sentence: Some atoms hold together quite well, which is what makes some solid materials hard.
sentences_before: Every material is made up of atoms which are very small parts.
sentences_after: Something made of pure iron is softer than steel because the atoms can slip over one another. If other atoms like carbon are added, they are different from iron atoms and stop the iron atoms from sliding apart so easily. This makes the metal stronger and harder.
concept_name: atom
headings: Steel Iron and steel chemistry
categories: Basic English 850 words Construction Alloys