1. Electron Configuration:
* Barium (Ba) has an atomic number of 56, meaning it has 56 electrons.
* Its electron configuration is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²
2. Identifying Core Electrons:
* Core electrons are the electrons in all the energy levels below the outermost (valence) shell.
* Valence electrons are the electrons in the outermost shell.
* For barium, the outermost shell is the 6th shell (6s²).
3. Counting Core Electrons:
* All the electrons in the first five energy levels are core electrons.
* Count the electrons in the 1st through 5th shells: 2 + 8 + 18 + 18 + 8 = 54
Therefore, a barium atom has 54 core electrons.
