Atoms and bonding
Introduced| English | Meaning |
|---|---|
| atom/ˈætəm/ | The smallest unit of an element that retains its chemical identity, consisting of a nucleus and electrons. |
| atomic number/əˈtɒmɪk ˈnʌmbə/ | The number of protons in the nucleus of an atom. |
| mass number/mæs ˈnʌmbə/ | The total number of protons and neutrons in an atomic nucleus. |
| isotopes/ˈaɪsətəʊps/ | Atoms with the same proton number but different neutron numbers. |
| periodic table/ˌpɪərɪˈɒdɪk ˈteɪbl/ | An arrangement of chemical elements by atomic number that displays recurring patterns in their properties. |
| ionic bonds/aɪˈɒnɪk bɒndz/ | The electrostatic attraction between oppositely charged ions. |
| covalent bonds/ˈkəʊvələnt bɒndz/ | A chemical bond involving a shared pair of electrons attracted to the nuclei of bonded atoms. |
An ion changes electrons, not element identity
- Neutral sodium-23 has 11 protons, 12 neutrons and 11 electrons. After it loses one electron, Na⁺ has 10 electrons but still has 11 protons and remains sodium.
- An atom 原子 contains a nucleus and electrons in the introductory model. Atomic number 原子序数 counts protons; mass number 质量数 counts protons plus neutrons.
What determines which element an atom is?
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The atomic number is the element. Change the neutrons and you have an isotope of the same element.
Find neutrons from the two numbers
- Neutron number follows $N=A-Z$. For sodium-23, $N=23-11=12$; for sodium-24, $N=24-11=13$.
- Isotopes 同位素 share proton number and differ in neutrons. Hydrogen-1 has one proton and no neutrons, so “every nucleus contains neutrons” is false.
Sodium-24 has atomic number 11. How many neutrons does it contain?
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N = A - Z = 24 - 11 = 13.
Connect table position to bounded patterns
- The periodic table 元素周期表 orders elements by atomic number. Introductory main-group patterns connect similar outer-electron arrangements with similarities in chemical behaviour.
- A group does not make all its elements identical. Use the model at its stated level rather than extending a simple outer-electron rule to every element and property.
In an introductory main-group model, what helps explain similarities within periodic-table groups?
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Outer arrangements help explain main-group patterns. They do not make group elements identical or replace more detailed models for every element.
Distinguish ion formation from the ionic bond
- Electron transfer can explain ion formation. Ionic bonds 离子键 are electrostatic attractions between opposite ion charges; the transfer itself is not the attraction.
- In the sodium/chlorine model, Na⁺ has 11 protons and 10 electrons, while Cl⁻ has 17 protons and 18 electrons. Opposite charges attract; bulk sodium chloride forms an ionic structure, not separate NaCl molecules in this model.
Match each bonding model to its defining feature.
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Electron transfer can explain formation of ions; the ionic bond is their electrostatic attraction. Covalent models use shared pairs.
In the supplied model, Na⁺ retains 11 protons after neutral sodium loses one electron. How many electrons remain?
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Neutral sodium has 11; losing one leaves 10.
Use a different model for shared electron pairs
- Covalent bonds 共价键 involve shared electron pairs. For a simple H₂ molecule, the bond is described by a pair of electrons shared between the hydrogen atoms.
- A compound's properties depend on structure and bonding, not just a list of constituent elements. Do not infer that a compound must behave like the separate elements or must be unreactive.
Atoms do not “want” electrons. Explain particle counts, electrostatic attraction and the appropriate bonding model; follow approved procedures for any real chemical work.
A compound must have the same properties as its separate constituent elements.
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Structure and bonding matter. Do not infer identical properties from the constituents or describe compounds as universally unreactive.
Check all three particle counts
- Changing neutrons gives a different isotope of the same element; changing electrons gives a different charge state. Changing proton number changes element identity.
- For each supplied example, write protons, neutrons and electrons before naming its isotope or charge. A positive charge means fewer electrons than protons, not necessarily extra protons.
Use $Z=\text{protons}$, $A=\text{protons}+\text{neutrons}$ and charge balance separately. Keep ion formation, ionic attraction and covalent sharing distinct.
Explain why Na⁺ has ten electrons but is still sodium in the supplied particle model.
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Example: “Losing one electron leaves ten, but the unchanged eleven protons still identify sodium.”
Losing an electron changes sodium into a different element.
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False: unchanged proton number keeps the element identity.