  The metric approach to studying 2-step nilpotent Lie algebras by making use
of non-degenerate scalar products is realised. We show that any 2-step
nilpotent Lie algebra is isomorphic to its standard pseudo-metric form, that is
a 2-step nilpotent Lie algebra endowed with some standard non-degenerate scalar
product compatible with Lie brackets. This choice of the standard pseudo-metric
form allows to study the isomorphism properties. If the elements of the centre
of the standard pseudo-metric form constitute a Lie triple system of the
pseudo-orthogonal Lie algebra, then the original 2-step nilpotent Lie algebra
admits integer structure constants. Among particular applications we prove that
pseudo $H$-type algebras have bases with rational structural constants, which
implies that the corresponding pseudo $H$-type groups admit lattices.
