34
Se
Selenium
78.971
Selenium
Nonmetal
Selenium is element 34, symbol Se. It sits in period 4, group 16, and weighs 78.971.
Poor conductors that tend to take on electrons and become negative ions.
- Atomic number
- 34
- Atomic weight
- 78.971
- Period
- Period 4
- Group
- Group 16
- Block
- p block
- Electron configuration
- [Ar] 4s2 3d10 4p4
- Outer-shell electrons
- 6
- Neutrons
- 45
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 Selenium's number and symbol?
Element 34, symbol Se, atomic weight 78.971.
Q. Where does it sit in the table?
Period 4, group 16 — a nonmetal.
Q. What is its electron configuration?
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4, or [Ar] 4s2 3d10 4p4 for short.
Q. How many neutrons does it have?
Rounding the atomic weight and subtracting the atomic number gives 45. The exact count depends on the isotope.