1) They have valence electrons in the same energy level.
2) Helium atoms have 2 valence electrons, while atoms of the other elements in the group all have 8 valence electrons.
3) cesium (Cs)
4) There are two electrons in the first energy level and seven electrons in the second energy level.
In the periodic table, elements are arranged in order of increasing atomic number into groups and periods.
In the periodic table, we have an arrangement of atoms based on their respective groups and periods.
If I pick two representative elements in the same period, their number of valence electrons will differ by 1 but they will all be accommodated in the same energy level.
Helium is the first member of group 18. Unlike the other members of the group, its first shell is also its valence shell. It has two and not eight valence electrons like other members of the group.
The electronic configuration of Ca is [Ar] 4s², that of Li is [He] 2s1, that of Cs is [Xe] 6s1, that of Br is [Ar] 4s² 3d¹⁰ 4p⁵ and that of oxygen is [He] 2s² 2p⁴.
We can see that only Cs has valence electrons in higher energy orbitals than those of Ca.
Lastly, Fluorine is in group 17. There are nine electrons in this atoms. They arranged in such a way that there are two electrons in the first shell and seven electrons in the second shell.
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