An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 picometers across. They are so small that accurately predicting their behavior using classical physics—as if they were tennis balls, for example—is not possible due to quantum effects.

Quotes

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We must be clear that when it comes to atoms, language can be used only as in poetry. ~ Niels Bohr

There is a very interesting analogy between the evolution of the atom and of man in the two methods of unfoldment that are followed. We have seen that the atom has its own atomic life, and that every atom of substance in the solar system is likewise a little system in itself, having a positive centre, or central sun, with the electrons, or the negative aspect, revolving in their orbits around it. Such is the internal life of the atom, its self-centred aspect.
- Alice Bailey, The Consciousness of the Atom (1922)

The atom is now being studied along a newer line... Much of the earlier teaching of physical science has been revolutionised by the discovery of radium, and the more scientists find out, the more it becomes apparent (as they themselves realise), that we are standing on the threshold of very great discoveries, and are on the eve of profound revelations.
In the human being, as he evolves and develops, these two stages can equally be seen. There is the early or atomic stage, in which a man's whole centre of interest lies within himself, within his own sphere, where self-centredness is the law of his being, a necessary protective stage of evolution. He is purely selfish, and concerned primarily with his own affairs. This is succeeded by a later stage, in which a man's consciousness begins to expand, his interests begin to lie outside his own particular sphere, and the period arrives in which he is feeling for the group to which he belongs. This stage might be viewed as corresponding to that of radio-activity. He is now not only a self-centred life, but he is also beginning to have a definite effect upon his surroundings. He is turning his attention from his own personal selfish life, and is seeking his greater centre. From being simply an atom he is, in his turn, becoming an electron, and coming under the influence of the great central Life which holds him within the sphere of Its influence.
- Alice Bailey, The Consciousness of the Atom (1922)

Life is a partial, continuous, progressive, multiform and conditionally interactive self-realization of the potentialities of atomic electron states.
- John Desmond Bernal, The Origin of Life (1967) Preface, p. xv.

The Atoms of Democritus
And Newton's Particles of light
Are sands upon the Red sea shore
Where Israel's tents do shine so bright.
- William Blake, Mock On, Mock On, Voltaire, Rousseau

We have grasped the mystery of the atom and rejected the Sermon on the Mount.
- Omar Nelson Bradley Speech on Armistice Day (1948). In Robert G. Torricelli, Quotations for Public Speakers (2002), 237.

We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.
- Niels Bohr, in his first meeting with Werner Heisenberg in early summer 1920, in response to questions on the nature of language, as reported in Discussions about Language (1933); quoted in Defense Implications of International Indeterminacy (1972) by Robert J. Pranger, p. 11, and Theorizing Modernism : Essays in Critical Theory (1993) by Steve Giles, p. 28

We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice. Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation.
- Niels Bohr, in statements after the Solvay Conference of 1927, as quoted in Physics and Beyond (1971) by Werner Heisenberg

In all chemical investigations, it has justly been considered an important object to ascertain the relative weights of the simples which constitute a compound. But unfortunately the enquiry has terminated here; whereas from the relative weights in the mass, the relative weights of the ultimate particles or atoms of the bodies might have been inferred, from which their number and weight in various other compounds would appear, in order to assist and to guide future investigations, and to correct their results. Now it is one great object of this work, to shew the importance and advantage of ascertaining the relative weights of the ultimate particles, both of simple and compound bodies, the number of simple elementary particles which constitute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle.
If there are two bodies, A and B, which are disposed to combine, the following is the order in which the combinations may take place, beginning with the most simple: namely,
1 atom of A + 1 atom of B = 1 atom of C, binary
1 atom of A + 2 atoms of B = 1 atom of D, ternary
2 atoms of A + 1 atom of B = 1 atom of E, ternary
1 atom of A + 3 atoms of B = 1 atom of F, quaternary
3 atoms of A and 1 atom of B = 1 atom of G, quaternary
- John Dalton, A New System of Chemical Philosophy (1808)

δοκεῖ δὲ αὐτῶι τάδε· ἀρχὰς εἶναι τῶν ὅλων ἀτόμους καὶ κενόν, τὰ δ'ἀλλα πάντα νενομίσθαι [δοξάζεσθαι].
- Now his principal doctrines were these. That atoms and the vacuum were the beginning of the universe; and that everything else existed only in opinion.
- Diogenes Laërtius, on the opinions of Democritus in Democritus, Vol. IX, 44 as translated by Yonge (1853)
- Variant translation: The first principles of the universe are atoms and empty space; everything else is merely thought to exist.
  - As translated by Robert Drew Hicks (1925)

If the motion to be discussed here can actually be observed, together with the laws it is expected to obey, then classical thermodynamics can no longer be viewed as applying to regions that can be distinguished even with a microscope, and an exact determination of actual atomic sizes becomes possible. On the other hand, if the prediction of this motion were to be proved wrong, this fact would provide a far-reaching argument against the molecular-kinetic conception of heat.
- Albert Einstein, "On the Motion of Small Particles Suspended in Liquids at Rest Required by the Molecular-Kinetic Theory of Heat"

The unleashed power of the atom has changed everything save our modes of thinking and we thus drift toward unparalleled catastrophe.
- Albert Einstein, Telegram (24 May 1946) sent to prominent Americans. Quoted in New York Times (25 May 1946). In Robert Andrews Famous Lines: a Columbia Dictionary of Familiar Quotations (1997), 340. Variations exist due to different translations from the original German.

… Several of Thomson’s colleagues thought he was joking when he argued that the electron was smaller than the atom and was a constituent of every atom; to many scientists, the idea that there could exist matter smaller than the atom was inconceivable. Yet he was proved right.
- Graham Farmelo, in The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom (2009)

If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.
- Richard Feynman, The Feynman Lectures on Physics (1963)

Another way to remember their size is this: if an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple.
- Richard Feynman, The Feynman Lectures on Physics (1963)

"But what if we could see them?"
"We shall never be able to see atoms themselves, only their effects."
"That's a poor excuse of an answer. For the same remark a pplies to things in general. In the case of a cat, too, all you can see is the reflection of light rays, i.e., the effects of the cat, and not the cat itself. And when you stroke its fur, the situation is much the same!"
- Werner Heisenberg, Physics and beyond

Hynes here too: account of the funeral probably. Thumping. Thump. This morning the remains of the late Mr Patrick Dignam. Machines. Smash a man to atoms if they got him caught. Rule the world today. His machineries are pegging away too. Like these, got out of hand: fermenting. Working away, tearing away. And that old grey rat tearing to get in.
- James Joyce, in Ulysses

And in the corner by the window gable was a shelf with some books, and some from a circulating library. She looked. There were books about Bolshevist Russia, books of travel, a volume about the atom and the electron, another about the composition of the earth’s core, and the causes of earthquakes: then a few novels: then three books on India. So! He was a reader after all.
- D. H. Lawrence, Lady Chatterley's Lover

A man concerned about the state of his soul will not usually be much helped by thinking about the spheres or the structure of the atom.
- C. S. Lewis, The Discarded Image Ch. 2. Reservations

I don't believe that atoms exist!
- Ernst Mach, in an exclamation after a lecture by Ludwig Boltzmann, as quoted in notes by Boltzmann in Ernst Mach; His Work, Life, and Influence (1972), by John T. Blackmore, Ch. 14 : Mach vs. Boltzmann, Planck, Stumpf, and Külpe, p. 206

In the very beginnings of science, the parsons, who managed things then,
Being handy with hammer and chisel, made gods in the likeness of men;
Till commerce arose, and at length some men of exceptional power
Supplanted both demons and gods by the atoms, which last to this hour.
Yet they did not abolish the gods, but they sent them well out of the way,
With the rarest of nectar to drink, and blue fields of nothing to sway.
From nothing comes nothing, they told us—naught happens by chance, but by fate;
There is nothing but atoms and void, all else is mere whims out of date!
Then why should a man curry favour with beings who cannot exist,
To compass some petty promotion in nebulous kingdoms of mist?
But not by the rays of the sun, nor the glittering shafts of the day,
Must the fear of the gods be dispelled, but by words, and their wonderful play.
So treading a path all untrod, the poet-philosopher sings
Of the seeds of the mighty world—the first-beginnings of things;
How freely he scatters his atoms before the beginning of years;
How he clothes them with force as a garment, those small incompressible spheres!
Nor yet does he leave them hard-hearted—he dowers them with love and with hate,
Like spherical small British Asses in infinitesimal state;
Till just as that living Plato, whom foreigners nickname Plateau,
Drops oil in his whisky-and-water (for foreigners sweeten it so);
Each drop keeps apart from the other, enclosed in a flexible skin,
Till touched by the gentle emotion evolved by the prick of a pin:
Thus in atoms a simple collision excites a sensational thrill,
Evolved through all sorts of emotion, as sense, understanding, and will
(For by laying their heads all together, the atoms, as councillors do,
May combine to express an opinion to every one of them new).
There is nobody here, I should say, has felt true indignation at all,
Till an indignation meeting is held in the Ulster Hall;
Then gathers the wave of emotion, then noble feelings arise,
Till you all pass a resolution which takes every man by surprise.
Thus the pure elementary atom, the unit of mass and of thought,
By force of mere juxtaposition to life and sensation is brought;
So, down through untold generations, transmission of structureless germs
Enables our race to inherit the thoughts of beasts, fishes, and worms.
We honour our fathers and mothers, grandfathers and grandmothers too;
But how shall we honour the vista of ancestors now in our view?
First, then, let us honour the atom, so lively, so wise, and so small;
The atomists next let us praise, Epicurus, Lucretius, and all.
Let us damn with faint praise Bishop Butler, in whom many atoms combined
To form that remarkable structure it pleased him to call—his mind.
Last, praise we the noble body to which, for the time, we belong,
Ere yet the swift whirl of the atoms has hurried us, ruthless along,
The British Association—like Leviathan worshipped by Hobbes,
The incarnation of wisdom, built up of our witless nobs,
Which will carry on endless discussions when I, and probably you,
Have melted in infinite azure—in English, till all is blue.
- James Clerk Maxwell, Notes of the President's Address, British Association (1874) published in Blackwell's Magazine as a critically amusing and poetic paraphrase of Association President John Tyndall's Belfast address arguing for the superiority of scientific over religious or non-rationalist explanation, as quoted by Sir Richard Glazebrook, James Clerk Maxwell and Modern Physics (1896)

As for the atoms, they have been in far worse rows before they became naturalised in my brain, but they forget the days before, etc. At any rate the atoms are a very tough lot, and can stand a great deal of knocking about, and it is strange to find a number of them combining to form a man of feeling.
- James Clerk Maxwell, Letter to Professor Lewis Campbell (26 Sept 1874) in Lewis Campbell, William Garnett, The Life of James Clerk Maxwell (1882)

In your letter you apply the word imponderable to a molecule. Don't do that again. It may also be worth knowing that the aether cannot be molecular. If it were, it would be a gas, and a pint of it would have the same properties as regards heat, etc., as a pint of air, except that it would not be so heavy.
Under what form (right or light) can an atom be imagined? Bezonian! speak or die! Now I must go to post with two dogs in the rain.—Your afft. friend
- James Clerk Maxwell, Letter to Professor Lewis Campbell (26 Sept 1874) in Campbell & Garnett, The Life of James Clerk Maxwell (1882)

We have now got what seems to be definite proof that an X ray which spreads out in a spherical form from a source as a wave through the aether can when it meets an atom collect up all its energy from all round and concentrate it on the atom. It is as if when a circular wave on water met an obstacle, the wave were all suddenly to travel round the circle and disappear all round and concentrate its energy on attacking the obstacle. Mechanically of course this is absurd, but mechanics have in this direction been for some time a broken reed.
- Henry Moseley, Letter to Margery Moseley (2 Feb 1913). In J. L. Heilbron (ed.), H. G. J. Moseley: The Life and Letters of an English Physicist 1887-1915 (1974), 201.

As far as materialistic atomism goes: this is one of the most well-refuted things in existence. In Europe these days, nobody in the scholarly community is likely to be so unscholarly as to attach any real significance to it, except as a handy household tool (that is, as an abbreviated figure of speech).
- Friedrich Nietzsche, Beyond Good and Evil (1886), Part I. On the prejudices of philosophers

Over the last century, physicists have used light quanta, electrons, alpha particles, X-rays, gamma-rays, protons, neutrons and exotic sub-nuclear particles for this purpose [scattering experiments]. Much important information about the target atoms or nuclei or their assemblage has been obtained in this way. In witness of this importance one can point to the unusual concentration of scattering enthusiasts among earlier Nobel Laureate physicists. One could say that physicists just love to perform or interpret scattering experiments.
- Clifford G. Shull, Nobel Banquet Speech (10 Dec 1994), in Tore Frängsmyr (ed.), Les Prix Nobel. The Nobel Prizes 1994 (1995).

Atoms are round balls of wood invented by Dr. Dalton.
- Answer given by an anonymous pupil to a question on atomic theory, as reported by Sir Henry Enfield Roscoe, Reports of the British Association for the Advancement of Science, 57th report, 1887, 7.

… A particle, on the other hand, has discrete properties which it carries with it. And so the remarkable thing was the discovery, in investigations of various atomic phenomena, of an apparent paradox.
- Julian Schwinger, Prologue in Quantum Mechanics: Symbolism of Atomic Measurements (2001)

Nothing exists, save atoms and their combinations. There is no atom, which wouldn't periodically take part in life.
- Konstantin Tsiolkovsky from Новая этика (из монизма) [1]

[The] structural theory is of extreme simplicity. It assumes that the molecule is held together by links between one atom and the next: that every kind of atom can form a definite small number of such links: that these can be single, double or triple: that the groups may take up any position possible by rotation round the line of a single but not round that of a double link: finally that with all the elements of the first short period [of the periodic table], and with many others as well, the angles between the valencies are approximately those formed by joining the centre of a regular tetrahedron to its angular points. No assumption whatever is made as to the mechanism of the linkage. Through the whole development of organic chemistry this theory has always proved capable of providing a different structure for every different compound that can be isolated. Among the hundreds of thousands of known substances, there are never more isomeric forms than the theory permits.
- Nevil Vincent Sidgwick Presidential Address to the Chemical Society (16 Apr 1936), Journal of the Chemical Society (1936), 533

For every atom belonging to me as good belongs to you.
- Walt Whitman, "Song of Myself"

The history of science shows that sharp definitions lead to trouble. Dogmatism in science is usually mistaken, because the conviction of certainty expresses a psychological compulsion, never any truly compelling reasons or facts. When a view attains wide popularity and seems obviously beyond question, its decline has usually begun or will begin very soon.
In 1892 W. W. Rouse Ball of Trinity College, Cambridge, who was well informed on competent opinion—J. J. Thomson was also at Trinity—wrote: "The popular view is that every atom of any particular kind is a minute indivisible article possessing definite qualities, everlasting in its properties, and infinitely hard." Rouse Ball wisely added descriptions of two rival atomic doctrines: Boscovich's point centres, and another based on twists in an elastic solid aether.
Four years later the hard everlasting atom began its rapid exit from physics.
- Lancelot Law Whyte, Essay on Atomism: From Democritus to 1960 (1961)

The matter which we suppose to be the main constituent of the universe is built out of small self-contained building-blocks, the chemical atoms. It cannot be repeated too often that the word "atom" is nowadays detached from any of the old philosophical speculations: we know precisely that the atoms with which we are dealing are in no sense the simplest conceivable components of the universe. On the contrary, a number of phenomena, especially in the area of spectroscopy, lead to the conclusion that atoms are very complicated structures. So far as modern science is concerned, we have to abandon completely the idea that by going into the realm of the small we shall reach the ultimate foundation of the universe. I believe we can abandon this idea without any regret. The universe is infinite in all directions, not only above us in the large but also below us in the small. If we start from our human scale of existence and explore the content of the universe further and further, we finally arrive, both in the large and in the small, at misty distances where first our senses and then even our concepts fail us.
- Emil Wiechert "Die Theorie der Eliktrodynamik und die Röntgensche Entdeckung," Schriften der Physikalisch-Ökonomischen Gesellschaft zu Königsberg in Preussen (1896) 37: 1-48, address to the Physics and Economics Society of Königsberg, East Prussia, as quoted by Freeman Dyson, Infinite in All Directions (1988)