Detailed on Atoms and Molecules

7/27/2019 Detailed on Atoms and Molecules

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1.1aATOM

(i) Even as far back as ancient Greece ~500BC philosophers had considered the concept of what wouldbe formed on continuously dividing matter i.e. what was the smallest 'bit' left of any substance. In 1808the English scientist-chemist Dalton proposed his 'atomic theory' - that all matter was made up of tiny

individual units called atoms which could NOT be subdivided into simpler substances. What is more, heproposed the idea that there were different types of atoms which we now call 'elements' andcombinations of them produce all the different substances which exist.The different types of atoms arecalled elements(examples below).An ATOM is the smallest particle of a substance which can have its own characteristic propertiesAND cannot be split into simpler substances.(ii)BUT, remember atoms are built up of even more fundamental sub-atomic particles - theelectron, proton and neutron. The centre of an atom, called the nucleus, consists of proton andneutron particles and the electrons move around the nucleus in 'orbital' energy levels

1.1bMOLECULES and

their representationA MOLECULE is a larger particle formed by the chemical combination of two or more atoms.The molecule may be anelemente.g. hydrogen formula H2 (H-H, two atoms combined) ora compound(more examples below) e.g. carbon dioxide formula CO2 (O=C=O, three atomscombined) and in each case the atoms are held together by chemical bonds. (detailed GCSEbonding notes and examples)You can represent molecule in various styles of diagram. For example, you can colour and size codethe atoms of different elements, so in the molecule pictured on the upper left, you can tell there are fivetypes of atom (elements) and six atoms in total in the molecule.The second molecule (lower left) shows the molecular structure ofethanol ('alcohol') which consists oftwo carbon atoms, six hydrogen atoms and one oxygen atoms.You will also come across shorthand versions of this diagrammatic style written like
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CH3CH2OH and even shorterC2H5OH but using these styles requires much more experience than isrequired when first learning the basic principles of chemistry.Why they are combined in this particular number and order depends primarily on an atoms combiningpower (its valency) an advanced concept dealt with in Part 3 equations, formula and valency.There are also styles to give a much greater '3D' impression of the shape of a molecule and theyattempt to show the '3D' spatial arrangement of the atoms in a molecule and how the bonds connectthem together.

Below are two '3D' representations of the hydrocarbon molecule called ethane, C2H6

This image is an example of a 'ball-and-stick' diagram and shows the

chemical bonds which hold the atomstogether.

is an example of a 'space-filling'diagram which gives a more accurate

representation of the space themolecule actually occupies.
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2. Ionic Bonding - electron transferIonic bonds are formed by one atom transferring electrons to another atom to form ions.Elements consist of neutral atoms or molecules, the electrical neutrality is because the number of positive protons equals the number ofnegative electrons.Ions are atoms, or groups of atoms, which have lost or gained electrons to have a net electrical charge overall .The atom losing electrons forms a positive ion (a cation) and is usually a metal. The overall charge on the ion is positive due to excess positivenuclear charge (protons do NOT change in chemical reactions).The atom gaining electrons forms a negative ion (an anion) and is usually a non-metallic element. The overall charge on the ion is negativebecause of the gain, and therefore excess, of negative electrons.


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The examples below combining a metal from Groups 1 (Alkali Metals), 2 or 3, with a non-metal from Group 6 or Group 7 (The Halogens). Theelectron structures are shown in () or []. Only the outer electrons of the original atoms, and where they end up in the ions, are shown in the dot andcross (ox) diagramsIonic bonding is not directional like covalent bonding, in the sense that the force of attraction between the positive ions and the negative ions act inevery direction around the ions.

Example 1: A Group 1 metal + a Group 7 non-metal e.g. sodium + chlorine ==> sodium chloride NaCl or ionic formula Na+Cl

-In terms of

electron arrangement, the sodium donates its outer electron to a chlorine atom forming a single positive sodium ion and a single negative chlorideion. The atoms have become stable ions, because electronically, sodium becomes like neon and chlorine like argon.

Na (2.8.1) + Cl (2.8.7) ==> Na+

(2.8) Cl-(2.8.8)

can be summarised electronically to give the stable 'noble gas' structures as [2,8,1] + [2,8,7] ==> [2,8]+

[2,8,8]-

ONE combines with ONE to form

The valencies of Na and Cl are both 1, that is, the numerical charge on the ions. sodium fluoride NaF, potassium bromide KBr and lithium iodideLiI etc. will all be electronically similar.Note:
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would represent the full electronic structure of the sodium ion.

would represent the full electronic structure of the chloride ion and note that the 'blob' and 'x' e lectrons are identical,but their use is just a useful visual device to show how the ion is formed.Only the outer valency electrons of the chloride ion are shown, the 'blob' electron represents the electron from the sodium atom which is acceptedby the chlorine atom to form the chloride ion.The charge on the sodium ion Na

+is +1 units (shown as just +) because there is one more positive proton than there are negative electrons in

the sodium ion.The charge on the chloride ion Cl

-is -1 units (shown as just -) because there is one more negative electron than there are positive protons in

the chloride ion.SeeExample 6. aluminium oxidefor more highly charged ion analysis.

Li is 2.1, K is 2.8.8.1, F is 2.7, rest of dot and cross diagram is up to you.
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Reminder: How to work out formula of ionic compounds without going through some demanding electronic thinking is described on the"Elements, Compounds and Mixtures" page and it is followed by a section on naming compounds.

Example 2: A Group 2 metal + a Group 7 non-metal e.g. magnesium + chlorine ==> magnesium chloride MgCl2 or ionic formula Mg2+

(Cl-)2 In

terms of electron arrangement, the magnesium donates its two outer electrons to two chlorine atoms forming a double positive magnesium ion andtwo single negative chloride ions. The atoms have become stable ions, because electronically, magnesium becomes like neon and chlorine likeargon.

Mg (2.8.2) + 2Cl (2.8.7) ==> Mg2+

(2.8) 2Cl-(2.8.8)

can be summarised electronically as [2,8,2] + 2[2,8,7] ==> [2,8]2+

[2,8,8]-

2

ONE combines with TWO to form see *NOTE* you can draw two separate chloride ions, but in these examples square brackets and a number subscript have been used, as in ordinarychemical formula.The valency of Mg is 2 and chlorine 1, i.e. the numerical charges of the ions.Beryllium fluoride BeF2, magnesium bromide MgBr2, calcium chloride CaCl2 or barium iodide BaI2 etc. will all be electronically similar.
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represents the full electronic structure of the magnesium ion.Ca is 2.8.8.2, F is 2.7 rest of dot and cross diagrams are up to you.

Example 3: A Group 3 metal + a Group 7 non-metal e.g. aluminium + fluorine ==> aluminium fluoride AlF3 or ionic formula Al3+

(F-)3 In terms of

electron arrangement, the aluminium donates its three outer electrons to three fluorine atoms forming a triple positive aluminium ion and threesingle negative fluoride ions. The atoms have become stable ions, because aluminium and fluorine becomes electronically like neon. Valency of Alis 3 and F is 1, i.e. equal to the charges on the ions.

Al (2.8.3) + 3F (2.7) ==> Al3+

(2.8) 3F-(2.8)

can be summarised electronically as [2,8,3] + 3[2,7] ==> [2,8]3+

[2,8]-3

ONE combines with THREE to form

Solid aluminium chloride/bromide/iodide have similar formula but are covalent when vapourised into Al2X6 dimer molecules, but AlCl3 has an ioniclattice in the solid, not sure on solid AlBr3 and AlI3, but these points are best left for an advanced AS-A2 chemistry discussion, not for GCSEstudents!
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Example 4: A Group 1 metal + a Group 6 non-metal e.g. sodium/potassium + oxygen ==> sodium/potassium oxide Na2O/K2O or ionicformula(Na

+)2O

2-/(K

+)2O

2-In terms of electron arrangement, the two sodium/potassium atoms donate their outer electron to one oxygen atom. This

results in two single positive potassium ions to one double negative oxide ion. All the ions have the stable electronic structures 2.8.8 (argon like) or2.8 (neon like). Valencies, K 1, oxygen 2. Lithium oxide, Li2O, sodium oxide Na2O, sodium sulphide Na2S and potassium K2S etc. will be similar.

sodium oxide2Na (2.8.1) + O (2.6) ==> 2Na

+(2.8.8) O

2-(2.8)

can be summarised electronically as 2[2,8,1] + [2,6] ==> [2,8]+

2 [2,8]2-

TWO combine with ONE to form

or

+ + ==>
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potassium oxide2K (2.8.8.1) + O (2.6) ==> 2K

+(2.8.8) O

2-(2.8)

can be summarised electronically as 2[2,8,8,1] + [2,6] ==> [2,8,8]+

2 [2,8]2-

TWO combine with ONE to form

The electronic similarities between the two examples are very obvious.Li is 2.1, Na is 2.8.1, S is 2.8.6 (for group 1 sulphide compound), rest of dots and crosses diagrams are up to you.

Example 5:A Group 2 metal + a Group 6 non-metal e.g. magnesium/calcium + oxygen ==> magnesium/calcium oxide MgO/CaO or ionicformula Mg

2+O

2-/Ca

2+O

2-In terms of electron arrangement, one magnesium/calcium atom donates its two outer electrons to one oxygen atom.

This results in a double positive calcium ion to one double negative oxide ion. All the ions have the stable electronic structures 2.8.8 (argon like) or2.8 (neon like). the valency of both calcium and oxygen is 2.

magnesium oxide
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For magnesium oxide: Mg (2.8.2) + O (2.6) ==> Mg2+

(2.8) O2-

(2.8)the stable 'noble gas' structures can be summarised electronically as [2,8,2] + [2,6] ==> [2,8,8]

2+[2,8]

2-

calcium oxideCa (2.8.8.2) + O (2.6) ==> Ca

2+(2.8.8) O

2-(2.8)

can be summarised electronically as [2,8,8,2] + [2,6] ==> [2,8,8]2+

[2,8]2-


Magnesium oxide MgO, magnesium sulphide MgS and calcium sulphide CaS will be similar electronically and give identical giant ionic latticestructures. Group 2 metals lose the two outer electrons to give the stable 2+ positive ion (cation) and S and O, both non-metals in Group 6, have 6

outer electrons and gain 2 electrons to form 2- negative ion (anion).Formagnesium sulphide: Mg (2.8.2) + S (2.8.6) ==> Mg

2+(2.8) S

2-(2.8.8)


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Forcalcium sulphide: Ca (2.8.8.2) + S (2.8.6) ==> Ca2+

(2.8.8) S2-

(2.8.8)The dot and cross (ox) diagrams will be identical to that for calcium oxide above, except Mg instead of Ca (same group) and S instead of O (same

group of Periodic Table).

Example 6: A Group 3 metal + a Group 6 non-metal e.g. aluminium + oxygen ==> aluminium oxide Al2O3 or ionic formula (Al3+

)2(O2-

)3 Interms of electron arrangement, two aluminium atoms donate their three outer electrons to three oxygen atoms. This results in two triple positivealuminium ions to three double negative oxide ions. All the ions have the stable electronic structure of neon 2.8. Valencies, Al 3 and O 2. 2Al (2.8.3) + 3O (2.6) ==> 2Al

3+(2.8) 3O

2-(2.8)

can be summarised electronically as 2[2,8,3] + 3[2,6] ==> [2,8]3+

2 [2,8]2-

3

TWO combine with THREE to form

Note:The charge on the aluminium ion Al

3+is +3 units (shown as 3+) because there are three more positive protons than there are negative electrons in

the aluminium ion.The charge on the oxide ion O

2-is -2 units (shown as 2-) because there are two more negative electrons than there are positive protons in

theoxide ion.on another web page ishow to work out an ionic formula given the ionic charges (combining power)
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The properties of Ionic Compounds
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The diagram on the right is typical of the giant ioniccrystal structure of ionic compounds like sodium chlorideand magnesium oxide.

The alternate positive and negative ions in an ionicsolid are arranged in an orderly way in a giant ionic latticestructure shown on the left.

The ionic bond is the strong electrical attractionbetween the positive and negative ions next to each other inthe lattice.

The bonding extends throughout the crystal in alldirections.

Salts and metal oxides are typical ionic compounds. This strong bonding force makes the structure hard (if

brittle) and have high melting and boiling points, so they are

not very volatile!

A relatively large amount of energy is needed to melt orboil ionic compounds. Energy changes for the physical changesof state of melting and boiling for a range of differently bondedsubstances are compared in a section of theEnergetics Notes.

The bigger the charges on the ions the stronger thebonding attraction e.g. magnesium oxide Mg

2+O

2-has a higher

melting point than sodium chloride Na+Cl

-.

Unlike covalent molecules, ALL ionic compounds arecrystalline solids at room temperature.

They are hard but brittle, when stressed the bonds are broken along planes of ions which shear away. They areNOT malleable like metals (see below).

Many ionic compounds are soluble in water, but not all, so don't make assumptions. Salts can dissolve in waterbecause the ions can separate and become surrounded by water molecules which weakly bond to the ions. Thisreduces the attractive forces between the ions, preventing the crystal structure to exist. Evaporating the water from asalt solution will eventually allow the ionic crystal lattice to reform.

The solid crystals DO NOT conduct electricity because the ions are not free to move to carry an electric current.However, if the ionic compound is melted ordissolved in water, the liquid will now conduct electricity, as the ionparticles are now free.
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ELEMENTS, COMPOUNDS, MIXTURE separation, CHEMICAL REACTIONS & EQUATIONS

NaOH + HCl ===> NaCl + H2OPART 3 How do we write chemical equations?, How do we construct equations from first principles? Examples of writing wordequations and balanced symbol equations, what is the valency of an element? What are ionic equations? How do we write ionicequations? How can we use valency to work out a formula? - index of keywords-terms-phrases below

Al2O3(s) + 3H2SO4(aq) ==> Al2(SO4)3(aq) + 3H2O(l)

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Detailed on Atoms and Molecules