Answer:
(A) first option
Explanation & Proof:
Bonds are formed to satisfy the octet rule, which states that an atom needs 8 valence electrons to be stable, with the exception of hydrogen and helium, which only need 2 valence electrons. Valence electrons are the outer electrons.
An easy way to solve this is to count the valence electrons in the options.
- a '-' is 2 valence electrons. Two '-' are equal to 4 electrons, three '-' are 6 electrons, etc.
- a '·' is one electron.
Count all the electrons surrounding one letter (which represents the atom). Each should have a full valence shell:
- Hydrogen (H) has 2 electrons → full
- Carbon (C) has 8 electrons → full
- oxygen (O) has 8 electrons → full
Also, make sure that the structure is correct***. In the dot diagrams:
- hydrogen has 1 valence electron, so it needs 1 more.
- carbon has 4 valence electrons, so it needs 4 more.
- oxygen has 6 valence electrons, so it needs 2 more.
Therefore, hydrogen should be sharing 1 electron, carbon should be sharing 4, and oxygen should be sharing 2:
In option A, carbon shares 1 of its electrons with the hydrogen atoms, and the hydrogen atoms share 1 electron with the carbon atom. The Hydrogen atoms are now full, and the carbon atom now has 2 more electrons, with 6 valence electrons.
Now both the carbon and oxygen atoms have 6 valence electrons. Each needs 2 more valence electrons to be stable, so they share these 2 with each other. All the atoms are now stable and happy with their full valence shells.
:Done
***This HAS to be done. For example, if O was the one sharing with the H atoms and C, O and H would have their full shells before C could get some, leaving C out with his sad little unfilled-shell. Or if H decided to share with C first, before the H atoms, either the H atoms wouldn't get any electrons or only one would get an electron, still leaving C unfilled. The structure is very important. Also make sure you have all the atoms described: 2 H, 1 C, and 1 O.