17
Cl
Chlorine
35.45
Chlorine
Halogen
Chlorine is element 17, symbol Cl. It sits in period 3, group 17, and weighs 35.45.
One electron short of a full shell, which makes them highly reactive.
- Atomic number
- 17
- Atomic weight
- 35.45
- Period
- Period 3
- Group
- Group 17
- Block
- p block
- Electron configuration
- [Ne] 3s2 3p5
- Outer-shell electrons
- 7
- Neutrons
- 18
The periodic table
Rows are periods, columns are groups. Tap any square to open that element.
1H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca21Sc22Ti23V24Cr25Mn26Fe27Co28Ni29Cu30Zn31Ga32Ge33As34Se35Br36Kr37Rb38Sr39Y40Zr41Nb42Mo43Tc44Ru45Rh46Pd47Ag48Cd49In50Sn51Sb52Te53I54Xe55Cs56Ba57La58Ce59Pr60Nd61Pm62Sm63Eu64Gd65Tb66Dy67Ho68Er69Tm70Yb71Lu72Hf73Ta74W75Re76Os77Ir78Pt79Au80Hg81Tl82Pb83Bi84Po85At86Rn87Fr88Ra89Ac90Th91Pa92U93Np94Pu95Am96Cm97Bk98Cf99Es100Fm101Md102No103Lr104Rf105Db106Sg107Bh108Hs109Mt110Ds111Rg112Cn113Nh114Fl115Mc116Lv117Ts118Og
Others in the same group
Neighbours by number
How to read this
- A row (period) is the number of electron shells. Elements in period 3 carry three of them.
- A column (group) tells you how many electrons sit in the outer shell — which is why a group behaves alike.
- Atomic weight runs at roughly twice the atomic number: about as many neutrons as protons.
- The 28 lanthanides and actinides are drawn below the table. Slotting them in would stretch it to 32 columns.
Frequently asked questions
Q. What are Chlorine's number and symbol?
Element 17, symbol Cl, atomic weight 35.45.
Q. Where does it sit in the table?
Period 3, group 17 — a halogen.
Q. What is its electron configuration?
1s2 2s2 2p6 3s2 3p5, or [Ne] 3s2 3p5 for short.
Q. How many neutrons does it have?
Rounding the atomic weight and subtracting the atomic number gives 18. The exact count depends on the isotope.