What is a sodium ion?
The keyword sodium ion refers to the positively charged ion of sodium, written as Na+. It is a cation formed when a neutral sodium atom loses one electron. Sodium has atomic number \(Z=11\), meaning every sodium atom has \(11\) protons in its nucleus.
The nucleus (protons) does not change during ordinary chemical ion formation; the charge change comes from changing the number of electrons.
How Na becomes Na+: electron loss (oxidation)
A neutral sodium atom has \(11\) electrons. Its outer (valence) electron is in the \(3s\) orbital. Losing that one valence electron produces the sodium ion:
\[ \mathrm{Na \rightarrow Na^+ + e^-} \]
- Protons: \(11\) (fixed for sodium)
- Electrons (Na): \(11\)
- Electrons (Na+): \(10\)
- Net charge: \(11 - 10 = +1\)
Electron configuration of the sodium ion
Neutral sodium has the configuration \[ \mathrm{Na:}\ 1s^2\,2s^2\,2p^6\,3s^1 \] Removing the highest-energy, outermost electron (the \(3s^1\) electron) gives: \[ \mathrm{Na^+:}\ 1s^2\,2s^2\,2p^6 = [Ne] \]
In simple main-group ion formation, electrons are removed from the outermost principal energy level \(n\) first.
Lewis symbol for sodium ion
Sodium (Group 1) has one valence electron, so the Lewis symbol for neutral sodium is Na· (one dot). After losing that electron, Na+ is written without dots because it has no valence electrons in the \(n=3\) shell.
In ionic compound diagrams, Na+ is commonly shown as [Na]+ to emphasize it is a cation.
Visualization: electron-shell change from Na to Na+
Key properties of Na vs Na+
| Quantity | Na (neutral atom) | Na+ (sodium ion) |
|---|---|---|
| Atomic number \(Z\) | 11 | 11 |
| Protons | 11 | 11 |
| Electrons | 11 | 10 |
| Net charge | 0 | \(+1\) |
| Electron configuration | \(1s^2\,2s^2\,2p^6\,3s^1\) | \(1s^2\,2s^2\,2p^6 = [Ne]\) |
| Typical chemical role | reactive alkali metal | stable cation in ionic compounds and solutions |
Where sodium ions appear in general chemistry
Sodium ions commonly occur in ionic compounds (for example, NaCl, Na2SO4) where Na+ pairs with anions to form an electrically neutral formula unit. In aqueous solutions, Na+ is a strongly solvated ion and contributes to electrical conductivity when salts dissociate.
- Ionic compound charge balance: one Na+ pairs with one Cl− to make a neutral unit.
- Electrolyte behavior: dissolved sodium salts produce mobile ions that carry current in solution.
Common checks and misconceptions
- Electron count: Na+ has \(10\) electrons, not \(11\).
- Charge source: the \(+1\) charge comes from losing \(1\) electron; the number of protons remains \(11\).
- Configuration shortcut: Na+ is \([Ne]\), meaning a filled second shell \(2s^2\,2p^6\).
Answer
A sodium ion is Na+, formed when a sodium atom (with \(11\) protons) loses its \(3s\) valence electron so it has \(10\) electrons and electron configuration \(1s^2\,2s^2\,2p^6\) (=\([Ne]\)), giving a net \(+1\) charge commonly found in ionic compounds and conducting aqueous solutions.