chemistry: atomic structure & the periodic table atoms, elements, & compounds:

the smallest part of an element that exists and makes up all substances

the smallest part of an element that exists and makes up all substances

atom

a substance made of atoms that all have the same number of protons in their nucleus. there are many elements, which can all be represented by a one/two letter symbol

element

when elements react, the atoms combine to form compounds, which are substances with two (or more) elements. most of the time, it’s quite difficult to separate compounds, usually a chemical reaction is necessary

compound

consists of two or more elements or compounds not chemically combined together

mixture

filtration separates insoluble solids from liquids:

• quite simply involves the liquids passing through the filter paper, leaving the solids in the cone.

filtration (what it separates & steps)

crystallisation separates soluble solids from liquids:

• 1). pour the solution into an evaporating dish and slowly heat the solution.

• 2). once some of the solvent has evaporated, remove the dish from the heat and leave the solution to cool.

• 3). when the crystals have formed, filter them out and leave them to dry. 

crystallisation (what it separates & steps)

evaporation separates soluble solids from liquids:

• 1). pour the solution into an evaporating dish and slowly heat the solution until crystals begin to form.

• 2). continue heating until all that is left is dry crystals.

evaporation (what it separates & steps)

simple distillation is used to separate liquids:

• 1). the solution is heated, so the solution with the lowest boiling will evaporate first.

• 2). the vapour is then turned back into a liquid in the condenser and is collected in a separate beaker.

• 3). the remaining solution is left behind in the flask. the problem with simple distillation is you can only separate liquids with very different boiling points.

simple distillation (what it separates & steps)

fractional distillation is used to separate a mixture of liquids:

• 1). put the mixture in a flask and put a fractionating column on top.

• 2). heat the flask, so the liquid with the lowest boiling point begins to reach the top of the column.

• 3). once the first liquid has been collected, raise the heat until the next one reaches the top of the column.

fractional distillation (what it separates & steps)

paper chromatography can be used to separate different dyes in ink:

• 1). draw a line near the bottom of a sheet of filter paper using a pencil.

• 2). add a spot of ink to the line and place the sheet in a beaker of solvent, making sure that the ink isn’t touching the solvent.

• 3). place a lid on the beaker to stop the solvent from evaporating.

• 4). as the solvent seeps up the paper, the dyes should begin to separate.

paper chromatography (what it separates & steps)

• at the start of the 19th century, john dalton described atoms as solid spheres, with different spheres making up different elements.

• however, in 1897, jj thomson discovered electrons. he theorised that the atom was a ball of positive charge with electrons within it, he called it the ‘plum pudding’ model.

• then, in 1909, rutherford and marsden conducted the alpha particle scattering experiment, in which they fired positively charged alpha particles at a thin sheet of gold. due to the plum pudding model, they were expecting the particles to pass through the sheet (or be ever so slightly deflected) because of the spread out positive charge. however, whilst most particles did pass through, a fair few were deflected. therefore, the nuclear model was born, in which there is a positively charged nucleus at the centre (where the mass was concentrated) with a cloud of negative electrons surrounding the nucleus.

• scientists realised that if electrons were in a cloud around the nucleus, the atom would collapse. bohr suggested that electrons were contained in fixed shells that orbit the nucleus.

• chadwick also discovered neutrons whilst carrying out an experiment.

development of the model of the atom (explanation)

• protons = +1

• neutrons = 0

• electrons = -1

electrical charges of subatomic particles

• protons = 1

• neutrons = 1

• electrons 0

relative mass of subatomic particles

different form of the same element, which has the same number of protons but a different number of neutrons.

isotope (definition)

the average mass taking into account the different masses and abundances of all isotopes that make up an element

relative atomic mass (definition)

relative atomic mass = sum of (isotope abundance × isotope mass number) ÷ sum of abundances of all isotopes

relative atomic mass (formula)

• first shell = 2

• second shell = 8

• third shell = 8

electron shell numbers

• arranged the elements by atomic weight. when they did this, they realised that there was a periodic pattern in the properties. early periodic tables were incomplete and had certain elements in the wrong group

• in 1869, mendeleev arranged his own ‘table of elements’, in which he mainly kept it the same (in order of atomic weight) but did change a few elements due to their properties. mendeleev left gaps in his table to make sure elements with similar properties stayed in the same group. some gaps indicated that there were elements missing.

development of periodic table

• metals = positive ions (lose electrons)

• non-metals = negative ions (gain electrons)

metal & non-metal ions

• almost all metals have similar properties: strong but also malleable, conductors of heat and electricity, and high melting and boiling points.

• non-metals usually have varied properties but they tend to be brittle, dull, low density, and don’t conduct electricity.

properties of metals & non-metals

• metals lose electrons, forming positive ions, this is because the electrons in the outer shell have a weak attraction to the nucleus (metals to the bottom) and they also don’t have many electrons to lose (metals to the left).

• non-metals, on the other hand, gain electrons, forming negative ions, this is because the electrons in the outer shell have a strong attraction to the nucleus (non-metals to the top) and they also have lots of electrons to remove (non-metals to the right).

electronic/atomic structure & arrangement of metals & non-metals

• full outer shell, making them energetically stable, which means they don’t need to lose or gain electrons to become stable.

• not having to react to form a full outer shell makes them inert (chemically inactive).

• they are monatomic (just one atom), colourless, & non-flammable.

properties of noble gases (group 0)

the boiling points of the noble gases increase moving down the group, it is the same for their relative atomic masses. the increase in boiling point is due to an increase in the number of electrons in each atom. more atoms means a stronger intermolecular force that needs to be overcome.

property trends of noble gases (group 0)

when an alkali metal is put into water, there is a vigorous reaction, in which hydrogen gas and metal hydroxides are produced. the reactivity and energy given out increases down group 1, in fact potassium releases enough energy to ignite hydrogen. the equation that is formed is as follows: [metal] + water → [metal] hydroxide + hydrogen.

alkali metals (group 1) reactions with water

when an alkali metal is heated in chlorine, they react vigorously and form a white metal chloride salt. the reaction becomes more intense down the group. the equation that is formed is as follows: [metal] + chlorine → [metal] chloride.

alkali metals (group 1) reactions with chlorine

when an alkali metal reacts with oxygen, they form a metal oxide. different types of oxides will form depending on the metal: lithium reacts to form lithium oxide, sodium reacts to form a mixture of sodium oxide & sodium peroxide, & potassium reacts to form a mixture of potassium peroxide & potassium superoxide.

alkali metals (group 1) reactions with oxygen

• they all have one electron in their outer shell. this makes them reactive and gives them all similar properties.

• the alkali metals are soft with low densities.

properties of alkali metals (group 1)

going down the group, reactivity increases since the outer electron becomes more easily lost due to the little attraction between the nucleus and electron. there is also a lower melting and boiling point going down.

property trends of alkali metals (group 1)

• halogens form ionic bonds with metals, becoming 1– ions called halides.

• whereas, when they react with other non-metals, a displacement reaction occurs, meaning a more reactive halogen displaces a less reactive halogen.

halogens (group 7) reaction with metals & non-metals

• halogens carry seven electrons in their outer shell. they can form covalent bonds with other non-metals to achieve a full outer shell. when this is done, a simple molecular structure is made.

• most of the halogens are poisonous gases, forming a range of colours, from yellow to green to red.

• they all exist as molecules which are in pairs.

properties of halogens (group 7)

going down the group, the halogens become less reactive, since its harder to gain an electron when the nucleus is so far from the outer shell, and also have higher melting & boiling points as they group goes down.

property trends of halogens (group 7)