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Topic 1 – Atomic structure and the periodic table
In the 1800s, a chemist called Dmitri Mendeleev used the properties of elements
known at the time to organise them into a table:
o Mendeleev arranged the elements in order of increasing atomic mass (i.e in
order of increasing numbers of protons and neutrons – see below)
o Mendeleev organised elements with similar properties into vertical
columns (called ‘groups’)
Unlike other chemists before him, Mendeleev:
o sometimes broke the ‘increasing atomic mass rule’
 e.g he switched tellurium and iodine around so that they would be
in the same groups as elements with similar properties (i.e by
switching them, iodine was next to bromine, chlorine, fluorine…)
o realised from the big jumps in atomic mass that there were still some
elements to discoverleft some gaps in his table
 E.g he left two gaps between zinc and arsenic
 Based on the known elements around them, Mendeleev predicted
the properties of the elements which should go in the gaps
 In the modern periodic table, these gaps have been filled by
gallium and germanium – they have very similar properties
to those predicted by Mendeleev
Atoms are the smallest particles of an element that can take part in chemical
The 3 subatomic particles – protons, neutrons, electrons:
At the centre of each atom is a nucleus containing protons and neutrons
Note: the nucleus is about 20,000 times smaller than the overall size of the atom
Electrons are arranged in shells (or ‘energy levels’) at different distances from the
The masses and charges of subatomic particles are very smallthey’re compared
to those of a proton – these are called the ‘relative mass’ and the ‘relative charge’
Subatomic particle
Relative mass
Relative charge
Negligible (i.e 0)
Atoms and elements:
All atoms contain the same number of protons and electronsatoms have no
overall charge
No two elements have the same number of protons in their atoms…e.g hydrogen
has 1 proton, helium has 2 protons etc…
Elements can have more than one atom…:
o Different atoms of the same element always have the same number of
protons (the same number of electrons), but a different number of
neutrons – these are called isotopes (see below)
Atomic number – number of protons in the nucleus of an atom
Mass number – total number of protons and neutrons in the nucleus of an atom
In a chemical symbol, e.g
, the atomic number is the bottom number (the
smaller one) and the mass number is the top number (the bigger one)
From the symbol, we can calculate the number of protons, neutrons and electrons
in an atom…e.g for
o Atomic number is 1313 protons
o Mass number – atomic number = number of neutrons…27 - 13 = 14
o Number of protons = number of electrons13 electrons
The elements in the modern periodic table are arranged in order of increasing
atomic number rather than in order of increasing atomic mass as Mendeleev did
The horizontal rows are called periods
The vertical columns are called groups:
o Each group contains elements with similar properties
o The main groups are numbered 1-7 from left to right
o The group on the far right is called group 0
Diagram of periodic table below:
o Elements to the left of the jagged line are metals
o Elements to the right of the line are non-metals
Relative atomic mass (Ar):
Atoms have very small massesrelative atomic mass is used instead of its actual
mass in kilograms
Relative atomic mass is the mass of an atom compared to that of carbon:
o Carbon has a relative atomic mass of 12
o Mass of a helium atom is one third that of carbonits relative atomic
mass is 4
In the periodic table, each element has its atomic number (the bottom number) and
its relative atomic mass (the top number) shown
Isotopes are different atoms of an element with the same number of protons and
electrons, but different numbers of neutrons (i.e same atomic number, different
mass number)
E.g chlorine atoms always have 17 protons, but some can have 18 neutrons and
others can have 20 neutrons – these are
The presence of isotopes means that some relative atomic masses aren’t whole
o Relative abundance (proportion) of
is 75% (or 0.75)
o Relative abundance of
is 25% (or 0.25)
o Relative atomic mass = (proportion in decimals x mass number) +
(proportion in decimals x mass number)
o relative atomic mass of Cl = (0.75 x 35) + (0.25 x 37) = 35.5
Relative atomic mass vs mass number:
The mass number is the mass of an atom – i.e the number of protons and neutrons
(because electrons have negligible relative mass)
The relative atomic mass is the average mass of all the different atoms (isotopes)
of an element (relative to carbon), taking their abundance into account
In most cases, the relative atomic mass of an element is very similar to the mass
number of the most common isotope (e.g in the above example, relative atomic
mass of Cl is 35.5…mass number of most common isotope is 35)
Electrons are arranged in shells around the nucleus of the atom
Each shell is shown as a circle drawn around the chemical symbol for the atom
The way in which electrons are arranged in an atom is called its electronic
Finding configurations:
Different shells can contain different numbers of electrons…
For the first 20 elements:
o The first shell can contain up to 2 electrons
o The second and third shells both hold up to 8 electrons
The first shell fills up first, then the second shell, and so on…
Elements which have the maximum number of electrons in their outer shells are
said to have ‘full outer shells’
Electronic configurations can be worked out using atomic numbers…e.g:
o Atomic number of sodium is 11it has 11 protons11 electrons
o 2 electrons in first shell
o 8 electrons in second shell
o 1 electron in third (outer) shell
This can be represented by a diagram:
Its electronic configuration can also be written in the form ‘2.8.1’, whereby…:
o The numbers show how many electrons are in each shell
o The dots separate each shell
Spotting connections:
1. The number of occupied shells is the same as the period number:
o E.g magnesium is in period 3 of the periodic table…its configuration is
‘2.8.2’…has 3 occupied shells
2. The number of outer electrons is the same as the group number (apart from
elements in group 0 which all have full outer shells):
o E.g magnesium is in group 2 of the periodic table…its configuration is
‘2.8.2’…has 2 electrons in its outer shell