Chemical Bonding Explained Simply
Why do atoms bond at all? The answer is already in atomic structure: almost every atom would like a full outermost shell, the way the noble gases have one by nature. There are exactly three routes to that - an atom can give electrons away, it can take some on, or two atoms can share them. Those three possibilities give you the three types of chemical bond, and which one happens depends simply on what kinds of atoms meet.
On the left an electron moves across completely, in the middle a pair is shared, on the right the electrons belong to all the atoms together. The dots stand for outer electrons in each case.
In an ionic bond, a metal meets a non-metal. Sodium has a single outer electron and is happy to lose it; chlorine is short of exactly one for a full shell. So the electron moves across. After that nothing is neutral any more: sodium is one electron short and carries a positive charge, chlorine has one too many and carries a negative one. Charged particles like these are called ions, and the actual bond is nothing but the electrical attraction between them. That attraction works equally strongly in every direction, which is why two particles do not pair off - instead millions of them stack into a regular ionic lattice. This is also why table salt is written NaCl and not Na₂Cl₂: the formula only gives the ratio inside the lattice, not a single molecule.
In a covalent bond, two non-metals meet. Neither will give anything away, because both want to take. The way out is a compromise: each contributes one electron, and the resulting pair counts towards a full shell for both atoms. Two chlorine atoms form a Cl₂ molecule this way. If two pairs are shared you get a double bond, as in oxygen O₂; with three pairs a triple bond, as in nitrogen N₂. This bond acts only between the atoms involved and not in all directions - which is why real, self-contained molecules form here.
In a metallic bond, finally, every atom present wants to give electrons away. And they all do - after which the released electrons no longer belong to any single atom but move freely through the whole piece of metal. What is left behind are positively charged atomic cores sitting in that electron sea like marbles in syrup. This one picture explains three properties at once: metals conduct electricity because the electrons are free to move; they conduct heat for the same reason; and they are shiny because the free electrons throw light back.
Electronegativity says how strongly an atom pulls on shared electrons. What decides the type of bond is not the individual value but the difference between the two partners.
The rule of thumb about metals and non-metals works well, but it is only a shortcut. The clean decision goes via electronegativity, EN for short. It describes how strongly an atom pulls on a shared electron pair. Fluorine pulls hardest, the metals at the bottom left of the periodic table pull weakest. What matters is the difference between the two partners: if it is very small, both pull equally and the pair sits exactly in the middle - a non-polar bond. If it is moderate, the pair shifts towards the stronger partner without moving over completely, and you get a polar bond with a slightly negative and a slightly positive end. If the difference is very large, the electron moves across entirely and you are back at an ionic bond. The boundaries at roughly 0.5 and 1.7 are not laws of nature but practical conventions - the transition is gradual.
This polarity is exactly what makes water the special case it is. Oxygen pulls considerably harder than hydrogen, the molecule is bent rather than straight, and so a water molecule has a negative end and a positive end. Molecules like that are called dipoles, and they attract one another. This attraction is the hydrogen bond. The distinction matters: it is not a fourth type of bond inside a molecule but a force between molecules - far weaker than the three real bonds, but strong enough to explain water's high boiling point, its surface tension and the anomaly of water.
💬 Mia asks Grandpa Theo
In an exercise, how do I know which type of bond I am looking at?
Look at what elements are involved first. Metal plus non-metal is almost always an ionic bond, two non-metals means a covalent bond, two metals means a metallic bond. If you need to be more precise, look up the electronegativities and take the difference. Above 1.7 it is ionic, below that it is covalent - and from about 0.5 upwards, polar covalent.
So is a hydrogen bond just a weak ionic bond?
No, and that is a popular trap. In an ionic bond an electron really has moved over, so there are genuine ions. In a hydrogen bond nothing has moved over at all - the shared pair just sits slightly off-centre, which creates small partial charges. The force is also only about a tenth as strong. And it acts between molecules, not inside one.
Then why write NaCl at all, if it is not a molecule?
Because you are writing down a ratio, not a headcount. In the crystal there is exactly one chloride ion for every sodium ion, so NaCl. For calcium chloride you need two singly negative chloride ions for every doubly positive calcium ion, hence CaCl₂. That is called an empirical formula. For real molecules like H₂O the numbers really are the atoms per particle.
A salt crystal is hard - and yet it shatters when you hit it with a hammer. A piece of copper simply deforms. Both follow directly from the type of bonding. In an ionic lattice, positive and negative ions alternate. If a blow shifts one layer by exactly one position, identical charges suddenly sit on top of each other. They repel, and the crystal splits apart along that plane - which is why salts are brittle and break with flat faces. In a metal, by contrast, all the cores are positive and the electron sea holds them together no matter how they are arranged. Shift a layer and nothing changes about the attraction - the piece deforms instead of breaking. That is exactly why metal can be rolled, bent and drawn into wire. And one more property falls out of this: salts conduct no electricity as a solid crystal, because the ions are locked in place. Dissolve them in water or melt them, and the ions become mobile - at which point they conduct very well indeed.
Where outer electrons come from and why the column in the periodic table tells you so much is covered in atomic structure and the periodic table. What happens when charged particles end up in water is shown by acids, bases and the pH scale. And what bond strength means for melting and boiling points is explained by the three states of matter.
The sentence to remember: a bond forms whenever it brings the atoms involved closer to a full outer shell. Whether an electron moves across, is shared, or belongs to everyone depends only on how hard the partners pull on it.
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