Chemistry December Assessment Revision
Relative atomic mass
Relative atomic mass is the mass of the element based off of the mass it has of its isotopes. For
example, 75% of the chlorine atoms in existence is Cl 35. Around 25% of the chlorine atoms in
existence is Cl37.
Relative Mass Formula:
Relative Atomic Mass = (Ar x %) + (Ar2 x %) / 100
e.g. Relative atomic mass = (35 x 75%) + (37 x 25%)/100
= 35.5
Therefore Chlorine has a relative atomic mass of 35.5.
Isotopes
Isotopes are atoms which have the same number of protons and electrons but different number of
neutrons. Isotopes only have different masses and apart from that they are the exact same to
another isotope of the same element.
Nearly all the elements in the Periodic Table have at least 1 isotope.
History of the atom
Democritus, a Greek philosopher, thought that stuff was made out of tiny particles with no space
around it. He called it ‘Atomos,’ Greek for indivisible. However, popular philosopher Aristotle
opposed his theory, stating instead that there are only four elements: air, water, wind and fire. His
theory became forgotten until 1808.
In 1808, a quaker with the name John Dalton sought to challenge the Aristotelian theory. Even
though Democritus’ theory was theoretical, Dalton found out that common substances always broke
down into same elements in the same proportion. He said that they each had a particular size and
mass and could not be created or destroyed. He made the Dalton model of the atom: hard spheres
that could not be broken down. Atomic theory had now been accepted into scientific community.
J.J. Thompson, a physicist, who in 1897 had discovered the electron. He made the plum pudding
model, which contained a ball of positive charge with negative charges studded in it.
Rutherford had performed the Rutherford Scattering where he shot alpha particles at a very thing
gold foil. He expected for all the particles to pass through. Some of the particles did go through,
however some particles were sent bouncing off nearly reaching 180°. He concluded that atoms
mostly consisted of empty space and that majority of the mass of the atom was in the centre, which
he called the nucleus nicknaming his model the ‘Nuclear Model’.
Relative atomic mass
Relative atomic mass is the mass of the element based off of the mass it has of its isotopes. For
example, 75% of the chlorine atoms in existence is Cl 35. Around 25% of the chlorine atoms in
existence is Cl37.
Relative Mass Formula:
Relative Atomic Mass = (Ar x %) + (Ar2 x %) / 100
e.g. Relative atomic mass = (35 x 75%) + (37 x 25%)/100
= 35.5
Therefore Chlorine has a relative atomic mass of 35.5.
Isotopes
Isotopes are atoms which have the same number of protons and electrons but different number of
neutrons. Isotopes only have different masses and apart from that they are the exact same to
another isotope of the same element.
Nearly all the elements in the Periodic Table have at least 1 isotope.
History of the atom
Democritus, a Greek philosopher, thought that stuff was made out of tiny particles with no space
around it. He called it ‘Atomos,’ Greek for indivisible. However, popular philosopher Aristotle
opposed his theory, stating instead that there are only four elements: air, water, wind and fire. His
theory became forgotten until 1808.
In 1808, a quaker with the name John Dalton sought to challenge the Aristotelian theory. Even
though Democritus’ theory was theoretical, Dalton found out that common substances always broke
down into same elements in the same proportion. He said that they each had a particular size and
mass and could not be created or destroyed. He made the Dalton model of the atom: hard spheres
that could not be broken down. Atomic theory had now been accepted into scientific community.
J.J. Thompson, a physicist, who in 1897 had discovered the electron. He made the plum pudding
model, which contained a ball of positive charge with negative charges studded in it.
Rutherford had performed the Rutherford Scattering where he shot alpha particles at a very thing
gold foil. He expected for all the particles to pass through. Some of the particles did go through,
however some particles were sent bouncing off nearly reaching 180°. He concluded that atoms
mostly consisted of empty space and that majority of the mass of the atom was in the centre, which
he called the nucleus nicknaming his model the ‘Nuclear Model’.
