleus (原子核變化)
Alpha ($\alpha$) Decay 阿爾法衰變	$$ ^A_Z X \rightarrow ^{A-4}_{Z-2} Y + ^4_2 \text{He} $$	Mass number decreases by 4. Atomic number decreases by 2. (質量數減 4，原子序數減 2)
Beta ($\beta$) Decay 貝他衰變	$$ ^A_Z X \rightarrow ^{\ \ A}_{Z+1} Y + ^{\ \ 0}_{-1} e $$	Mass number remains unchanged. Atomic number increases by 1. (A neutron turns into a proton). (質量數不變，原子序數加 1。中子變成質子)
Gamma ($\gamma$) Decay 伽馬衰變	$$ ^A_Z X^* \rightarrow ^A_Z X + \gamma $$	No change in mass or atomic number. Nucleus loses excess energy. (質量數和原子序數不變。原子核失去多餘能量)

4. Applications of Radioactivity | 放射性的應用

🦴 1. Carbon-14 Dating | 碳-14 測年法

Used to estimate the age of ancient organic materials (e.g., wood, bones). Living organisms maintain a constant ratio of C-14 to C-12. When they die, C-14 decays (beta decay) with a half-life of about 5730 years. By measuring the remaining C-14 activity, the age can be calculated. (用於估計古代有機物（如木材、骨骼）的年齡。生物死亡後，體內的碳-14會發生貝他衰變（半衰期約5730年）。通過測量剩餘的碳-14活度，可計算其年齡。)

🏭 2. Thickness Gauge | 厚度計

Used in factories to monitor the thickness of materials like paper, plastic, or metal sheets. A Beta ($\beta$) source is typically used for paper/aluminium because its penetration changes significantly with slight thickness variations. If the material is too thick, less radiation reaches the detector, and the rollers adjust automatically. (用於工廠監測紙張、塑膠或金屬板的厚度。通常使用貝他射源，因為其穿透力會隨厚度微小變化而顯著改變。若材料太厚，到達探測器的輻射減少，滾軸會自動調整。)

🚨 3. Smoke Detector | 煙霧探測器

Contains a weak Alpha ($\alpha$) source (e.g., Americium-241). The alpha particles ionize the air inside a chamber, creating a small, steady electric current. When smoke enters the chamber, it absorbs the alpha particles, causing the current to drop and triggering the alarm. Alpha is used because it is easily blocked by smoke and has a short range, making it safe. (內含微弱的阿爾法射源（如鎇-241）。阿爾法粒子使腔內的空氣電離，產生微小且穩定的電流。當煙霧進入腔體時，會吸收阿爾法粒子，導致電流下降並觸發警報。使用阿爾法射線是因為它容易被煙霧阻擋，且射程短，十分安全。)

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