Topic 1- Atomic structure and the periodic table Flashcards
What are all substances made of?
All substances are made of atoms.
What is an atom?
An atom is the smallest part of an element that can exist-so it cannot be divided.
What is the radius of an atom?
Atoms have a radius of about 0.1 nanometers
thats 1 x 10-10 m
What do atoms contain?
Atoms contain a NUCLEUS-
1) It’s in the middle of the atom.
2) It contains PROTONS an NEUTRONS.
3) The nucleus has a radius of around 1 x 10-14 m).
4) It has a positive charge because of the protons.
5) Almost the whole mass of the atom is concentrated in the nucleus.
and ELECTRONS-
1) Move around the nucleus in electron SHELLS.
2) They’re negatively charged and tiny, but they cover lot of space.
3) The volume of their orbits determines the size of the tom.
4) Electrons have virtually NO mass.
What is the relative mass and charge of protons, neutrons and electrons?
Particle Relative Mass Charge
Proton 1 +1
Neutron 1 0
Electron Very small -1
What is the overall charge of an atom which contains and equal number of protons and electrons and why?
1) Atoms are NEUTRAL- they have NO CHARGE overall (unlike ions).
2) This is because they have the SAME NUMBER of PROTONS as ELECTRONS.
3) The charge on the electrons is the SAME size as the charge of the PROTONS, but OPPOSITE- so the charges CANCEL OUT.
What is an ion?
An ion is an atom or group of atoms that has lost or gained electrons.
What is the charge of an ion?
In an ion, the number of protons DOESN’T EQUAL the number of electrons. This means it has an OVERALL CHARGE.
For example, an ion with a 2- charge, has two more electrons than protons.
How are atoms represented?
Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium.
What can you learn from the chemical symbol?
The chemical (or nuclear) symbol of an atom tells you its ATOMIC (proton) NUMBER and MASS NUMBER.
What does the mass and atomic number tell us?
Tip- the mass number is always larger than the atomic number except from hydrogen.
1) The ATOMIC NUMBER tells you how many PROTONS there are.
2) The MASS NUMBER tells you the TOTAL number of PROTONS AND NEUTRONS in the atom.
3) To get the number of neutrons, just subtract the atomic number from the mass number.
What is an element?
An element is a substance that contains only one type of atom.
So an element is a substance made up of atoms that all have the same number of protons in their nucleus.
N.b - all elements of the same type have the same atomic number i.e. the same number of protons.
How do atoms differ?
Atoms can have different numbers of protons, neutrons and electrons.
It’s the number of PROTONS in the nucleus that decides what type of atom it is.
E.g. an atom with one proton in its nucleus is hydrogen and an atom with two protons is helium.
What must a substance contain the same number of to make it an element?
A substance must only contain atoms with the same number of protons to make it an element.
How many elements are there?
Where are they shown?
There are about 100 different elements.
Elements are shown in the periodic table.
What are isotopes?
1) ISOTOPES are different forms of the same element, which have the SAME NUMBER of PROTONS but a different number of NEUTRONS.
2) So isotopes have the same atomic number but different mass numbers.
N.B- the number of neutrons is just the mass number minus the atomic number.
What is a common example of an isotope?
A very popular example of a pair of isotopes are carbon -12 and carbon-13.
Carbon-12 contains 6 protons
6 neutrons
12 6 electrons
C
6
Whereas Carbon -13 contains 6 protons
6 electrons
7 NEUTRONS 13
C
6
Therefore the IDENTITY of an atom depends on the number of PROTONS in its nucleus.
What is relative atomic mass and what is it used for?
Because many elements can exist as a number of different isotopes, RELATIVE ATOMIC MASS (Ar) is used instead of mass number when referring to the element as a whole.
The relative atomic mass (Ar) of an element is an average value that takes account of the abundance of the isotopes of the element.
In other words, the Ar is an average value for all the isotopes of that element. Numerically it is the same as the mass number.
How can you work out the relative atomic mass of an element?
Example:
Bromine atoms exist as the isotopes 79 and 81- there are 50% of each present in the universe, which is why in the periodic table, an average mass number of 80 is quoted.
Other isotopes are 35 with 37
Cl Cl
In this case, 25% are the 37 isotope, and 75% are the 35 isotope, and that is why the periodic table quotes the relative atomic mass number of Cl as 35.5:
% x mass % x mass
(25 x 37) + (75 x 35)
__________________ = 35.5
100
What is the relative atomic mass (Ar) formula?
relative atomic
mass =
(Ar) (isotope abundance x isotope mass number)
____________________________________
100
What are compounds?
How are they bonded?
Compounds are substances formed from TWO OR MORE elements, the atoms of each element are in fixed proportions throughout the compound and can be represented by formulae using the symbols of the atoms from which they were formed.
The elements are held together by chemical bonds.
How are compounds formed and how are bonds made?
Compounds are formed from elements by chemical reactions. When elements react, atoms combine with other atoms to form compounds.
Making bonds involves atoms giving away, taking or sharing ELECTRONS. Only the ELECTRONS are involved, the nuclei of the atoms aren’t affected at all when a bond is made.
How can compounds be separated?
It’s usually difficult to separate the original elements of a compound out again-compounds can ONLY be separated into elements by CHEMICAL REACTIONS (breaking and making bonds). Compounds cannot be separated using physical processes.
N.b- during a chemical reaction at least one new substance is made. You can usually measure a change in energy such as temperature change.
How can Chemical reactions be represented?
Chemical reactions can be represented by word equations or equations using symbols and formulae.
How are compounds made of metals and non-metals formed?
A compound which is formed from a metal and a non-metal consists of IONS. The METAL atoms LOSE electrons to form POSITIVE IONS and the non-metal atoms GAIN electrons to form NEGATIVE IONS.
The OPPOSITE CHARGES (positive and negative) of the ions mean that they’re strongly ATTRACTED to each other. This is called IONIC BONDING.
What do compounds made of only non-metals consist of?
How are they formed?
A compound formed from non-metals consists of molecules. Each atom SHARES an ELECTRON with another atom- this is called COVALENT BONDING.
What are the properties of compounds?
The PROPERTIES of a compound are usually TOTALLY DIFFERENT from the properties of the ORIGINAL ELEMENTS.
Compounds can be small molecules or great bug structures called lattices.
What is used to display the different types of atoms in a compound?
Compounds can be represented by formulas. The formulas are made up of elemental symbols in the same proportions that the elements can be found in the compound.
What occurs during chemical reactions?
During a chemical reaction, bonds between atoms break and the atoms change places- the atoms from the substances you start off with (the reactants) rearrange themselves to form different chemicals. These new chemicals are called the products. You can show what happens in a chemical reaction using equations.
What do word equations show?
These show what happens in a chemical reaction using the full names of the substances involved. They show the reactants and the products.
What do symbol equations show?
Symbol equations show exactly the same as word equations but using chemical symbols and formulas. However, when balanced correctly they also show the ratio of the amounts of substances involved in the reaction.
What are valencies?
The valence of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. It can be used to work out the formula of compounds.
How can you work out a valency?
The valencies of metals are found by which group number they are in. So hydrogen is in group 1 so has a valency of 1.
However, to find the valency of a non-metal between group 5-8 you have to subtract their group number by 8. So oxygen has the group number 6 so has 6 electrons in its outer shell. The valence of oxygen is 8-6 which is 2. So oxygen needs to gain 2 electrons to have a full outer shell and become stable.
Why is it necessary to balance equations?
Equations must be balanced because mass doesn’t disappear during a reaction- there are always the same atoms present at the end of a reaction as there are at the start. This is known as the law of conservation of mass.
How do you balance equations?
The number of atoms on the left hand side must equal the number of atoms on the right hand side. Rules:
1) Write the different types of elements below the left and right hand side.
2) Count up the numbers of atoms on each side and write them in.
3) Look for something that doesn’t balance and add large numbers to the equation so that it does balance.
4) Recount the number of atoms again and return to rule 2 until all the elements are balanced.
Tip-symbol equations show how many atoms of one element there are compared to the number of elements. So it’s fine to double, or triple, or quadruple the number of atoms in a balanced equation, as long as you do the same to every term in the equation.
Tip- You don’t have to use whole numbers to balance an equation.
What is a molecule?
A molecule is two or more atoms chemically bonded together. The atoms may be the same type (an elemental molecule) or of a different types (a compound e.g. H2O).
What is a mixture?
1) Unlike in a compound, there’s NO CHEMICAL BOND between the different parts of a mixture.
2) The parts of a mixture can either be ELEMENTS or COMPOUNDS, and they can be easily separated out by PHYSICAL METHODS such as filtration, crystallisation and chromatography.
N.b- a physical method is one that doesn’t involve a chemical reaction, so doesn’t form any new substances.
What are the properties of a mixture?
The PROPERTIES of a mixture are just a MIXTURE of the properties of the SEPARATE PARTS- the chemical properties of a substance aren’t affected by it being part of a mixture.
What is chromatography?
CHROMOTOGRAPHY is a method to separate mixtures made up of liquids of different colours. An example of this is the use of paper chromatography to separate different dyes in an ink?
How do you carry out paper chromatography?
1) Draw a LINE near the bottom of a sheet of FILTER PAPER.
2) Add a SPOT of the ink to the line and place the sheet in a beaker of solvent, e.g. water.
3) The SOLVENT used depends on what’s being tested. Some compounds dissolve well in WATER, but sometimes other solvents, like ethanol are needed.
4) Make sure the ink isn’t touching the solvent-you don’t want it to DISSOLVE into it.
5) Place a LID on top of the container to stop the solvent EVAPORATING.
6) The solvent SEEPS up the paper, carrying the ink with it.
7) Each different DYE in the ink will move up the paper at a DIFFERENT RATE so the dyes will SEPARATE OUT. Each dye will form a SPOT in a different place- 1 spot per dye ink.
8) If any of the dyes in the ink are INSOLUBLE (won’t dissolve) in the solvent you’ve used, they’ll stay on the BASELINE.
9) When the SOLVENT has nearly reached the TOP of the paper, take the paper out of the beaker and leave it to DRY.
10) The end result is a pattern of spots called a CHROMATOGRAM.
N.b. - The point the solvent has reached as it moves up the paper is the solvent front.
How can you interpret chromatograms?
On some chromatograms, the number of spots you end up with tells you the number of different substances there were in a mixture. For example , if you end up with two spots, that might mean the ink contains two dyes.
This isn’t always the case though-sometimes the umber of visible spots on the chromatogram does’t match the number of dyes in the ink. For example, two dyes could travel the same distance up the filter paper so would only show one spot.
So all we can say is there are at least as many substances in the mixture as there are spots on the chromatogram.
What is filtration?
Filtration is often used if your desired product is an INSOLUBLE SOLID that needs to be separated from a LIQUID mixture. It’s also a useful technique for purification. For example, solid impurities in a reaction mixture can be removed using filtration.
Explain the process of filtration…
1) Fold a piece of filter paper into a cone.
2) Place the filter paper point down into a filter funnel that is sitting in the neck of a container such as a conical flask.
3) Pour the mixture containing the insoluble solid into the funnel lined by the filter paper. Make sure that none of the mixture goes over the top or down the side of the filter paper.
4) The liquid will pass through the filter paper but the solid won’t - it will be left behind.
Tip-insoluble means the solid can’t be dissolved in the liquid.
What two methods can be used to separate a soluble solid and a solution?
If a solid can be dissolved it is said to be soluble.
There are 2 methods used to remove a soluble product from a solution- EVAPORATION and CRYSTALLISATION.
What is evaporation and how long does the process take?
Evaporation is a really QUICK way of separating a soluble salt from a solution, but you can only use it if the salt DOESN’T DECOMPOSE when it is heated. Otherwise you’ll have to use CRYSTALLISATION.
Explain the process of evaporation…
1) Pour the solution into an EVAPORATING DISH.
2) Place the evaporating dish on top of a TRIPOD and gauze and place a BUNSEN BURNER underneath.
Tip-if the solvent is flammable you shouldn’t use a Bunsen burner to heat it, as it may catch fire. You’d need to use a different heating method, such as using a hot plate.
3) Slowly HEAT the solution. The SOLVENT will evaporate and the solution will get more CONCENTRATED. Eventually the SOLID CRYSTALS will start to form.
4) Keep heating the evaporating dish until all you have left is dry solid crystals.
What is crystallisation and how long does the process take?
Crystallisation takes more time than evaporation, however it can produce nice big crystals, that would decompose if heated.
Explain the process of crystallisation…
1) Place an EVAPORATING DISH on top of a TRIPOD with a GAUZE MAT. Place a BUNSEN BURNER underneath the TRIPOD.
2) Pour the solution into the EVAPORATING DISH and gently HEAT it. Some of the SOLVENT will evaporate and the solution will get more CONCENTRATED.
3) Once some of the solvent has evaporated, or when you see crystals start to form (the POINT OF CRYSTALLISATION), remove the dish from the heat and leave the solution to COOL.
Tip-Make sure you use tongs when removing the very hot evaporating dish.
4) The salt should start to form CRYSTALS as it becomes INSOLUBLE in the cold, highly concentrated solution.
5) FILTER the crystals out of the solution, and leave them in a warm place to dry. You could also use a DRYIN OVEN or a DESICCATOR.
Tip- the longer you the solution is left to cool following heating, the larger the crystals will be.
Exam tip- You may be asked to suggest what technique could be used to separate a certain mixture, so you need to know which techniques separate what.
Tip-You can use crystallisation INSTEAD of evaporation to form salt crystals. This is useful if you want to form large salt crystals.
What is rock salt and which technique is used to separate the mixture?
ROCK SALT is:
simply a MIXTURE of SALT and SAND (they spread it on roads in winter).
Salt and sand are both COMPOUNDS-but SALT DISSOLVES in water and SAND DOESN’T. This VITAL DIFFERENCE in their PHYSICAL PROPERTIES gives a great way to separate them.
Chemists don’t usually use one of these techniques in isolation, but will often use two or more in oder to separate a mixture.
Explain the process of separating rock salt using filtration and evaporation…
1) GRIND the mixture to make sure the salt crystals are small, so will dissolve easily.
2) Put the mixture in water and stir. The SALT will DISSOLVE, but the SAND WON’T. (Tip-heating the mixture will help to dissolve the salt).
3) FILTER the mixture. The grains of SAND won’t fit through the tiny holes in the filter paper, so they collect on the PAPER instead. The SALT passes through the filter paper as it’s part of the solution.
4) EVAPORATE the water from the salt so that it forms DRY CRYSTALS. (Tip-you could also use crystallisation here if you wanted nice big crystals).
What is distillation?
Distillation is used to separate mixtures which contain LIQUIDS. There are 2 different kinds- simple and fractional distillation. (page 40)
What is simple distillation?
SIMPLE DISTILLATION is used to separate out a liquid from a mixture. (It’s used in industry to get pure water from sea water).
Explain the process of simple distillation…
- Set up apparatus. Attach a clamp stand to a flask of sea water and place a safety mat, bunsen burner, tripod and gauze under it. Then place a thermometer inside the flask and attach a condenser. Heat the mixture.
Tip- the thermometer will show the boiling point of the vaporised liquid. The condenser turns the vapour back into a liquid as it it cooled by the water. - HEAT the solution. The part of the solution that has the lowest boiling point EVAPORATES first.
- The VAPOUR is the COOLED, CONDENSES (turns back into a liquid) and is collected.
- The rest of the SOLUTION is left behind in the flask.
- You can use simple distillation to get PURE WATER from SEAWATER. The WATER evaporates and is condensed and collected. Eventually you’ll end up with just SALT left in the flask.
What is a disadvantage of using simple distillation to separate out solutions?
The PROBLEM with simple distillation is that you can only use it to separate things with VERY DIFFERENT boiling points- if the temperature goes higher than the boiling point of the substance with the higher boiling point, they will MIX again.
So if you have a MIXTURE OF LIQUIDS with SIMILAR BOILING POINTS you need another method to separate them- like fractional distillation.
