Atoms are tiny invisible particles of substances.
Usually they are indivisible (a-tomos means indivisible).
White beads may help you understand how hydrogen atoms are looking like, red beads how oxygen atoms are formed.                                                                                                              more about atoms
Nearly all atoms join other atoms to form molecules.

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Molecules are bonded collections of two or more atoms of the same or of different elements.

Hydrogen molecules (H₂) consist of 2 hydrogen atoms (H),
oxygen molecules    (O₂) consist of 2 oxygen atoms (O),
water molecules (H₂O)   consist of 2 H and 1  O atom.
To understand how the collection of 2 hydrogen atoms and 1 oxygen atom is bonded, you may combine 2 white beads and 1 red one by welding.