Topic 1 Atomic Structure and Periodic Table Flashcards
C1 and C2
What is an atom?
An atom is the smallest part of an element. The atom is the smallest part of an element that exists even though it can be divided into smaller particles.
This is because the atom is the smallest part of an element that still retains the properties of the element - the subatomic particles do not.
What is an element?
It is a substance made of only one type of atoms
How many elements are there?
Approx 100
Elements can be classified in 2 groups based on their properties. What are these groups?
Metals and non metals
What is a compound?
A compound is a pure substance made up of two or more elements chemically combined and which cannot be separated by physical means. The properties of compounds are usually quite different from the elements that form them
What is a mixture?
- A mixture consists of two or more elements or compounds not chemically combined together;
- Each constituent of the mixture retains its chemical properties
- The parts of a mixture are not chemically bonded together and so they can be separated by physical means
- The choice of the method of separation depends on the nature of the substances being separated
- All methods rely on there being a difference in a physical property such as the boiling point or solubility, between the substances being separated
What are the 5 methods through which mixtures can be separated? Do these involve chemical reactions?
- Filtration
- Crystallisation
- Simple distillation,
- Fractional distillation
- Chromatography
They DO NOT involve chemical reactions
What is Filtration?
- Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g., sand from a mixture of sand and water)
- Centrifugation and decanting can also be used for this mixture
- A piece of filter paper is placed in a filter funnel above a beaker
- A mixture of insoluble solid and liquid is poured into the filter funnel
- The filter paper will only allow small liquid particles to pass through as filtrate
- Solid particles are too large to pass through the filter paper so will stay behind as a residue
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What is crystallisation?
- Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g., copper sulphate from a solution of copper (II) sulphate in water)
- The solution is heated which allows the solvent to evaporate and leaves a saturated solution
- To test if the solution is saturated a clean, dry, cold glass rod is dipped into the solution.
If the solution is saturated, crystals will form on the glass rod. - The saturated solution is then allowed to cool slowly
- Crystals begin to grow as solids come out of solution due to decreasing solubility
- The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry
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What is simple distillation?
This is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids
- The solution is heated and a liquid evaporates producing a vapour which rises through the neck of the round-bottomed flask (e.g. for saltwater, this would be
water boiling at 100 C)
- The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker
After all the liquid is evaporated from the solution, only the solid solute will be left behind
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What is Fractional Distillation?
- This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two)
- The solution is heated to the temperature of the substance with the lowest boiling point
- This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be
collected in a beaker - All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
- For water and ethanol: Ethanol has a boiling point of 78 ºC and water of 100 ºC; The mixture is heated until it reaches 78 ºC, at which point the ethanol boils
and distills out of the mixture and condenses into the beaker - When the temperature starts to increase to 100 ºC heating should be stopped.
Water and ethanol are now separated
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What is paper chromatography?
- This technique is used to separate substances that have different solubilities in a given solvent (e.g., different coloured inks that have been mixed to make black ink)
- A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along
with the samples - The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent, so the samples don’t wash into the solvent
container - The solvent travels up the paper by capillary action, taking some of the coloured substances with it
- Different substances have different solubilities so will travel at different rates, causing the substances to spread apart
- Those substances with higher solubility will travel further than the others
- This will show the different components of the ink / dye
- If two or more substances are the same, they will produce identical chromatograms
- If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
- An impure substance will show up with more than one spot, a pure substance should only show up with one spot
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Chromatography Exam Tip
Exam Tip
Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the visual output of a chromatography run. This is the piece of chromatography paper with the
visibly separated components after the run has finished.
The initial line must be drawn in pencil because if you used ink this would smudge or run in the water!
The solvent level must not start above the pencil line, or this will ruin the chromatogram.
Describe the plum-pudding model
- In 1897, physicist J.J. Thomson discovered the electron
Using a cathode-ray tube, he conducted an experiment which identified the electron as a negatively charged subatomic particle, hence proving that atoms are divisible - Based on his investigations, Thomson proposed a model of the atom known as the plum pudding model which depicted negative electrons spread throughout soft globules of positively charged material
Describe the Bohr model
- In 1913, Niels Bohr further developed the nuclear model by proposing that electrons orbit the nucleus in fixed shells or orbitals located at set distances from
the nucleus - Each orbital has a different energy associated with it, with the higher energy orbitals being located further away from the nucleus
- This model solved the question of why the atom does not collapse inwards due to the attraction between the positive nucleus and negative electrons circling the
nucleus - Bohr’s theory and calculations agreed with experimental results
- Further investigation and experimentation revealed that the nucleus could be divided into smaller particles, each one having the same mass and charge
- This work led to the discovery of the proton
How was the neutron discovered?
- In 1920, Rutherford put forward the idea of the existence of large, neutral particles within the nucleus
- His idea was based on the differences between the atomic mass and the atomic number of atoms
- In 1932, James Chadwick published a paper based on an experiment carried out by Frédéric and Irène Joliot-Curie which provided evidence for the existence of these neutral particles which were called neutrons
Describe the structure of the atom
- Protons, neutrons and electrons are the subatomic particles that make up atoms
- Each of the subatomic particles has a different electric charge, which is: Protons +1, Neutron 0, Electron -1
Why are atoms neutral overall?
Even though they contain charged subatomic particles, the charges cancel each other out in an atom, as they have the same number of electrons and protons
What is the radius of an atom?
0.1 nm
What is the name given to the number of protons in the nucleus?
Atomic number
What is the relative mass of protons, neutrons and electrons?
Protons and Neutrons 1, Electrons very small
Most of the atom is actually empty space, with the mass being concentrated in the nucleus and the electrons orbiting in shells around it.
What is the atomic number?
The atomic number (or proton number) is the number of protons in the nucleus of an atom.
The symbol for this number is Z.
It is also the number of electrons present in an atom and determines the position of the element on the periodic table
The proton number is unique to each element, so no two elements have the same number of protons
Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction
What is the Mass Number?
The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
The symbol for this number is A
The nucleon number minus the proton number gives you the number of neutrons of an atom
Note that protons and neutrons can collectively be called nucleons
The atomic number and mass number for every element is on the periodic table
How do you calculate the number of neutrons using mass number and atomic number?
Mass Number - Atomic number = Number of Neutrons
What is an isotope?
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
What is the chemical symbol for isotopes?
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8).It can also be written as C 14
Do Isotopes display the same chemical characteristics?
Isotopes display the same chemical characteristics.
This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry.
The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only.
How are PEN numbers calculated?
- The atomic number is equal to the number of protons (p) in an atom
- Since atoms are neutral, then it is also the same as the number of electrons (e)
Atomic number = number of protons AND
Atomic number = number of electrons - The mass number is the number of protons plus neutrons
- The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
For example, beryllium has an atomic number of 4, therefore it has 4 protons and 4 electrons.
The mass number of beryllium is 9, so it has 9 - 4 = 5 neutrons
The PEN numbers for beryllium are thus:
p = 4
e = 4
n = (9 - 4 =) 5
How are PEN numbers calculated for Ions?
- The same process can be applied to ions but the charge of the ion has to be considered
- For example, the fluoride ion can be represented as 9
19 F− - The fluoride ion has an atomic number of 9, therefore it has 9 protons and 9 electrons
- But, it has a 1- charge which means that it has gained one more electron
- The mass number of the fluoride ion is 19, so it has 19 - 9 = 10 neutrons
- The PEN numbers for the fluoride ion are thus:
p = 9
e = (9 + 1 =) 10
n = (19 - 9 =) 10 - For example, the magnesium ion can be represented as 12 24Mg2+
- The magnesium ion has an atomic number of 12, therefore it has 12 protons and 12 electrons
- But, it has a 2+ charge which means that it has lost two electrons
- The mass number of the magnesium ion is 24, so it has 24 - 12 = 12 neutrons
-The PEN numbers for the magnesium ion are thus:
p = 12
e = (12 - 2 =) 10
n = (24 - 12 =) 12
