-
all elements are made up of
atoms
-
atoms are made up of 3 types of particle
protons , neutrons and electrons
-
draw a diagram to show how protons , neutrons and electrons are arranged in the atom
x
-
electrons have a .... charge
1-
-
electrons whizz around the nucleus in
orbitals . the orbitals take up most of the volume of the atom
-
most of the mass of the atom is concentrated in the
nucleus
-
the nucleus of the atom is .... compared to the whole atom
small
-
the nucleus is where you find
the protons and neutrons
-
the mass and charge of these subatomic particles is
really small so relative mass and relative charge is used instead
-
draw a table to show the relative masses and charges or protons , neutrons and electrons
x
-
you can figure out the number of protons , neutrons and electrons from the
nuclear symbol
-
annote a diagram of the nuclear symbol
x
-
mass number
this is the total number of protons and neutrons in the nucleus of an atom
-
atomic number
this is the number of protons in the nucleus of an atom - it identifies the element
-
all atoms of the same element have the same number of
protons
-
sometimes the .... ... is let out of the nuclear symbol . you don't really need it because the elements symbol tells you its value
atomic number
-
the model of the atom is useful for
understanding loads of ideas in chemistry
-
in the past completely different models of the atom were accepted because they
fitted the evidence available at the time . as scientists did more experiments , new evidence was found and the models were modified to fit it
-
at the start of the 19th century john dalton described atoms
as solid spheres made up of the different elements
-
in 1897 JJ Thompson concluded from his experiment
that atoms weren't solid and indivisible . his measurements of charge and mass showed that an atom must contain even smaller , negatively charged particles - electrons . the 'solid sphere' idea of atomic structure had to be changed the new model . the new model was known as the "plum pudding model"
-
draw a diagram to show Dalton's model of the atom
-
draw a diagram to show Thomson's model of the atom
-
in 1909 Ernest Rutherford an his students Hans Geiger and Ernest Marsden conducted the famous
- gold foil experiment
- they fired alpha particles which are positively charged at an extremely thin sheet of gold .
-
what were the scientists expected from the gold foil experiment
from the plum pudding model they were expecting most of the alpha particles to be deflected very slightly by the positive pudding that made up most of an atom . In fact most of the alpha particles passed straight through the gold atoms , and a very small number were deflected backwards . so the plum pudding model couldn't be right . So Rutherford came up with a model that could explain this new evidence - the nuclear model of the atom . In this there's a tiny positively charged nucleus at the centre , surrounded by a cloud of negative electrons - most of the atom is empty space
-
draw a diagram to show what happened in the gold foil experiment and draw another diagram to show rutherford's model of the atom
-
how does scientific knowledge develop
new evidence prompts people to come up with new , improved ideas . Then other people go through each new improved idea with a fine tooth-comb as well - modern peer review is a part of this process
-
there were quite a few other modifications to the model before we got to our currently accepted versions . Neils Bohr got pretty close though . Scientists realised that electrons in a cloud around the nucleus of an atom , as Rutherford described , would
quickly spiral down into the nucleus , causing the atom to collapse .
-
Niels Bohr proposed a new model of the atom with four basic principals
- electrons only exist in fixed orbitals (shells) and not anywhere between
- each shell has a fixed energy
- when an electron moves between shells electromagnetic radiation is emitted or absorbed
- because the energy of the shells is fixed the , the radiation will have a fixed frequency
- the frequencies of radiation emitted and absorbed by atoms were already known from experiments . The Bohr model fitted these observations - it looked good
-
annotate a diagram to show Borhs model of the atom
-
one of the things that makes a theory scientific is that it's
falsifiable - you can make predictions using the theory , then if you test predictions and they turn out to be wrong , you know the theory's wrong
-
the refined Bohr model
scientists discovered that not all the electrons in a shell had the same energy . this meant that the Bohr model wasn't quite right . So , they refined it to include sub-shells . The Bohr model also explained why some elements (the noble gases) are inert . Borh said that the shells of an atom can only hold fixed numbers of electrons , and that an elements reactivity is due to its electrons . so when an atom has full shells of electrons it's stable and doesn't react
-
we know that the Borh model isn't prefect - but it's still widely used to describe atoms because
it's simple and explains many observations from experiments , like bonding and ionisation trends
-
the most accurate model we have today involves
complicated quantum mechanics . Basically you can never know where an electron is or which direction it's going in at any one moment , but you can say how likely it is to be at a certain point in the atom . Also electrons can act as waves as well as particles
-
the quantum model might be more accurate , but
it's a lot harder to get your head around and visualise . It does explain some observations that can't be accounted for by the Bohr model though . So scientists use whichever model is most relevant to whatever they're investigating
-
draw a diagram to show the quantum model
-
ionisation
when electrons have been removed from an atom or molecules , it's been ionised .
-
the energy you need to remove the first electron is called the
first ionisation energy
-
the first ionisation energy is the
energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous uni positive ions
-
here are a few points about ionisation energies
- you must use the gas state symbol , because ionisation energies are measured for gaseous atoms
- always refer to 1 mole of atoms , as stated in the definition , rather than to a single atom
- the lower the ionisation energy the easier to form an ion
-
a high ionisation energy means there's a
high attraction between the electron and the nucleus .
-
there are three things which can affect ionisation energy
- nuclear charge : the more protons there are in the nucleus , the more positively charged the nucleus is and the stronger the attraction for the electrons (you can only see the effect of nuclear charge on ionisation energy if you're looking at atoms with outer electrons that are the same distance from the nucleus and with equal shielding effects . This only really happens when you're looking at elements in the same period of the periodic table)
- distance from the nucleus : attraction falls rapidly with distance . An electron close to the nucleus will be more strongly attracted than one further away
- shielding : as the number of electrons between the outer electron and the nucleus increases , the outer electrons feel less attraction towards the nuclear charge. This lessening pull of the nucleus by inner shells of electrons is called shielding or screening
- repulsion : the repulsion between two electrons in an orbital means that electrons are easier to remove from shared orbitals (only take into consideration repulsion if shielding , nuclear charge and distance have the same effect on both moles of elements)
-
why does lithium have a higher first ionisation energy than sodium
- there are only two electrons between the nucleus and the outer electron in a lithium atom whereas there are 10 electrons between the nucleus and outer electron in a sodium atom- this means the shielding effect is greater for sodium
- also the distance between the nucleus and electron being removed is greater in the sodium atom
-
the second ionisation energy is the
energy needed to remove an electron from each ion in 1 mole of gaseous 1+ ions
-
just like the first ionisation energy , the value of the second ionisation energy depends on nuclear charge , the distance of the electron from the nucleus and shielding effect of inner electrons . second ionisation energies are .... than first ionisation energies because
- higher
- the electron is being removed from a positive ion and not an atom , which will require more energy . The electron configuration of the atom will also play a role in how much larger the second ionisation energy is than the first
-
explain why the second ionisation energy of lithium is much higher than the first
the first electron removed from lithium is in the second shell ( 2s1 ) and the second is in the first shell ( 1s2 ) . so the electron being removed during the second ionisation is closer to the nucleus and will experience a stronger nuclear attraction than the electron that is removed during the first ionisation . this means that the second ionisation energy of lithium is much higher than the first
|
|
