OCR AS Chemistry F331: Atoms

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Relative Atomic Mass
The link between the mass of an element and the number of atoms it contains is the relative atomic
mass of the element.
The relative atomic mass of an element is the mass of its atom relative to carbon-12
The relative atomic mass scale is used to compare the masses of different atoms.
Approximate relative atomic masses are used most of the time.
12 relative atomic mass
C
6
Relative Formula Mass
The relative formula mass of a substance is the mass of its formula unit relative to carbon-12
The relative formula mass of a substance can be worked out by first writing the formula of the
substance and then adding together the relative atomic masses of each of the atoms in the formula
e.g. C2H6O = (2x12) + (6x1) + 16 = 46
For methane: CH4
relative atomic mass = (1x12) + (4x1) = 16
For calcium nitrate: Ca(NO3) 2
relative formula mass = (1x40) + (2x14) + (6x16) = 164

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Moles
Amount of substance is measure using a unit called the mole.
A mole of a substance is the amount of substance which contains as many formula units as there are
atoms in 12g of carbon.
One mole of atoms or molecules of any substance will have a mass in grams equal to the relative
formula mass for that substance.
E.g.…read more

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Empirical and Molecular Formulas
The empirical formula gives just the smallest whole number ratio of atoms in a compound
The molecular formula gives the actual numbers of atoms in a molecule
The molecular formula is made up of a whole number of empirical units
E.g. a molecule has an empirical formula of C4H3O2 and a molecular mass of 166
Work out its molecular formula
1.…read more

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Atoms
The Atom
Atoms are made up of protons, neutrons and electrons
Electrons move around the nucleus in shells. These shells take up
most of the volume of an atom.
Most of the mass of the atom is concentrated in the nucleus.
The nucleus is where you find the protons and electrons.…read more

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Atomic Models
Dalton's Theory ­ 1803
Formed the atomic theory, which stages that all matter is composed of tiny
indestructible particles called atoms that are all alike and have the same atomic weight.…read more

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Nuclear Reactions
Some isotopes of some elements are unstable. Their nuclei break down spontaneously, and they
are described as being radioactive. These isotopes are called radioisotopes. As these nuclei break
down, they emit rays and particles called emissions. This breakdown (or radioactive decay) occurs
of its own accord ­ it isn't triggered off by something we do.
Some isotopes decay quickly; for others the process takes thousands of years.
Not all unstable atoms decay in the same way.…read more

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Nuclear Equations
Alpha Decay
Involves the emission of alpha particles which helium nuclei.
The isotope produced by alpha decay will have a mass number four units lower and a
nuclear charge two units lower than the original atom
E.g. 238 234 4
U Th + He
92 90 2
Beta Decay
Involves the emission of electrons
During Beta decay, the mass number remains constant but the nuclear charge
increases by one unit
E.g.…read more

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Nuclear Fusion
In a nuclear fusion reaction, two light atomic nuclei fuse together to form a single,
heavier nucleus of a new element. The process release enormous quantities of energy.
Fusion reactions only occur at very high temperatures, such as those found in the sun
and other stars. For two nuclei to fuse, the must come very close together.
At normal temperatures found on earth, the positive nuclei repel one another so strongly
that this cannot happen.…read more

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Electronic Structure
Electrons move around the nucleus in shells (sometimes called energy levels)
Shells further from the nucleus have a greater energy level than shells closer to the
nucleus
Shells can only hold so many electrons:
Electron Shell 1st 2nd 3rd 4th
Maximum No. 2 8 18 32
of electrons
A shell which contains its maximum number of electrons is called a filled shell.…read more

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