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IGCSE Chemistry 0620 · Chapter 2 notes · ~18 min read

Atoms, elements, compounds & bonding

Inside the atom, how electrons are arranged, and why atoms join up — ionic, covalent and metallic bonding — with the structures and properties that follow from each.

Atomic structureIsotopesElectronic configurationIonic bondingCovalent bondingGiant structuresMetallic bonding
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Elements contain one type of atom. Compounds are different atoms chemically bonded. Mixtures are not bonded at all.
  • An atom has a tiny nucleus (protons + neutrons) with electrons in shells around it. Electrons decide the chemistry.
  • Atoms bond to get a full outer shell. Metals give electrons → ionic. Non-metals share → covalent. Metals also pool electrons → metallic.
  • The type of bonding decides the structure, and the structure decides the properties. EXT

By the end you can

  • Tell elements, compounds and mixtures apart.
  • Describe an atom, and work out protons, neutrons and electrons from the nuclide symbol.
  • Write electronic configurations for the first 20 elements and link them to groups and periods.
  • Define isotopes, and calculate relative atomic mass from isotope data. EXT
  • Draw dot-and-cross diagrams for ionic and covalent compounds.
  • Link structure and bonding to melting point, conductivity and uses.

1Elements, compounds and mixtures

Three kinds of substance

ELEMENTCOMPOUNDMIXTUREone kind of atom(argon, Ar)different atomschemically joined(water, H₂O)different substancesnot joined(argon + water)compound: fixed ratio, new properties · mixture: any proportions, easy to separate
Left: in a compound the atoms are bonded in a fixed ratio. Right: in a mixture the particles are just jumbled together.
ElementCompoundMixture
made ofone type of atomtwo or more elements chemically bondedtwo or more substances not bonded
formulafixed (Fe, O₂)fixed (H₂O, NaCl)no formula — amounts can vary
separationcannot be broken down chemicallyonly by a chemical reactionby physical methods (filtering, distilling …)
propertiesthose of the elementvery different from the elements in itproperties of each part are kept
exampleiron, oxygenwater, saltair, sea water, brass
  • Iron + sulfur: a mixture can be separated with a magnet (iron is magnetic). Heat the mixture strongly and it reacts to form the compound iron(II) sulfide, which is not magnetic and has new properties.
Picture itA compound is a cake — you cannot pick the flour back out. A mixture is a fruit salad — you can pick out each fruit.
Exam tipCompounds have a fixed ratio (water is always H₂O). Mixtures have no fixed ratio (air can have more or less water vapour).

2Inside the atom

Protons, neutrons, electrons

6p6nelectron (−1)moves in a shell round the nucleusshells1st holds 2 electrons, 2nd holds 8nucleusprotons (+1) and neutrons (0)carbon atom: 6 protons, 6 electrons → electronic configuration 2,4
An atom. The nucleus holds the protons and neutrons. The electrons move in shells outside it.
ParticleRelative chargeRelative massWhere
proton+11nucleus
neutron01nucleus
electron−11/1836 (almost nothing)shells
  • An atom is neutral: the number of protons equals the number of electrons.
  • Almost all the mass is in the nucleus — it is tiny but very dense. Almost all the volume of the atom is empty space.

Proton number (atomic number, Z) — the number of protons in the nucleus. It identifies the element.

Nucleon number (mass number, A) — the total number of protons and neutrons in the nucleus.

neutrons = A − Z
nucleon number minus proton number
  • The nuclide symbol shows both numbers: 2311Na has A = 23 (top) and Z = 11 (bottom).
Worked example

How many protons, neutrons and electrons are in an atom of 2713Al?

  1. protons = Z = 13
  2. electrons = protons = 13 (the atom is neutral)
  3. neutrons = A − Z = 27 − 13 = 14
Worked example

EXT How many protons, neutrons and electrons are in the ion 2412Mg²⁺?

  1. protons = Z = 12
  2. neutrons = 24 − 12 = 12
  3. the ion has a 2+ charge, so it has lost 2 electrons: 12 − 2 = 10 electrons
Common mistakeMixing up which number is on top. The bigger number (A) is on top.

Electronic configuration

sodium2,8,1Group IPeriod 3magnesium2,8,2Group IIPeriod 3chlorine2,8,7Group VIIPeriod 3argon2,8,8Group VIIIPeriod 3outer electrons = group number · number of shells = period number
Electrons fill the shells from the inside outwards. Sodium, magnesium, chlorine and argon are shown — all in Period 3, so all have 3 shells.

Filling the shells

  • Electrons sit in shells (energy levels). Fill them from the inside out:
  • shell 1 holds up to 2 electrons; shell 2 holds up to 8; shell 3 holds up to 8 (for the first 20 elements).
  • Write it as numbers separated by commas: sodium (11 electrons) = 2,8,1; chlorine (17) = 2,8,7.
  • The electrons in the outer shell decide how the element reacts.
PeriodIIIIIIIVVVIVIIVIII
1H 1––––––He 2
2Li 2,1Be 2,2B 2,3C 2,4N 2,5O 2,6F 2,7Ne 2,8
3Na 2,8,1Mg 2,8,2Al 2,8,3Si 2,8,4P 2,8,5S 2,8,6Cl 2,8,7Ar 2,8,8
4K 2,8,8,1Ca 2,8,8,2––––––

Groups, periods and noble gases

  • Group number = number of outer-shell electrons (Groups I to VII). Group VIII (also called Group 0) has a full outer shell.
  • Period number = number of shells that contain electrons.

Noble gases — the Group VIII elements (He, Ne, Ar …). They have a full outer shell, so they are very unreactive. Atoms of other elements bond to reach this stable arrangement.

Picture itSodium (2,8,1) has one lonely electron in its outer shell, so it is easy to lose. Chlorine (2,8,7) is one short of a full set, so it is eager to gain one. Put them together and both get what they want.
Exam tipSodium is in Group I (1 outer electron) and Period 3 (3 shells) — you can read both straight from 2,8,1.

Isotopes

Isotopes — atoms of the same element with the same number of protons but different numbers of neutrons.

17 p18 n3517Clchlorine-3517 protons, 18 neutrons17 electrons (2,8,7)17 p20 n3717Clchlorine-3717 protons, 20 neutrons17 electrons (2,8,7)=same electrons = same chemistry · different neutrons = different mass
Two isotopes of chlorine. Both have 17 protons and 17 electrons. Chlorine-35 has 18 neutrons; chlorine-37 has 20.
  • Isotopes have the same proton number but different nucleon numbers. Examples: carbon-12 and carbon-14; chlorine-35 and chlorine-37; hydrogen-1, hydrogen-2 and hydrogen-3.
  • EXT Isotopes of an element have the same chemical properties, because chemistry depends on electrons and they have the same number. They have different physical properties that depend on mass, for example density.
EXT Ar = Σ (isotope mass × % abundance)100
relative atomic mass is the weighted average of the isotope masses
Worked example

EXT Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass.

  1. Ar = (75 × 35 + 25 × 37) ÷ 100
  2. = (2625 + 925) ÷ 100
  3. = 3550 ÷ 100 = 35.5
Worked example

EXT Magnesium is 79% ²⁴Mg, 10% ²⁵Mg and 11% ²⁶Mg. Calculate its relative atomic mass to 1 decimal place.

  1. (79 × 24) + (10 × 25) + (11 × 26) = 1896 + 250 + 286 = 2432
  2. 2432 ÷ 100 = 24.32
  3. Ar = 24.3
Exam tipAlways check that your answer lies between the lightest and heaviest isotope, and closer to the one that is most abundant. That is why chlorine is 35.5, not 36.

3Ions and ionic bonding

How ions form

Ion — a charged particle formed when an atom (or group of atoms) loses or gains electrons.

Cation — a positive ion, formed when an atom loses electrons. Metals form cations.

Anion — a negative ion, formed when an atom gains electrons. Non-metals form anions.

  • Atoms lose or gain electrons to get a full outer shell, like a noble gas.
  • Group I atoms lose 1 electron (+1); Group II lose 2 (2+); Group III lose 3 (3+). Group VII gain 1 (−1); Group VI gain 2 (2−); Group V gain 3 (3−).
Picture itA sodium atom has one outer electron it does not need. A chlorine atom is one electron short. Sodium hands over the electron and both end up with a full outer shell — sodium has a + charge and chlorine has a − charge, and they stick together.

Ionic bond — the strong electrostatic attraction between oppositely charged ions.

Dot-and-cross diagrams for ionic compounds

● electron from Na ✕ electron from ClNaClNa+Cl−Na atom2,8,1loses 1 e⁻Cl atom2,8,7gains 1 e⁻sodium ionNa⁺ (2,8)chloride ionCl⁻ (2,8,8)oppositely charged ions attract each otherthis electrostatic attraction is the ionic bond
Sodium chloride. Sodium loses one electron to chlorine. Both ions end up with a full outer shell. Draw the outer shell only.
  • Step 1: draw the outer shell of each atom. Use dots for one atom's electrons and crosses for the other's.
  • Step 2: move the electron(s) from the metal to the non-metal.
  • Step 3: draw each ion in square brackets with its charge at the top right.
  • Na⁺ has 2,8 and Cl⁻ has 2,8,8 — both now have a noble-gas arrangement.

Magnesium compoundsEXTENDED

MgO2 e⁻Mg2+O2−magnesium oxide MgO: one Mg²⁺ and one O²⁻ (ratio 1 : 1)ClMgCl1 e⁻ to each ClCl−Mg2+Cl−magnesium chloride MgCl₂: one Mg²⁺ and two Cl⁻ (ratio 1 : 2)
Top: magnesium oxide, one Mg²⁺ and one O²⁻. Bottom: magnesium chloride, one Mg²⁺ and two Cl⁻ — each chlorine takes one electron.
  • The number of electrons lost must equal the number gained, so the total charge is zero.
  • Mg loses 2 electrons. Each Cl can only gain 1, so you need two chlorine atoms: MgCl₂.
Common mistakeForgetting the brackets and the charge on each ion, or drawing the shared electrons between the ions. In ionic bonding electrons are transferred, not shared.

The giant ionic latticeEXTENDED

−+−+−+−+−+−+−+−+−+−+−+−+−+Na⁺ sodium ion−Cl⁻ chloride ioneach ion is held by strongelectrostatic attraction in everydirection → giant ionic latticesolid: ions fixed, no conductionmolten or dissolved: ions move,so it conductssodium chloride, NaCl
Part of a sodium chloride crystal. Every ion is attracted to all of its neighbours with the opposite charge.
  • An ionic solid is not made of molecules. It is a huge, regular, giant ionic lattice of alternating positive and negative ions.
  • Each ion is held by strong electrostatic attraction in every direction.

Properties of ionic compounds

PropertyWhy
high melting and boiling points (NaCl melts at 801 °C)lots of energy is needed to overcome the strong attraction between ions in the whole lattice
hard but brittlehitting the crystal shifts the layers, so like charges line up, repel and the crystal splits
do not conduct when solidions are held in fixed positions
conduct when molten or dissolved in waterthe ions are free to move and carry the charge
  • Many ionic compounds dissolve in water, which breaks the lattice apart into separate ions.
Exam tip"Why can't solid sodium chloride conduct electricity?" → the ions cannot move. Never say "no electrons". Conduction in ionic compounds is by ions, not electrons.

4Covalent bonding

Sharing electrons

Covalent bond — a pair of electrons shared between two atoms. It forms between non-metal atoms.

  • Each atom shares electrons so that it gets a full outer shell. The shared pair is attracted to the nuclei of both atoms, and this holds the atoms together.
  • In a dot-and-cross diagram, the shared electrons sit in the overlap of the two outer shells.
  • Number of bonds an atom makes: H = 1, Cl = 1, O = 2, N = 3, C = 4. (It is just the number of electrons it needs for a full shell.)
● electron from one atom ✕ electron from the otherHHClClHClhydrogen H₂chlorine Cl₂hydrogen chloride HClOHHNHHHCHHHHwater H₂Oammonia NH₃methane CH₄
Core covalent molecules: H₂, Cl₂, HCl, H₂O, NH₃ and CH₄. Only the outer electrons are drawn. Pairs that are not shared are called lone pairs.
  • Lone pairs (not shared) are drawn on the atom. Oxygen in water has 2 lone pairs; nitrogen in ammonia has 1.

Double and triple bonds, and larger moleculesEXTENDED

● electron from one atom ✕ electron from the otherOONNoxygen O₂ (double bond)nitrogen N₂ (triple bond)OCOCCHHHHcarbon dioxide CO₂ethene C₂H₄ (C=C double bond)
O₂ has a double bond (2 shared pairs). N₂ has a triple bond (3 shared pairs). CO₂ has two double bonds. Ethene C₂H₄ has a C=C double bond.

Check your diagram

CHHHOHmethanol, CH₃OHC forms 4 bondsO forms 2 bonds and has 2 lone pairsH forms 1 bond each● and ✕ = electrons from different atoms
Methanol, CH₃OH. Carbon makes 4 bonds, oxygen makes 2 and keeps 2 lone pairs, each hydrogen makes 1.
  • Check: count the electrons around each atom. Every atom except hydrogen should have 8 (octet); hydrogen has 2.
Common mistakeForgetting the lone pairs on the outside of O and N atoms.

Properties of simple molecular substances

  • Low melting and boiling points: many are gases or liquids at room temperature.
  • Do not conduct electricity: there are no ions and no free electrons.
  • EXT The weak forces between molecules (intermolecular forces) are overcome when they melt or boil. The strong covalent bonds inside the molecules stay intact.
covalent bondstrong; stays intactforces betweenmolecules: weakweak forces → little energyneeded → low melting andboiling pointssimple molecules (e.g. iodine, water, CO₂)no free ions or electrons → do not conduct electricity
Strong covalent bonds hold the atoms inside each molecule. Only the weak forces between the molecules are overcome when the substance melts or boils.
Exam tip"Why does iodine have a low melting point?" → weak forces between molecules need little energy to overcome. Never say the covalent bonds break — that is the most common mark lost on this topic.

5Giant covalent structures

Diamond, graphite and silicon dioxide

  • Some covalent substances do not form small molecules. Millions of atoms join in one giant network held together by strong covalent bonds.
diamond (carbon)silicon dioxide (silica)CCCCCSiSiSiSiOOOOSieach C bonded to 4 other Cstrong bonds throughout → very hardeach Si bonded to 4 O, each O to 2 Sisimilar structure → very hardgiant covalent: millions of strong bonds, so very high melting points
Diamond: each carbon is bonded to four others. Silicon dioxide: each silicon is bonded to four oxygen atoms, and each oxygen to two silicons. Silicon dioxide is Extended.
strong covalent bondswithin each layer(each C bonded to 3 others)weak forces between thelayers → layers slideover each othersoft and slippery: lubricant,pencils · delocalised electrons→ conducts: electrodesgraphite (carbon)
Graphite. Each carbon is bonded to three others in flat layers. The layers are held together only by weak forces.
DiamondGraphiteSilicon dioxide EXT
structureeach C bonded to 4 otherslayers; each C bonded to 3 otherseach Si bonded to 4 O; each O to 2 Si
melting pointvery highvery highvery high
hardnessvery hardsoft and slipperyvery hard
conducts electricity?no — no free electronsyes — one delocalised electron per carbonno
usescutting tools, drill tipslubricant, pencils, electrodesglass, sand, abrasives
  • Diamond and graphite are both pure carbon — different structures give different properties.
  • In graphite, the weak forces between layers let the layers slide, making it a good lubricant. The delocalised electrons are free to move along the layers, so it conducts.
  • EXT Silicon dioxide has a structure like diamond, so it has similar properties: hard, high melting point, does not conduct.
Exam tip"Why is diamond so hard?" → each carbon forms four strong covalent bonds in all directions through the whole structure.

6Metallic bonding

A lattice in a sea of electrons

Metallic bonding — the electrostatic attraction between positive metal ions and a sea of delocalised (free-moving) electrons.

+++++++++++++++layers can slide● delocalised electronsmetal ions in a regular latticemetallic bond: attractionbetween the + ions and thesea of delocalised electronsfree electrons carry chargeand thermal energy: conductslayers slide without breaking:malleable and ductile [E]
A metal. The positive ions sit in a regular lattice. The outer electrons are free to move through the whole structure.
  • Metal atoms lose their outer electrons. These become delocalised — they belong to the whole piece of metal, not to one atom.
  • The attraction between the + ions and the − electrons holds the metal together.

Properties of metalsEXTENDED

  • Good electrical conductors: the delocalised electrons carry charge through the metal.
  • Malleable and ductile: the layers of ions can slide over each other without breaking the bonding, because the electrons keep holding the ions together.
  • High melting points: the attraction between the ions and the electrons is strong, and it takes a lot of energy to break it.
Common mistake"The metal ions are free to move and carry the current." In a metal the electrons carry the current; the ions stay in place.

7Compare the structures

Which type of substance is it?

TypeMade ofHeld together byMelting pointConducts?Examples
ionicmetal + non-metal ionsstrong attraction between + and − ionshighonly when molten or dissolvedNaCl, MgO
simple molecularsmall moleculesweak forces between moleculeslowneverH₂O, CO₂, I₂
giant covalenthuge network of atomsstrong covalent bondsvery highno (graphite does)diamond, SiO₂
metallic+ ions and electrons+ ions attracted to a sea of electronsusually highyes, solid and liquidCu, Fe, Na
  • Quick decision: metal on its own → metallic. Metal + non-metal → ionic. Two non-metals → covalent (small molecule — unless it is C, Si or SiO₂).
Memory trickIonic = give and take. Covalent = share. Metallic = pool.

8Test yourself

Test yourself — cover the answers first

Quick check

  1. What is the difference between a compound and a mixture?
  2. An atom has 15 protons and a nucleon number of 31. How many neutrons does it have?
  3. Write the electronic configuration of calcium (Z = 20).
  4. What do all atoms of the same element have in common?
  5. Why are noble gases unreactive?
  6. Why does graphite conduct but diamond does not?

Exam-style

  1. Draw (or describe) the dot-and-cross diagram for sodium chloride. [3]
  2. Explain why sodium chloride has a high melting point but does not conduct when solid. [3]
  3. EXT Explain why iodine has a low melting point but diamond has a very high one. [3]
  4. EXT Calculate the relative atomic mass of boron: 20% ¹⁰B and 80% ¹¹B.
Show answers
  1. a compound has atoms chemically bonded in a fixed ratio; a mixture is not bonded and the amounts can vary
  2. 31 − 15 = 16
  3. 2,8,8,2
  4. the same number of protons
  5. they have a full outer shell
  6. graphite has delocalised electrons; every carbon in diamond uses all 4 outer electrons in bonds
  7. Na loses its one outer electron to Cl
    Na⁺ has 2,8 and Cl⁻ has 2,8,8
    each ion shown in brackets with its charge (+ and −)
  8. it is a giant lattice of ions with strong electrostatic attraction in all directions
    so a lot of energy is needed to overcome it
    in the solid the ions cannot move, so no charge can flow
  9. iodine is simple molecular with weak intermolecular forces between molecules, which need little energy to overcome
    diamond is a giant covalent structure, so every strong covalent bond must be broken
    that needs a lot of energy
  10. (20 × 10 + 80 × 11) ÷ 100 = (200 + 880) ÷ 100 = 10.8
Practise this chapter
Elements, compounds & mixturesAtomic structure & the Periodic TableIsotopesIons & ionic bondingCovalent bonding & giant structuresMetallic bonding
Next chapterStoichiometry — formulae & moles →
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