Key Concepts Flashcards
How can chemical reactions be shown?
Using word equations or symbol equations
What do state symbols show?
The physical state of the reactants and products
What are compounds?
Combinations of atoms of different elements
What do symbol equations show?
Symbols or formulas of the reactants and products
Chemical formula for ammonia?
NH3
Chemical formula for ammonium
NH4+
Chemical formula for nitrate
NO3-
Chemical formula for hydroxide
OH-
Chemical formula for carbonate
2-
CO3
Chemical formula for sulfate
2-
SO4
How do ions form?
When atoms, or groups of atom, gain or lose electrons to form charged particles
Hazard
Anything with the potential to cause harm or damage
What are atoms?
Tiny particles of matter which make up everything
The smallest pieces of an element
How did John Dalton describe atoms at the start of the 19th century?
Solid spheres that cannot be split
Different spheres made up the different elements
Who concluded that atoms weren’t solid spheres? What did he discover?
JJ Thomson in 1897
He discovered electrons
What did JJ Thomson conclude from his experiments on atoms?
Atoms weren’t solid spheres that cannot be divided
Atoms have an internal structure
How did JJ Thompson show atoms have an internal structure?
His measurements of charge and mass showed an atom must contain even smaller, negatively charged particles- electrons
What did JJ Thomson change the ‘solid sphere’ idea of atomic structure to?
The plum pudding model
A ball of positive charge with negatively charged electrons mixed in
What proved the plum pudding model wrong?
The gold foil experiment in 1909 by Ernest Rutherford
What was the gold foil experiment?
Firing positively charged alpha particles at a thin sheet of gold
What were they expecting to see during the gold foil experiment because of the plum pudding model?
The particles to pass through the sheet or be slightly deflected because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom
What happened during the gold foil experiment?
Most alpha particles went through the gold sheet, some were deflected more than expected, and a small number were deflected backward
In the alpha particle experiment, what did some particles bouncing back tell the scientists?
The centre of the atom must contain a lot of mass- we call this the nucleus
In the alpha particle experiment, what did most particles passing through tell the scientists?
Atoms are mainly just empty space
In the alpha particle experiment, what did some particles getting deflected tell the scientists?
The centre of the atom must have a positive charge
What theory did Rutherford come up with after the gold foil experiment?
The theory of the nuclear atom
In this there’s a tiny, positively charged nucleus at the centre, surrounded by a ‘cloud’ of negative electrons
Most of the atoms empty space
What did scientists realise about Rutherford’s idea of electrons in a cloud?
The electrons would be attracted to the nucleus, causing the atom to collapse
What did Niels Bohr propose?
A new model of the atom where all the electrons were contained in shells
He suggested the electrons only existed in shells and each shell has a fixed energy
What did James Chadwick discover in 1932?
Neutrons
What are atoms made up of?
3 subatomic particles:
Protons- heavy and positively charged
Neutrons- heavy and neutral
Electrons- hardly any mass and negatively charged
Characteristics of the nucleus
Middle of the atom Protons and neutrons Positive charge due to protons Almost whole mass of atom concentrated here Tiny compared to overall size of atom
Characteristics of electrons
Move around nucleus in shells
Negatively charged
Tiny but shells cover a lot of space
Size of shells determines size of atom
-10
Radius of about 10 m
Tiny massWhy are atoms neutral?
They have the same amount of protons and electrons
What does the nuclear symbol of an atom tell us?
It’s atomic (proton) number and mass number
What does the mass number tell us?
Total number of protons and neutrons
Isotopes
Different forms of the same element
Same number of protons, different number of neutrons
Same atomic number, different mass number
Relative atomic mass
The average mass of one atom of the element, compared to half the mass of one atom or carbon-12
Why might the relative atomic mass not be a whole number?
It is the average of the mass numbers of all the different isotopes of an element, taking into a count their abundance
How to find out relative atomic mass
Multiply each relative isotopic mass by its isotopic abundance and add up the results
Divide the sum by its abundances
Differences between the modern periodic table and Mendeleev’s
Modern- element ordered in atomic number, Mendeleev- protons hadn’t been discovered so ordered them by atomic mass
Modern- has noble gases, Mendeleev- left gaps for undiscovered elements
Who made the first proper periodic table?
Dimitri Mendeleev in 1869
How many elements did Mendeleev arrange?
50
How did Mendeleev order the elements?
He sorted them into groups, based on properties and the properties of their compounds
Ordered them in order of relative atomic mass so he could put elements with similar chemical properties in columns
Why was the atomic mass Mendeleev had sometimes wrong?
The presence of isotopes
What did Mendeleev do when some elements didn’t fit the pattern of similar chemical properties being in columns?
He switched the order of the elements to keep those with the same properties in the same columns
What did Mendeleev also have to do to keep elements with the same properties together?
Left gaps for undiscovered elements
Used properties of other elements in the column to predict their properties
They were found and matched his pattern, confirming his ideas
Why did Mendeleev order his table by atomic mass?
Protons hadn’t been discovered
What are elements on the modern periodic table ordered by?
Ascending atomic number
They fit the patterns Mendeleev worked out
How is the modern periodic table laid out?
In columns called groups where elements with similar chemical properties are together
What determines which column the element belong to on the periodic table?
The number of electrons in its outer shell
What are rows called?
Periods
Each period represents another full shell of electrons
What corresponds to the period an element belongs to?
The number of shells of electrons an element has
What are 3 electron shell rules?
They always occupy shells
Lowest energy levels are always filled first
Only a certain number of electrons are allowed in each shell (1st= 2, 2nd= 8, 3rd= 8
What does the group number tell us?
How many electrons occupy the outer shell of the element
Ions
Charged particles- they can be single atoms or groups of atoms
Why do atoms lose or gain electrons?
To gain a full outer energy level
To gain a stable electronic structure
Anions
Negative ions that form when atoms gain electrons
More electrons than protons
Cations
Positive ions
Form when atoms lose electrons
More protons than electrons
Which groups are most likely to form ions?
1,2,6,7
Which groups lose electrons?
1,2
Metals
Form cations
Which groups gain electrons?
6 and 7
Non metals
Form anions
Why do elements in the same group all form ions with the same charges?
They all have the same number of outer electrons
They have to lose or gain the same number to achieve a full outer shell
What are ionic compounds made up of?
A positively charged part and a negatively charged part
What is the overall charge of any ionic compound?
0
Positive charges must balance negative ones
What are the three types of bonding?
Ionic
Covalent
Metallic
What is an ionic bond?
The transfer of electrons between a metal and a non metal. The metal loses electrons and becomes a cation, the non metal gains electrons to form and anion. These oppositely charged ions are strongly attracted to one another by electrostatic forces
What do dot and cross diagrams show?
How ionic compounds form
Show the arrangement of electrons in an atom or ion
What structure do ionic compounds have?
Giant lattice- very strong electrostatic forces of attraction between oppositely charged ions
Properties of ionic compounds
High melting and boiling points due to strong attraction between ions- lots of energy needed to overcome this
Solid ionic compounds don’t conduct electricity because the ions are fixed in place and cannot move
Liquid ionic compounds carry an electric current because the electrons are free to move
Soluble in water because ions separate and are free to move in the solution, they’ll carry an electric current
2D representations advantages and disadvantages
Simple Show what atoms something contains Shows how atoms connect Don’t show shape or substance No idea about size
Dot and cross diagram advantages and disadvantages
Show how compounds or molecules are formed
Show where electrons in bonds or ions came from
Don’t show size or arrangement of atoms or ions
3D models advantages and disadvantages
Show arrangement of ions
Only show outer layer
Ball and stick model advantages and disadvantages
Visual Show shape of lattice or molecule in 3D More realistic than 2D Show gaps between atoms- misleading Don’t show correct scales of atoms or ions
Covalent bond
A strong bond that forms when a pair of electrons is shared between 2 atoms
What are simple molecular substances made from?
Molecules containing a few atoms joined by covalent bonds
Properties of simple molecular substances
Atoms within molecules held together by strong covalent bonds
Forces of attraction between molecules are weak
Low melting and boiling points because the intermolecular forces are feeble and easily broken
Gases or liquids at room temperature
As they get bigger, strength of intermolecular forces increases and melting and boiling points increase
Don’t conduct electricity because they don’t contain free electrons or ions
Some soluble and some aren’t
What are polymers?
Molecules made of long chains of covalently bonded carbon toms
Eg polyethene
Form when small molecules called monomers join together
Properties of giant covalent structures
All atoms bonded by strong covalent bonds
High melting and boiling points
Usually don’t conduct electricity (apart from graphite and graphene) as they normally don’t contain charged particles
Aren’t water soluble
Examples of carbon based giant covalent structures
Diamond
Graphite
Graphene
Diamond properties
Made of a network of carbons that each form 4 covalent bonds
Strong covalent bonds- high melting point
Strong covalent bonds hold atoms in a rigid lattice structure making it really hard. Used to strengthen cutting tools
Doesn’t conduct electricity as it has no free electrons or ions
Graphite properties
Each carbon atom forms 3 covalent bonds creating sheets of carbon atoms arranged in hexagons
No covalent bonds between layers- held together weakly so move over each other making graphite soft and slippery. Used as a lubricating material
High melting point as covalent bonds in layers need loads of energy to break
1 out of each carbons 4 outer electrons used in bonds do each carbon atom has one electron delocalised and can move. Therefore graphite conducts electricity and is often used to make electrodes
Graphene properties
A fullerene One layer of graphite Sheet of carbon atoms joined together in hexagons One atom thick 2d compound
Fullerenes
Molecules of carbon shaped like closed tubes or hollow balls
Properties of fullerenes
Made up of carbon atoms arranged in hexagons but also contain pentagons or heptagons
Cage other molecules- could be used to deliver a drug directly to cells in the body
Huge surface area so could help make industrial catalysts
What are nanotubes?
Fullerenes
Tiny cylinders of graphene- conduct electricity
High tensile strength so can strengthen materials without adding weight
Used in sports equipment
Example of a fullerene
Buckminsterfullerene C60 Forms a hollow sphere made of 20 hexagons and 12 pentagons Stable molecule Forms soft brownish black crystals
Metallic bonding
Forces of electrostatic attraction between positive metal ions and shared negative electrons that hold the atoms together in a regular lattice
What does metallic bonding involve?
Delocalised electrons in the outer shell of metal atoms
Physical properties of metals
Very strong electrostatic forces between metal ions and delocalised seas of electrons
High melting and boiling points due to this- shiny solids at room temperature
Not water soluble
Denser than non metals as ions in metallic structure are packed closely together
Malleable- layers of atoms can slide over each other
Conduct heat and electricity- delocalised electrons carry an electric current and thermal heat energy through the material
Why do metals have similar basic physical properties?
They all have metallic bonding
Why don’t non metals have the same properties as metals?
They don’t have metallic bonding
Why do non metals have a variety of chemical and physical properties?
They form a variety of different structures
Properties of non metals
Dull looking Brittle Lower boiling points- not solids at room temp Don’t conduct electricity Lower density
How do non metals and metals have different chemical properties?
Non metals gain electrons to form full outer shells
Metals lose electrons to gain full outer shells
Why is mass always conserved in a chemical reaction?
No atoms are destroyed or created
There are the same number and type of atoms on each side of a reaction equation
Total mass of the system before and after doesn’t change
What is a precipitation reaction?
When two solutions react and an insoluble solid- called a precipitate- forms in the solution
How can we show conservation of mass?
Precipitation reaction
What is usually involved if the mass changes in an experiment?
A gas
Why might mass increase in an unsealed reaction vessel during a reaction?
If it increases its probably because at least one of the reactants is a gas found in air and the products are solids liquids or aqueous.
Before reaction gas is floating in air, its not contained in the reaction vessel so you can’t measure its mass
When gas reacts to form part of the product, it becomes contained inside the reaction vessel
Total mass inside reaction vessel increases
Why might mass decrease in an unsealed reaction vessel during a reaction?
Some or all the reactants are solids liquids or aqueous and at least one of the products is a gas
Before reaction, any solid liquid or aqueous reactants are contained in the reaction vessel
If vessel isn’t enclosed, gas can escape from reaction vessel as its formed. It’s no longer contained in the reaction vessel so you can’t measure its mass
Total mass of stuff inside the reaction vessel decreases
How are formulae made?
Combining chemical symbols of elements that combine to make compounds
How are molecules formed?
If 2 or more atoms of an element chemically join
Features of a chemical reaction (4)
Compounds broken up or formed
At least one new substance created
Measurable energy change
No atoms created or destroyed
What are ions made up of?
Groups of atoms
