(for′myŭ-lă)

[L. *formula*, a register]

1. A rule prescribing ingredients and proportions for the preparation of a compound.

2. In chemistry, a symbolic expression of the constitution of a molecule. It consists of the symbols of its component elements, each denoting one atom, with subscripted numbers denoting the number of atoms present. Water, or H_{2}O, consists of two molecules of the element hydrogen and one of oxygen. It may also be written HOH.

Collections of atoms that constitute a group by themselves (radical) are often separated by periods or parentheses. In this case, figures prefixed or appended to the parentheses, or prefixed to an expression contained within periods, apply to all the symbols embraced by the parentheses or periods. In all other cases, a figure prefixed to a symbolic expression for a molecule, such as a coefficient in an algebraic formula, is a multiplier of all the symbols following.

3. Any liquid diet containing variable amounts of protein, fat, carbohydrate, vitamins, and minerals administered to infants as an alternative or a supplement to breast milk.

Arneth's formula

An obsolete method of estimating the number of immature leukocytes by means of an elaborate differential blood count, on the basis of their shape and the number of lobes in the nucleus.

chemical formula

*SEE: formula (2)*

dental formula

A brief method of expressing the dentition of mammals in which the numbers of the teeth are given in the form of a fraction, each portion representing one quadrant; the numbers of the upper teeth form the numerator, and those of the lower teeth the denominator.

The first number listed represents the incisors; the second, the canines; the third, the premolars; and the fourth, the molars. The dental formula of the upper and lower right half of the mouth in humans is

empirical formula

A chemical formula that indicates the simplest numerical ratio of the elements within a molecule, without demonstrating the molecule's chemical bonds or structure. For example, the empirical formula for water is H_{2}O.

SYN: *SEE: molecular formula*

molecular formula

SEE: Empirical formula.

official formula

A formula in a pharmacopeia.

spatial formula

stereochemical formula

A method of depicting chemical formulas so that the elements and their number are depicted as well as their position in space in relation to each other.

SYN: *SEE: spatial formula*

structural formula

A formula of a compound that shows the relationship of the atoms in a molecule. The atoms are shown joined by valence bonds, e.g., H−O−H, as opposed to the empirical formula (H_{2}O).

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