Cl--Cl with three lone pairs around each
The electron configuration of Fr, or Francium starts with the base configuration of [Rn]. The outer shell is 7s1, giving 1 valence electron.
It becomes a negatively charged ion.
Sodium can lose its one valence electron to form noble gas configuration. Chlorine will accept one electron to form noble gas configuration. Hence a ionic bond will e formed between sodium and chlorine.
Because calcium has 2 valence electrons, it needs to get rid of them to have a stable noble gas conformation. Chlorine atom has 7 VE and only needs to gain one electron to have the noble gas conformation. Therefore one calcium will give one electron to two chlorine atoms, therefore resulting in an ionic bond.
It does so because it gains an extra electron in order to attain a noble gas state, i.e. in order to contain the same # of electrons as a noble gas. in the case of Chlorine, that gas is Argon which has an atomic # of 18, therefor having 18 electrons. Chlorine has an atomic # of 17. In order to be monoatomic with Argon, it gains an extra electron to have a total of 18 electrons. Since it now has 1 more electron than protons, it's charge is -1. Therefore, it is written as Cl-.
We can logically walk through this. Hydrogen has 1 proton, and 1 electron. Chlorine has 17. If these were to combine stably, Argon would be made. Argon is a noble gas and therefore has 8 full valence electron slots.
Chlorine gains 1 electron to achieve the noble gas electron configuration of argon.
Gain an electron.
Chlorine need one extra negatively charged electron to be a noble gas.
The electron configuration of Fr, or Francium starts with the base configuration of [Rn]. The outer shell is 7s1, giving 1 valence electron.
Chlorine Electron configuration - [Ne] 3s 2 3p 5
It becomes a negatively charged ion.
The element chlorine does not have noble gas configuration. But chloride ion formed (when chlorine accepts an electron) has noble gas configuration of argon.
The science of atoms and chemical bonding is extremely complicated. Real-life electron density diagrams in a molecule of hydrogen gas for example show how the electrons whizz around the two hydrogen nuclei in a very complex fashion. Obviously, when one is just learning about chemistry, or needs to present a model of atomic structure that is clear, the dot and cross diagram is superb. Let's use the element Lithium, Li as an example. It has one valence (outer-shell) electron, thus it would possess one dot in such a diagram. A molecule of chlorine gas, Cl2, will have a dot and cross diagram where the atoms' outer shells overlap, with two dots in between the overlap, representing the bond. This is a very hard topic to discuss in terms of prose alone; textbooks such as Winter- Chemical Bonding, will provide diagrams that will further embellish what I've discussed here.
Sodium can lose its one valence electron to form noble gas configuration. Chlorine will accept one electron to form noble gas configuration. Hence a ionic bond will e formed between sodium and chlorine.
He :The number of electrons in an atom's outer electron shell determines how many dots there are. Helium has 2 electrons in its outer electron shell, so 2 dots.
Because a chlorine atom only needs one more electron to achieve a noble gas configuration.