Water for life
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# Conclusion

You have learned about the following concepts in this course:

- Each type of atom contains a characteristic number of protons in a central nucleus and an equal number of electrons in layers surrounding the nucleus.

- Elements are substances that consist of only one type of atom. Compounds contain two or more elements combined together. There are two kinds of bond between atoms: covalent and ionic.

- Molecules are the smallest units in which elements and/or compounds can exist covalently bonded together.

- Ions are formed by atoms or groups of atoms gaining or losing electrons.

- Chemical formulas describe the number of atoms present in elements, ions and compounds.

- Balanced chemical equations describe, using chemical formulas, the reactants and products in a chemical reaction.

- Concentration is the mass of a substance dissolved in a given volume (usually 1 litre) of a liquid.

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