A particle is a minute or very small part, usually of matter or a substance (as in 'Forensic examination revealed particles of explosive on the suspect's clothing'), but sometimes of what is immaterial (as in 'I cannot see a particle of difference between the two views'). Particles small enough to be breathed into lungs, contained in smoke, dust, diesel emissions and various industrial processes, contribute seriously to atmospheric pollution: they are also known as particulates.
In Physics a subatomic particle is one of the constituent parts of an atom, while an elementary or fundamental particle is a constituent part of an atom which cannot (yet) be broken down any further. Particle physics is that branch of the subject which studies the nature and behaviour of these particles. (See also Boson.)
In Grammar a particle is a short word or affix which is without meaning in itself and has a function only when used with other words in a sentence. Grammarians differ about the (types of) words which fall into this category, but in the study of English grammar the following types of particle are generally recognised:
- adverbial particles, i.e., the words needed to complete phrasal or multi-word verbs, such as 'to look after' and 'to bring about'. In the sentences 'He looked after his elderly mother' and 'This campaign will bring about a change in public attitudes' 'after' and 'about' may be said to be adverbial particles, though in other contexts, of course, 'after' and 'about' (like other adverbial particles) may function straightforwardly as prepositions.
- the infinitival particle, i.e., in English, the word 'to' which precedes the infinitive in certain constructions - as in 'I wish to make a complaint', 'To travel hopefully is better than to arrive', 'He seems to be asleep'. (In other contexts 'to' may function straightforwardly as a preposition.)
- the negative particle, in English the word 'not', the use of which is the most common way of negating a sentence. ('Not' is always an adverb.) In French, most negatives are expressed by two words: the negative particle ne before the verb, and another particle, pas, or a more specific word such as jamais 'never' or personne 'no-one', after it.
- pragmatic particles or fillers, i.e., words like 'oh' and 'well' and (perhaps) sounds like 'er' and 'hem', which a speaker who hesitates or pauses may use to fill what would otherwise be a gap in the flow of their speech.
- affixes, i.e., prefixes, such as 're-' (indicating repetition, as in 'to retake an examination' or 'to resubmit an application') and 'de-' (indicating down, as in 'to devalue the currency' or 'to depose the king') and suffixes, such as '-ess' (indicating a female, as in 'princess', 'governess', 'heiress', 'tigress', or 'seamstress').
Note that none of the above types of particle can be inflected: some grammarians regard this (i.e., being uninflected) as a further distinguishing characteristic of a particle.
In languages other than English particles may have a variety of other functions.
- In Ancient Greek particles such as γάρ (gar, for) and οὖν (oun, therefore) function as conjunctions, relating a sentence or clause to what has gone before, while others such as γε (ge, at least, at any rate, certainly, indeed) and τοι (toi, let me tell you) emphasise the preceding word, and the pair μέν ... δέ (men ... de) serve to contrast two objects or statements. (Particles in Ancient Greek can never begin a sentence and are almost always the second word in their sentence or clause.)
- In Modern Greek verbal particles (e.g., θά (tha), νά (na),ἄς (as), γιά (yia)), which come immediately before a verb, may signify, e.g., time or mood. Thus θά κλείσω (tha kleiso) is 'I shall close' and ἄς γράφει (as graphei) is 'Let him write'.
- Many languages have interrogative particles. In Japanese, e.g., questions are asked by placing the particle ka at the end of a sentence, while in Arabic one may ask a question by prefixing the particle a to the first word of a sentence.