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simplified chemistry – atoms notes (cards)

ATOMS

THREE POSTULATES OF DALTON’S ATOMIC THEORY

1) An element is composed of identical atoms.

2) Atoms of different elements have different properties.

3) Compounds are atoms of 2 or more elements chemically combined.

THE LAW OF CONSERVATION OF MASS

During a physical change or reaction, or a chemical reaction, matter is neither created nor destroyed.

(Lavoisier – 1777)

THE LAW OF DEFINITE COMPOSITION

A compound always contains…

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Atomic number

Atomic number The number of elementary positive charges (protons) con­tained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom.

The concept of atomic number emerged from the work of G. Moseley, done in 1913-1914. He measured the wavelengths of the most energetic rays (Kand L lines) produced by using the elements…

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