BONDING

When an atom's valence shell is not full it tends to bond with other atoms to make it full. Depending on the strength of the atom's electrons due to the electron configuration, some atoms would attract better than others. When these atoms are attracted to each other, and share their electrons, they bond chemically. There are three different types of chemical bonds:

1.  Ionic Bonding
2. Covalent Bonding
3. Metallic Bonding

Ionic Bonding 

Ionic bonding occurs when a metallic and a non-metallic atom trade their electrons in order to make both their valence shells full.
For example, when lithium bonds with fluorine it makes lithium fluorine. Fluorine's valence shell needs one more electron  to make it full whereas lithium can lose it's extra electron. As fluorine has more electrons, making it stronger than lithium, its one extra electron is attracted to fluorine instead of seven of fluorine's electrons being attracted to lithium's valence shell. This is an example of ionic bonding.

(Technological Solutions Inc, 2015)

Covalent Bonding 

Covalent Bonding occurs when two non-metallic atoms share their electrons to make the third energy level (valence shell) full. 
For example, when oxygen bonds with itself it makes o2. Oxygen requires two more electrons and attracts to another oxygen atom which it can share with. So, by sharing two of its electrons with another oxygen atom, its valence shell becomes full and stable. An example of this is seen in the first 15 seconds of the video below. 

https://www.youtube.com/watch?v=TbyiNmH5UOg


Metallic Bonding

Metallic Bonding occurs when metallic elements have a strong attraction between positive ions and many delocalised electrons. The bond has  high melting and boiling points. Its strong electrostatic forces hold the particles together as seen in the image below. 

(BBC, 2014)

Nickel's Bonding

As nickel is a white transition metal and it bonds with non-metallic elements like sulphur, it is known to form an ionic bond. For example, when nickel bonds with oxygen, they trade their electrons. Nickel has 2 electrons in its valence (fourth) shell and can lose 2 electrons or gain 6 more electrons to make it stable. Oxygen has 6 electrons in it valence (second) shell and can lose 6 electrons or gain 2 electrons to make its valence shell full and stable. Therefore, in order for these atoms to bond they either lose or gain their electrons. As nickel is a stronger atom than oxygen, it gains  oxygen's electrons. This make both the atoms valence shells full and stable. This example can be seen in the diagram below:

Reference: https://nickel-atom.weebly.com/bonding.html