Joule

the unit for all ENERGY measurements. It is the mechanical equivalent of heat, and one joule (J) is equal to a force of one NEWTON moving one metre, i.e. 1J = 1Nm. It is named after James Prescott Joule (1818-1889), a British physicist who investigated the relationship between mechanical, electrical and heat energy, and, from such investigations, proposed the first law of THERMODYNAMICS, the conservation of energy.

Taken from Dictionary of Science

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The joule (symbol J), named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by the force of one newton acting to move an object through a distance of one metre. In terms of dimensions:

One joule is defined as the amount of work done by a force of one newton moving an object through a distance of one metre. Other relationships are:

• The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt (C·V). This relationship can be used to define the volt;

• The work required to continuously produce one watt of power for one second; or one watt second (W·s) (compare kilowatt hour). This relationship can be used to define the watt.

Taken from Wikipedia

Valency

is the bonding potential or combining power of an atom or group, measured by the number of hydrogen ions (H⁺, or equivalent) that the atoms could combine with or replace. In an ionic compound, the charge on each ion represents the valency, e.g., in NaCl, both Na⁺ and Cl^- have a valency of one. In covalent compounds, the valency is represented by the number of bonds formed. In carbon dioxide, CO₂, carbon has a valency of 4 and oxygen 2.

The electronic theory of valency explains bonds through the assumption that specific arrangements of outer electrons in atoms (outer shells of eight electrons) give stability (as with the inert gases, which have such a structure) through the transfer or sharing of electrons. Thus with the combination of sodium with chlorine, sodium has one electron in the outer shell, which it loses to chlorine to form the stable structure of the inert gas neon. Similarly, the gain of one electron by chlorine gives it the stable structure of argon.

Taken from Dictionary of Science

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In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958) and all the advanced methods of quantum chemistry.

Taken from Wikipedia