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Unit 2 Topic 3
What are Elements
The Law of Conservation of Mass
• In a chemical change, the total mass of the new substances is
always the same as the total mass of the original substances
Lavoisier was a pioneer in the field of alchemy.
He defined elements as pure substances that
cannot be decomposed into simpler substances
by means of a chemical change.
His careful measurement of mass, allowed him
to make accurate inferences about what was
happening to the substances.
The law of Definite Composition
• Compounds are pure substances that contain two or more
elements combined together in fixed (or definite) proportions.
Dalton’s Atomic Theory
• All matter is made up of small particles called
• Atoms cannot be created, destroyed, or
divided into smaller particles.
• All atoms of the same element are identical
in mass and size. Atoms of one element are
different in mass and size from the atoms of
other elements.
• Compounds are created when atoms of
different elements link together in definite
Dalton’s model is often referred to as the
“Billiard Ball Model”
A Brief History of Atomic Models
Thompson’s “plum pudding model”
• Thompson pictured the negatively charged electrons stuck in
the positively charged mass like raisins in a bun.
Rutherford’s “planetary model”
• Rutherford proposed that almost all of the mass of the atom
was in the centre of the atom, which he named the atomic
nucleus. The rest of the atom was mostly empty space
occupied by very small electrons.
Bohr’s Atomic model
• Both Bohr and Rutherford pictured the atom like a miniature solar
• Electrons rotated around the nucleus like planets rotating around
the sun.
• Bohr refined the model by suggesting that electrons move around
the nucleus in fixed pathways called electron shells. The exact
path and position of electrons could not be determined precisely,
but their energy level could be determined.
The Electron Cloud Model
This theory suggests that there is an area around the nucleus
where electrons are most likely to be found.
This area is called the electron cloud
This theory suggests that the electron’s behaviour can be
described with equations very similar to those that describe
The answers to the equations give the probability of finding
the electrons at certain places in the atom