Atoms form bonds in order to fill their outer electron shells and make them more stable. The type of chemical bond maximizes the stability of the atoms that form it. In ionic bonds, one atom loses its outer electrons for the other atom which takes them in to become stable. But in covalent bonds, both the atoms share electrons so as to get gain highest stability. Even though here I have mentioned two types of bonds, there are many other types of bonds besides the aforementioned.
Valence Electrons
Simply put, the electrons present at the outermost shell of an atom are said to be the valence electrons of that atom. For example, a hydrogen atom has 1 electron, so it can readily share its electron with the outer shell of any other atom. A helium atom, on the other hand, has 2 electrons in its outermost shell, which means its outermost shell has complete electrons. Therefore, Helium is pretty stable and hence you find it on the top of the noble gas group. This makes it unlikely to bond with another atom.
After hydrogen and helium, it's easy to predict whether two atoms will form bonds. Most atoms need 8 electrons to complete their valence shell(outermost shell). So, atoms will either share or gain as many electrons as needed to fill their valence shell and become stable.
A chlorine atom, for example, needs one electron to fill its outer shell. Sodium readily donates its outer electron (forming the Na+ ion, since it then has one more proton than it has electrons), while chlorine readily accepts a donated electron (making the Cl- ion, since chlorine is stable when it has one more electron than it has protons).
Sodium and chlorine form an ionic bond with each other to form table salt or sodium chloride.
After hydrogen and helium, it's easy to predict whether two atoms will form bonds. Most atoms need 8 electrons to complete their valence shell(outermost shell). So, atoms will either share or gain as many electrons as needed to fill their valence shell and become stable.
A chlorine atom, for example, needs one electron to fill its outer shell. Sodium readily donates its outer electron (forming the Na+ ion, since it then has one more proton than it has electrons), while chlorine readily accepts a donated electron (making the Cl- ion, since chlorine is stable when it has one more electron than it has protons).
Sodium and chlorine form an ionic bond with each other to form table salt or sodium chloride.
How To Predict Bonds Between Atoms
Predicting bonds gets easier when working with a periodic table. The far right-hand side of the modern periodic table includes a group of noble gases. This group consists of Helium, Krypton, etc. which have their valence shell filled up and are stable. Noble gases generally do not form chemical bonds with any atoms.
Another way is to compare the electronegativity of the atoms. Electronegativity is the measure of attraction an atom has to electrons in a chemical bond. In a modern periodic table, electronegativity increases as you move from left to right across the periodic table (except for the noble gases). It decreases as you move down a column or group of the table. Atoms on the left-hand side of the table readily form ionic bonds with atoms on the right side (except the noble gases). Atoms in the middle of the table often form metallic or covalent bonds with each other.
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