Isotopes
| English | Chinese | Pinyin |
|---|---|---|
| isotope | 同位素 | tóng wèi sù |
| nucleon number | 核子数 | hé zǐ shù |
| density | 密度 | mì dù |
| relative atomic mass | 相对原子质量 | xiāng duì yuán zi zhì liàng |
Same element, different weight
- Isotopes 同位素 are atoms of the same element with a different number of neutrons.
- Same protons ($Z$), different nucleon number 核子数 ($A$).
- They behave the same chemically — but differ slightly physically.
Isotope lab
Classify isotope facts by what changes and what stays the same.
Isotopes of an element have:
Isotopes share the proton number Z but differ in neutron number, so they have different nucleon numbers A.
Isotopes of an element have the same number of protons but different numbers of ______.
Same protons (so same element), different mass.
Isotopes of an element react chemically in the same way.
They have the same electron configuration, and chemistry depends on the electrons.
What makes an isotope
- Isotopes have the same proton number $Z$ but a different nucleon number $A$.
- We write one as $^{A}_{Z}\text{X}$ — e.g. chlorine has $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$.
Two chlorine isotopes: same protons, different neutrons
Same chemistry, different physics
- Chemical properties depend on the electrons — isotopes have the same electron arrangement, so they react identically.
- Some physical properties depend on mass — a heavier isotope (more neutrons) has a higher density 密度.
Isotopes of one element have the same chemical properties because:
Chemistry depends on the electrons; isotopes have identical electron arrangements, so they react the same.
Which property can differ between two isotopes?
A heavier isotope (more neutrons) has more mass and so a higher density; chemistry is unchanged.
Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass: (35×75 + 37×25) ÷ 100.
(35×75 + 37×25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5 — a weighted mean, closer to ³⁵Cl because it is more abundant.
Working out relative atomic mass 相对原子质量
Worked example. Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
Relative atomic mass is a weighted average — each isotope's mass weighted by how common it is
You've got it
- isotopes = same element, same $Z$, different $A$ (different neutrons)
- written $^{A}_{Z}\text{X}$ (e.g. $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$)
- same chemical properties (same electrons); different physical (mass → density)