--- title: Molecules: when atoms join up — a lesson of Mova's school · trees and pragmatics description: «Molecules: when atoms join up», a lesson of the chemistry course of Mova's primary school, sentence by sentence with dependency trees and pragmatics. It begins: Atoms rarely stay alone: they join up into groups called molecules, a bit like Lego bricks clicking together. --- [Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/) [home](https://md.mova.elamur.ai/free/en/) / [corpora](https://md.mova.elamur.ai/free/en/corpora/) / [school](https://md.mova.elamur.ai/free/en/corpora/school/) / [Chemistry](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/) / Molecules: when atoms join up # Molecules: when atoms join up Lesson 3 of the chemistry course of Mova's primary school: 20 sentences, each with its syntax tree and pragmatics. The lessons are written for Mova, and their agreed trees are part of Mova corpus 0.4 (CC BY-SA 4.0). How Mova does on its tests: [the lesson at school](https://md.mova.elamur.ai/free/en/school-run/chemistry/3-molecules-when-atoms-join-up/). - [0](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/0/) — Atoms rarely stay alone: they join up into groups called molecules, a bit like Lego bricks clicking together. - [1](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/1/) — "H2O" means two hydrogen atoms joined to one oxygen atom: that is one molecule of water. - [2](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/2/) — The same few kinds of atom, joined in different ways, give completely different substances: salt and sugar look alike but are made of different atoms. - [3](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/3/) — When atoms change partners, a new substance appears, which is what happens when a nail slowly rusts in water. - [4](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/4/) — No atom is lost in a reaction; the atoms are only rearranged, so a rusty nail actually weighs more than the clean one, because it has gained oxygen from the air. - [5](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/5/) — A molecule is a group of atoms joined together; a water molecule (H2O) is two hydrogen atoms joined to one oxygen atom. - [7](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/7/) — Your breath carries carbon dioxide (one carbon and two oxygen atoms) and water vapour; the water shows up as mist on a cold window. - [8](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/8/) — Plants take in carbon dioxide and use it to build sugar and wood. - [9](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/9/) — Salt is made of sodium and chlorine; sugar is made of carbon, hydrogen and oxygen. - [10](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/10/) — Sugar burns to black carbon in a hot pan; salt does not burn. - [11](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/11/) — In a mixture the ingredients are just mixed and can be separated; in a compound the atoms are joined and behave like a new substance. - [12](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/12/) — Mixed iron filings and sulphur powder can be separated with a magnet; after a teacher heats them, they form iron sulphide, which the magnet no longer picks out. - [13](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/13/) — Our noses sense particular molecules; natural gas has no smell, so companies add a smelly chemical to make leaks noticeable. - [14](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/14/) — Some molecules come in mirror-image pairs, like left and right gloves, and the two can smell different: one version of carvone smells of spearmint, its mirror twin of caraway seeds. - [15](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/15/) — In ball-and-stick models, balls stand for atoms and sticks for bonds; carbon is usually black, oxygen red and hydrogen white, by custom only. - [16](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/16/) — A water molecule has 3 atoms, a sugar molecule 45, and the DNA in one of your cells has billions; stretched out it would be about two metres long, but it is extremely thin and tightly coiled and packed. - [17](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/17/) — Everything is made of chemicals, including water, apples and you; what matters is which chemical and how much. - [18](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/18/) — A molecule is the same whether a plant or a lab made it, and natural does not always mean safe: the berries of deadly nightshade, a wild plant, are poisonous. - [19](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/19/) — Carbon atoms can link into long chains and rings, which is how huge molecules like sugar, wood and DNA are built. - [20](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/3-molecules-when-atoms-join-up/20/) — Rust forms when iron atoms join with oxygen and water, and a rusted nail weighs more than the clean nail. - ← previous: [Elements and the famous members of the periodic table](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/2-elements-and-the-periodic-table/) - next →: [Solids, liquids and gases](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/4-solids-liquids-and-gases/) - ↑ back to [Chemistry](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/)