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atomic structure and the periodic table (group 7 elements (Trends (become…
atomic structure and the periodic table
atoms
atoms contain protons, neutrons and electrons
atoms have radius of 0.1 nanometers (1 x 10-10m)
the nucleus
it contains neutrons and protons
the nucleus has radius of 1 x 10-14 (1/10 000 of the radius of whole atom
it has a positive charge
almost the whole mass of the atom is concentrated at the nucleus
the electrons
orbit nucleus in shells
they are negatively charged and tiny but cover a lot of space in atom
the volume of their orbits determines the size of the atom
electrons have a negligible mass
number of protons= number of electrons
atoms are neutral- have no overall charge
this is because they have the same number of electrons and protons
the charge on the electrons is the same size as the charge on the protons but opposite so they cancel out
in an ion the number of protons does not equal the number of electrons - so it has an overall charge
atomic number and mass number describes an atom
the nuclear symbol of an atom tells you its atomic (proton) number and mass number
the atomic number tells you how many protons there are
the mass number tells you the total number of protons and neutrons in the atom
to get the number of neutrons: mass number - atomic number
elements
elements consist of atoms with the same atomic number
atoms can have different numbers of protons, neutrons and electrons - its the number of protons in the nucleus that decides what type of atom it is
if a substance only contains atoms with the same number of protons its called an element
so all the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons
isotopes are the same except for extra neutrons
isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons
so isotopes have the same atomic number but a different mass number
relative atomic mass is used instead to refer to the element as a whole. this is an average mass taking into account the different masses and abundances of all isotopes of one element
relative atomic mass = sum of (isotope abundance x isotope mass number) / sum of abundances of all isotopes
compounds
atoms join together to make a compound
when elements react atoms combine with other atoms to form compounds
compounds= substances formed from two or more elements and are held together by chemical bonds
making bonds involves giving away, receiving or sharing electrons - only electrons are involved
its normally difficult to separate the original elements from compound again- a chemical reaction is needed to do this
a compound which is formed from a metal and a non metal consists of ions. the metal loses electrons to get positive ion and non metal gains electrons to get negative ion. the opposite charges of ions means they are strongly attracted - ionic bonding
a compound formed from non metals consists of molecules. each atom shares electrons with another atom- covalent bonding.
properties of compounds are usually different from original elements
mixtures and chromotography
mixtures are easily separated- not like compounds
unlike compounds theres no chemical bond between different parts of a mixture
can be separated by : filtration, crystallization, simple distillation, fractional distillation, chromatography
the properties of a mixture are just a mixture of properties of the separate parts- the chemical properties aren't effected by being in a mixture
chromotography
( look at practical rev cards)
more separation techniques
filtration- separates insoluble solids from liquids
filtration can be used if your product is an insoluble solid that needs to be separated from a liquid reaction mixture
it can be used in purification. e.g. solid impurities can be filtered out by filtration
two ways to separate soluble solids from solutions
evaporation
a really quick method of separating soluble salt from a solution but can only be used if the salt doesn't decompose when its heated
(look at method on practical rev cards)
crystallization
look at method on practical rev cards
filtration and crystallization can be used to separate rock salt
rock salt is a mixture of salt and sand
salt and sand are both compounds- but salt dissolves in water and sand doesn't.
look at method on rev cards
distillation
simple distillation is used to separate out solutions
simple distillation is used for separating out a liquid from a solution
problem= can only be used to separate things with different boiling points- if temp goes higher than boiling point of substance with the higher boiling point, they will mix again
look at practical rev cards
fractional distillation is used to separate a mixture of liquids
look at practical rev cards for method
the history of the atom
the theory of atomic structure has changed over time
start of 19th century- John Dalton- described atoms as solid spheres and said that different spheres make up different elements
in 1897- JJ Thompson- concluded that atoms weren't solid spheres. his measurements of charge and mass showed that an atom must contain even smaller negative particles- electrons
plum pudding model- showed an atom as a ball of positive charge with electrons embedded in it
Rutherford showed that plum pudding was wrong
in 1909- Ernest Rutherford- conduced the alpha particle scattering experiments (fired positive alpha particles at a thin sheet of gold)
from plum pudding they expected all particles to pass through. although most did got through, some were deflected slightly and a small number were deflected backwards
Rutherford created nuclear model of atom. positive charged nucleus (mass concentrated there) and negative electrons surrounding the nucleus - most of atom was empty space
further experiments showed existence of protons
Rutherford and others showed that the nucleus can divide into smaller particles with a positive charge- protons
James Chadwick- 20 years later- found existence of neutral particles in the nucleus (neutrons).
electronic structure
electron shell rules
electrons always occupy shells
the lowest energy levels are always filled first
only a certain number of electrons are allowed in each shell (1st shell= 2, 2nd shell = 8)
in most atoms the outer shell is not full and this makes the atom want to react to fill it
development of the periodic table
in the early 1800's elements were arranged by atomic mass
elements were categorized by : atomic mass, physical and chemical properties
1817- Dobereiner creates law of triads
1863- Newlands law of octaves: arranged table in order of atomic mass, found that each element was similar to the one 8 places along e.g. Li and Na, showed a repeating or periodic pattern of properties, was not accepted by scientists as he put iron in the same group as oxygen and sulfur
Dimitri Mendeleev left gaps and predicted new elements
in 1869
Mendeleev put elements in order of atomic mass but switched order if properties meant it should be changed.
gaps were left to make sure that elements with similar properties stayed in the same group- some gaps indicated the existence of future elements and when they were discovered they fitted pattern
the discovery of isotopes in the early 20th century showed that Mendeleev was not correct
in 1911- ordered by atomic number by ernest rutherford
modern periodic table
the periodic table helps you to see patterns in properties
there are 118 elements
elements are laid out in order of increasing atomic number- arranging it like this means there a repeating patterns in properties of the elements
metals are on left and non metals are on left
elements with similar properties form columns called groups
the group number tells you how many electrons are in the outer shell
the rows are called periods, each new period represents another full shell of electrons
elements in the same group have similar properties
metals and non metals
most elements are metals
metals are elements which can form positive ions when they react
they are located towards left and bottom of periodic table
non metals are at the far right and top of the periodic table
non metals don't generally form positive ions when they react
the electronic structure of atoms effects how they will react
atoms react to form a full outer shell, by sharing, gaining or losing electrons
metals to the left of the periodic table don't have many electrons to remove and metals towards the bottom have outer electrons which are a long way from the nucleus so feel a weaker attraction
both these effects means that not much energy is required to remove electrons to form a positive ion
for non metals forming positive ions is more difficult as they have lots more electrons in their outer shell and are closer to the nucleus so feel a strong force of attraction to the nucleus. so its easier for them to share or gain electrons
metals and non metals have different physical properties
all metals have metallic bonding which causes them to have similar basic properties
theyre strong, malleable, good conductors of heat and electricity, have high melting/ boiling points
non metals don't have metallic bonding so don't have same properties as metals
theyre dull looking, more brittle, aren't always solids, don't generally conduct heat or electricity, have a lower density
transition metals
are in centre of the periodic table
have typical metal properties: dense, strong, shiny, conduct heat and electricty
unique properties to transition metals: can have more than one ion, often form coloured compounds, make good catalysts
group 1 elements
are reactive, soft metals
they are alkali metals
they all have 1 electron in their outer shell which makes them very reactive and gives them similar properties
are soft and have low density
trends for the metals as you go down the group:
-increasing reactivity: the outer electron is more easily lost as the attraction between nucleus and electron decreases, because the electron is further away from the nucelus
alkali metals form ionic compounds with non metals
group 1 elements don't need much energy to lose their outer electron
the ionic compounds they form are generally white solids that dissolve in water to form colourless solutions
reaction with water
they react vigorously
produce hydrogen gas and metal hydroxides (salt that can dissolve in alkaline solutions)
the more reactive an element is the more violent the reaction
the amount of energy given out in the reaction increases down the group
reaction with chlorine (halogen)
react vigorously when heated in chlorine gas to form white metal chloride salts
reactivity increases down group
reaction with oxygen
react to form metal oxide
different types of oxide will form depending on group 1 metal
have different properties to transition metals
group 1 metals are more reactive that transition metals
theyre much less dense, strong and hard that transition metals and have lower melting/ boiling points
group 7 elements
the halogens are all non metals with coloured vapours
fluorine is very reactive, poisonous yellow gas
chlorine is a fairly reactive, poisonous dense green gas
bromine is a dense, poisonous red- brown volatile liquid
iodine is a dark grey crystalline solid or purple vapour
THEY ARE ALL DIATOMIC
Trends
become less reactive as you go down group- harder to gain extra electron because the outer shell is further from nucleus
have higher melting/ boiling points as you go down group
have higher relative atomic masses
halogens can form molecular compounds
halogen atoms can share electrons via COVALENT BODNING with other non metals to achieve a full outer shell
these compounds all have simple molecular structures
halogens form ionic bonds with metals
the halogens form 1- ions called halides when they bond with metals
the compounds that form have ionic structures
more reactive halogens will displace less reactive ones
a displacement reaction an occur between a more reactive halogen and a salt of a less reactive one
e.g. Cl2 + 2KI---> I2 + 2KCl
group 0 elements
group 0 elements are all inert, colourless gases
called the noble gases
they all have 8 electrons in their outer shell giving them a full outer shell. as their outer shell is energetically stable they don't need to give up or gain electrons - this means they are inert
they are all monatomic gases- single atoms not bonded to each other
all colourless gases at room temp
are non flammable
patterns in properties
the boiling points and relative atomic mass of the noble gases increase as you move down the group
they increase in boiling point due to an increase in the number of electrons in each atom leading to greater intermolecular forces between them which need to be overcome