JOHN DALTON (1766-1844)

1801 England

‘The total pressure of a mixture of gases is the sum of the partial pressures exerted by each of the gases in the mixture’

Partial pressures of gases:
Dalton stated that the pressure of a mixture of gases is equal to the sum of the pressures of the gases in the mixture. On heating gases they expand and he realised that each gas acts independently of the other.

Each gas in a mixture of gases exerts a pressure, which is equal to the pressure it would exert if it were present alone in the container; this pressure is called partial pressure.

Dalton’s law of partial pressures contributed to the development of the kinetic theory of gases.

His meteorological observations confirmed the cause of rain to be a fall in temperature, not pressure and he discovered the ‘dew point’ and that the behaviour of water vapour is consistent with that of other gases.

He showed that a gas could dissolve in water or diffuse through solid objects.

Graph demonstrating the varying solubility of gases

The varying solubility of gases

Further to this, his experiments on determining the solubility of gases in water, which, unexpectedly for Dalton, showed that each gas differed in its solubility, led him to speculate that perhaps the gases were composed of different ‘atoms’, or indivisible particles, which each had different masses.
On further examination of his thesis, he realised that not only would it explain the different solubility of gases in water, but would also account for the ‘conservation of mass’ observed during chemical reactions – as well as the combinations into which elements apparently entered when forming compounds – because the atoms were simply ‘rearranging’ themselves and not being created or destroyed.

In his experiments, he observed that pure oxygen will not absorb as much water vapour as pure nitrogen – his conclusion was that oxygen atoms were bigger and heavier than nitrogen atoms.

‘ Why does not water admit its bulk of every kind of gas alike? …. I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases ’

In a paper read to the Manchester Society on 21 October 1803, Dalton went further,

‘ An inquiry into the relative weight of the ultimate particles of bodies is a subject as far as I know, entirely new; I have lately been prosecuting this enquiry with remarkable success ’

Dalton described how he had arrived at different weights for the basic units of each elemental gas – in other words the weight of their atoms, or atomic weight.

Dalton had noticed that when elements combine to make a compound, they always did so in fixed proportions and went on to argue that the atoms of each element combined to make compounds in very simple ratios, and so the weight of each atom could be worked out by the weight of each element involved in a compound – the idea of the Law of Multiple Proportions.

When oxygen and hydrogen combined to make water, 8 grammes of oxygen was used for every 1 gramme of hydrogen. If oxygen consisted of large numbers of identical oxygen atoms and hydrogen large numbers of hydrogen atoms, all identical, and the formation of water from oxygen and hydrogen involved the two kinds of atoms colliding and sticking to make large numbers of particles of water (molecules) – then as water has an identity as distinctive as either hydrogen or oxygen, it followed that water molecules are all identical, made of a fixed number of oxygen atoms and a fixed number of hydrogen atoms.

Dalton realised that hydrogen was the lightest gas, and so he assigned it an atomic weight of 1. Because of the weight of oxygen that combined with hydrogen in water, he first assigned oxygen an atomic weight of 8.

There was a basic flaw in Dalton’s method, because he did not realise that atoms of the same element can combine. He assumed that a compound of atoms, a molecule, had only one atom of each element. It was not until Italian scientist AMADEO AVOGADRO’s idea of using molecular proportions was introduced that he would be able to calculate atomic weights correctly.

In his book of 1808, ‘A New System of Chemical Philosophy’ he summarised his beliefs based on key principles: atoms of the same element are identical; distinct elements have distinct atoms; atoms are neither created nor destroyed; everything is made up of atoms; a chemical change is simply the reshuffling of atoms; and compounds are made up of atoms from the relevant elements. He published a table of known atoms and their weights, (although some of these were slightly wrong), based on hydrogen having a mass of one.

Nevertheless, the basic idea of Dalton’s atomic theory – that each element has its own unique sized atoms – has proved to be resoundingly correct.

If oxygen atoms all had a certain weight which is unique to oxygen and hydrogen atoms all had a certain weight that was unique to hydrogen, then a fixed number of oxygen atoms and a fixed number of hydrogen atoms combined to form a fixed weight of water molecules. Each water molecule must therefore contain the same weight of oxygen atoms relative to hydrogen atoms.

Here then is the reason for the ‘law of fixed proportions’. It is irrelevant how much water is involved – the same factors always hold – the oxygen atoms in a single water molecule weigh 8 times as much as the hydrogen atoms.

Dalton wrongly assumed that elements would combine in one-to-one ratios as a base principle, only converting into ‘multiple proportions’ (for example from carbon monoxide, CO, to carbon dioxide, CO2) under certain conditions. Each water molecule (H2O) actually contains two atoms of hydrogen and one atom of oxygen. An oxygen atom is actually 16 times as heavy as a hydrogen atom. This does not affect Dalton’s reasoning.

The law of fixed proportions holds because a compound consists of a large number of identical molecules, each made of a fixed number of atoms of each component element.

Although the debate over the validity of Dalton’s thesis continued for decades, the foundation for the study of modern atomic theory had been laid and with ongoing refinement was gradually accepted.

A_New_System_of_Chemical_Philosophy - DALTON's original outline


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1808 – Manchester, England

‘All matter is made up of atoms, which cannot be created, destroyed or divided. Atoms of one element are identical but different from those of other elements. All chemical change is the result of combination or separation of atoms’

Dalton struggled to accept the theory of GAY-LUSSAC because he believed, as a base case, that gases would seek to combine in a one atom to one atom ratio (hence he believed the formula of water to be HO not H2O). Anything else would contradict Dalton’s theory on the indivisibility of the atom, which he was not prepared to accept.

The reason for the confusion was that at the time the idea of the molecule was not understood.
Dalton believed that in nature all elementary gases consisted of indivisible atoms, which is true for example of the inert gases. The other gases, however, exist in their simplest form in combinations of atoms called molecules. In the case of hydrogen and oxygen, for example, their molecules are made up of two atoms, described as H2 and O2 respectively.

Gay-Lussac examined various substances in which two elements form more than one type of compound and concluded that if two elements A and B combine to form more than one compound, the different masses of A that combine with a fixed mass of B are in a simple whole number ratio. This is the law of multiple proportions.

AVOGADRO’s comprehension of molecules helped to reconcile Gay-Lussac’s ratios with Dalton’s theories on the atom.

Gay-Lussac’s ratio for water could be explained by two molecules of hydrogen (four ‘atoms’) combining with one molecule of oxygen (two ‘atoms’) to result in two molecules of water (2H2O).

2H2 + O2 ↔ 2H2O

When Dalton had considered water, he could not understand how one atom of hydrogen could divide itself (thereby undermining his indivisibility of the atom theory) to form two particles of water. The answer proposed by Avogadro was that oxygen existed in molecules of two and therefore the atom did not divide itself at all.

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WILLIAM PROUT (1785-1850)

1815 – UK

‘Atoms are not the smallest thing’

After ANTOINE LAVOISIER had compiled his list of the then known elements, another 32 were added in the years following his death. Fifty kinds of fundamental building blocks for matter seemed excessive. In 1815 Prout, using AVOGADRO’s method of comparing the relative densities and weights of gases, proposed that all atoms appeared to have weights that were exact multiples of the weight of the lightest atom, hydrogen, and that the different atomic weights of elements are whole-number multiples of the atomic weight of hydrogen (Prout’s hypothesis).

Portreait of William Prout (c) The University of Edinburgh Fine Art Collection; Supplied by The Public Catalogue Foundation


He took this as proof that all atoms were actually made from hydrogen atoms and the idea was adopted as atomic theory and used for later investigations of atomic weights and the classification of the elements.

If all atoms are made from atoms of hydrogen, then it could be possible to transform an atom of one element into an atom of another.
If atoms had been assembled from other things, then they themselves could not be the smallest things in creation.

Apart from the method of weighing atoms being controversial, there are exceptions to the rule. Chlorine is 35.5 times as heavy as hydrogen.

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1897 – England

’Not only was matter composed of particles not visible even with the modern microscope, as scientists from DEMOCRITUS to DALTON had surmised, but those particles were themselves composed of even smaller components’

photo of JJ THOMSON at work in the laboratory ©


By the end of the nineteenth century scientists had cleared up much of the confusion surrounding atomic theory. The discovery of the sub-atomic particle was made in April 1897. They believed that they now largely understood the properties and sizes of the atoms of elements; without question, hydrogen was the smallest of all.

When JJ Thomson announced the discovery of a particle one thousandth the mass of the hydrogen atom the particles were named ‘electrons’ and have been a fundamental part of the understanding of atomic science ever since.

Thomson was investigating the properties of cathode rays, now known to be a simple stream of electrons, but at the time the cause of widespread debate. The rays were known to be visible, like normal light, but they were quite clearly not normal light. He devised a series of experiments, which would apply measurements to the cathode rays and clarify their nature. The rays were created by passing an electric charge through an airless or gasless discharge tube.

By improving the vacuüm in the tube, it was demonstrated that the rays could be deflected by electric and magnetic fields. Thomson drilled a hole in the anode of the tube to allow the mysterious rays from the cathode to pass through. In the space after the anode, he arranged that a magnetic force field from a magnet would tug the cathode rays in one direction, and an electric force field between two electrically charged metal plates would tug them in the opposite direction. The rays would eventually strike the glass wall of the tube to create a familiar greenish spot of light on the phosphor-coated tube.

Thomson concluded that the rays were made up of particles, not waves. He saw that the properties of the particles were negative in charge and didn’t seem to be specific to any one element; they were the same regardless of the gas used to transport the electric discharge, or the metal used at the cathode. From his findings he concluded that cathode rays were made up of a jet of ‘corpuscles’ and, more importantly, that these corpuscles were present in all elements. Thomson devised a method of measuring the mass of the particles and found them to be a fraction of the weight of the hydrogen atom.

The position of the spot indicated how much the beam of cathode rays had been deflected. The deflection could be made zero by adjusting the magnetic and electric forces so that they perfectly balanced. In such a situation, Thomson could read off the strength of the electric force. He knew in theory how the magnetic force on a charged particle depends on its speed. By equating the two forces, he was able to deduce the speed of the cathode rays. The deflection was also influenced by the electric charge carried by the cathode ray particles, and their mass. The larger the charge, the greater the force the particles felt and the greater their deflection, the smaller the mass, the easier it was for any force to push the particles about and again, the greater their deflection.

Independent evidence from electrolysis (passing electricity through liquids) that electric charge came in discreet chunks, which he assumed to be carried by individual cathode ray particles, enabled Thomson to calculate their mass.
He arrived at a figure that was a thousand billion billion billionth of a kilogram – a 1000th of the mass of a hydrogen atom.

Knowing the deflection of the dot and the velocity of the particles (the slower the particles, the longer they were exposed to the electric force and the greater the deflection of the glowing dot), Thomson expected to be able to deduce their charge and mass. What he actually deduced was a combination of their charge and mass.

Atoms were made of smaller things, but the fundamental building-blocks were not hydrogen atoms, as had been maintained by PROUT.

Thomson’s particles were christened ‘electrons’ and were the first subatomic entities. Thomson visualized a multitude of tiny electrons embedded ‘like raisins in a plum pudding’ in a diffuse ball of positive charge.

‘The atom is a sphere of positively charged protons in which negatively charged electrons are embedded in just sufficient quantity to neutralise the positive charge’

This was the accepted picture of the atom at the start of the twentieth century until RUTHERFORD found a way to probe inside the atom in 1911.

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1898 – France



‘1903 – Awarded the Nobel-Prize for Physics jointly with Marie and Pierre Curie’

picture of a rock displaying fluorescence under short wavelength radiation

The phenomenon of fluorescence – displayed under short wavelength radiation

Stimulated by WILHELM CONRAD ROENTGEN’s discovery of X-rays in 1895, Becquerel chanced upon the phenomenon that is now known as radioactivity in 1896. The Frenchman believed that Röntgen’s X-rays were responsible for the fluorescence displayed by some substances after being placed in sunlight. Although he was wrong to assume that fluorescence had anything to do with X-rays, he tested large numbers of fluorescent minerals.

He found that uranium, the heaviest element, caused an impression on a covered photographic plate, even after being kept in the dark for several days, and concluded that a phenomenon independent of sunlight induced luminescence.
Investigation isolated the uranium as the source of ‘radioactivity’, a name given to the occurrence by Mme. Curie.

The SI unit of radioactivity, the becquerel is named in his honour.

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NEXT button - MARIE CURIE (1867-1934) PIERRE CURIE (1859-1906)TIMELINE

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  • MARIE CURIE (1867-1934) PIERRE CURIE (1859-1906)

    1898-1902 – France

    ‘Pitchblende, the ore from which uranium is extracted, is much more radioactive than pure uranium. The ore must therefore contain unknown radioactive elements’

    Photograph of marie_curie ©


    Photograph of pierre_curie ©


    Following the discovery of radioactivity by HENRI BECQUEREL (1852-1908) in 1896, Marie Curie conclusively proved that radioactivity is an intrinsic property of the element in question and is not a condition caused by outside factors.

    She correctly concluded that pitchblende contained other, more radioactive elements than uranium.
    The Curies isolated two new radioactive elements, polonium and radium, from pitchblende. The discovery of new elements by their radioactivity was proof that radioactivity was a property of atoms.

    image of two pages from MarieCurie's notebook, which remains radioactive

    Even today, Marie Curie’s notebooks of her studies remain too radioactive to handle.

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    NEXT buttonTHE ATOM


    ALBERT EINSTEIN (1879-1955)

    1905 – Switzerland

    1. ‘the relativity principle: All laws of science are the same in all frames of reference.
    2. constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
    photo portrait of Albert Einstein &copy:


    The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.

    Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
    The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.

    ‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’

    This equation shows that mass and energy are mutually convertible under certain conditions.

    The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.

    Two of his early papers described Brownian motion and the ‘photoelectric’ effect (employing PLANCK’s quantum theory and helping to confirm Planck’s ideas in the process).

    1915 – Germany

    ‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’

    From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.

    Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.

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