Atoms and bonding
| English | Chinese | Pinyin |
|---|---|---|
| atom | 原子 | yuán zi |
| atomic number | 原子序数 | yuán zi xù shù |
| mass number | 质量数 | zhì liàng shù |
| isotopes | 同位素 | tóng wèi sù |
| periodic table | 元素周期表 | yuán sù zhōu qī biǎo |
| ionic bonds | 离子键 | lí zi jiàn |
| covalent bonds | 共价键 | gòng jià jiàn |
What an element is
- An atom 原子 has a nucleus of protons and neutrons, with electrons around it.
- The atomic number 原子序数 is the proton count, and it is the element. Change it and you have a different substance.
- The mass number 质量数 counts protons plus neutrons, and isotopes 同位素 of one element differ only in neutrons.
What determines which element an atom is?
The atomic number is the element. Change the neutrons and you have an isotope of the same element.
The periodic table
- The periodic table 元素周期表 orders elements by atomic number.
- Elements in the same column behave similarly because they have the same number of outer electrons.
- That is the whole logic of the table: chemistry is what the outer electrons do, so a shared outer arrangement means shared behaviour.
Why do elements in the same column of the periodic table behave similarly?
Chemistry is what the outer electrons do, so a shared outer arrangement means shared behaviour.
Bonds
- Ionic bonds 离子键 transfer electrons from one atom to another, leaving oppositely charged ions that attract.
- Covalent bonds 共价键 share electrons between atoms.
- Metals lose electrons easily and non-metals gain them, which is why a metal and a non-metal usually bond ionically and two non-metals covalently.
Match each bond to what happens to the electrons.
Metal with non-metal is usually ionic; two non-metals is usually covalent.
Why sodium and chlorine react so violently.
Sodium has one outer electron it holds weakly. Chlorine needs exactly one to complete its outer shell.
The transfer releases a large amount of energy, and the resulting ions attract strongly — which is table salt, an unreactive solid made from a metal that burns in air and a poisonous gas.
The compound's properties come from the bond, not from the elements. That is the point of the whole unit.
A compound's properties can be predicted from the properties of its elements.
Salt is an unreactive solid made from a metal that burns in air and a poisonous gas.
Chemistry is what the outer electrons do. Nearly every pattern in the periodic table follows from that one idea, and an answer that reaches for it will usually be the right kind of answer.
Do not say an atom "wants" a full outer shell. It is a description of which arrangements are stable, not a motive — and while the shorthand is convenient, a question about why a reaction happens is asking for energy, not for intention.
Rewrite "chlorine wants an electron" as a statement about energy and stability.
Example: "Gaining one electron completes chlorine's outer shell, giving a lower-energy, more stable arrangement and releasing energy."