Which of the following correctly describes a compound?

A compound is a pure substance formed by the chemical combination of two or more different elements.

A compound may be splitted into simpler substances by chemical reactions, and has different properties to those of the elements that form it.

The composition of a compound is fixed: every piece of a compound has the same kind of atoms, bonded in the same way and proportion.

Some examples of compounds are H₂O, NaCl, H₂O₂, CH₃COOH. As you see, they have a chemical formula which states the kind and number of the atoms that form them.

They are different to mixtures, which are formed by two or more compounds, in a variable proportion, and can be separated by physical media. Some examples of mixtures are the solutions (e.g. NaCl dissolved in H₂O), and some solid mixtures (e.g. a mixture of marbles and sand).

clouds form

solid ice forms on leaves.

water droplets form on grass.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.