Mendeleev · science by example

Chlorine: one electron changes the charge

Atomic number 17 is the anchor. From there, this chapter connects the particle picture to the property we actually observe, instead of asking you to memorize an isolated sentence.

ClZ = 17Open element →

D. Mendeleev

The starting question

Atomic number 17 is the anchor. From there, this chapter connects the particle picture to the property we actually observe, instead of asking you to memorize an isolated sentence.

Make a prediction

Before reading on, predict which particles or energy levels must change. Then ask which part of the atom must remain unchanged if the element is to keep the same identity.

What the model explains

Use Cl as the concrete case. Compare it with a neighbouring element or a member of the same group and decide which periodic change is likely to matter.

A concrete example

The first model should be useful, not sacred. Keep it while it predicts correctly; replace it with a deeper model when exceptions appear.

Practical method

Check the chlorine electron story

Which number fixes chlorine as chlorine, even after it becomes an ion?

What changes when a neutral Cl atom becomes Cl⁻: the nucleus, the electron count, or both?

Why does the stability of solid NaCl involve lattice interactions in addition to the electron-gain step?

Go deeper

From the simple model to deeper science

The simple model builds intuition. Then move to orbitals, energy levels, periodic trends and measured data.

Core Science Visual Science Cl · Z=17

17ClChlorine

Chlorine · Cl · Z = 17

Chlorine (Cl) is chemical element 17. It belongs to the halogens, and its reference state at room temperature is a gas. Those basic facts are a useful starting point for its chemistry and applications.

Understanding the element

Where do we encounter it?

In practice, it turns up most often in water treatment, PVC and chemical synthesis.

What its position in the table tells us

Chlorine sits in period 3, group 17 and the P block. That location is useful because neighbouring elements often share trends in valence electrons, oxidation states and bonding behaviour.

01Electron affinity makes the isolated first step favorable

02Charge comes from counting protons and electrons

03Ionic compounds add lattice or solvation stabilization

Electron affinity makes the isolated first step favorable

A gas-phase chlorine atom releases energy when it gains an electron, unlike many atoms for which electron addition is unfavorable.

Charge comes from counting protons and electrons

Chlorine keeps 17 protons. Chloride has 18 electrons, so its net charge is −1; no proton has been changed.

Ionic compounds add lattice or solvation stabilization

In sodium chloride, electron-transfer language is useful, but the solid exists because the complete crystal has a favorable energy balance, not because one isolated electron jump tells the whole story.

Worked example

How many electrons are in Cl−?

Eighteen: atomic number 17 fixes 17 protons, and the −1 charge means one extra electron.

Common traps to avoid

Forming chloride does not change chlorine into another element.
A filled shell is not by itself the complete energy explanation for NaCl formation.