Atom and molecule

In short: Atoms are the smallest building blocks of chemical elements. When atoms combine, molecules arise, for example water (H₂O) from two hydrogen atoms and one oxygen atom.

In more detail: An atom consists of a tiny nucleus of protons and neutrons and a shell of electrons. Protons are positively, electrons negatively charged.

In detail

• Size: An atom is about one ten-billionth of a metre across. The nucleus is another 10,000 times smaller.
• Elements: The number of protons determines the element (periodensystem-grundlagen).
• Bonding: Atoms share electrons (covalent bond) or give them up (ionic bond, as in table salt).
• Isotopes: Atoms of the same element with different numbers of neutrons.
• Reaction: In a chemical reaction atoms rearrange but are not lost.