Topic 2

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Particles and isotopes

Particle	Relative mass	Relative charge
proton	1	+1
neutron	1	0
electron	1/1840	−1

protons = Z; neutrons = A − Z; electrons = Z − charge (Inside the atom)
Isotopes: same protons, different neutrons. Same configuration so same chemistry; different mass so different physical properties (Isotopes).

Definitions to learn word for word

Atomic (proton) number: the number of protons in the nucleus of an atom.
Mass number: the total number of protons and neutrons in the nucleus of an atom.
Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass: the weighted mean mass of an atom of an element, relative to one twelfth of the mass of an atom of carbon-12.
First ionisation energy: the energy required to remove one electron from each atom in one mole of gaseous atoms, to form one mole of gaseous 1+ ions.
Second ionisation energy: the energy required to remove one electron from each ion in one mole of gaseous 1+ ions, to form one mole of gaseous 2+ ions.
Orbital: a region within an atom that can hold up to two electrons, with opposite spins.
Periodic property: a property that shows a repeating pattern when plotted against atomic number, the same shape being seen again in each period.

Mass spectra

Task	Rule	Page
read an element's spectrum	one peak per isotope; m/z = mass number for a 1+ ion; height = relative abundance	The mass spectrometer
$A_\text{r}$ from a spectrum	Σ(mass × abundance) ÷ Σ(abundance); divide by the total height, not 100	Relative atomic mass
abundances from $A_\text{r}$	let one isotope be x%, the other (100 − x)%, and solve	Relative atomic mass
$M_\text{r}$ of a molecule	the molecular ion peak is the one with the largest m/z, not the tallest	Molecules and 2+ ions
2+ ions	m/z = mass number ÷ 2; small peaks; leave out of $A_\text{r}$	Molecules and 2+ ions
X₂ with two isotopes	peaks at 2m₁, m₁ + m₂, 2m₂ in the ratio p² : 2pq : q²; Cl₂ is 9 : 6 : 1 at 70, 72, 74	Predicting spectra

The five stages of a mass spectrometer: vaporise, ionise (electron gun), accelerate (electric field), deflect (magnetic field), detect.

Ionisation energy

Question	What to say
Why endothermic?	Energy is needed to overcome the attraction between the nucleus and the electron.
Three factors	nuclear charge; shielding and distance; sub-shell and pairing (Ionisation energy)
Successive values	a big jump means the next electron is in an inner shell; electrons before the first jump = group (Successive ionisation energies)
Across a period	general rise: more protons, same shell. Be to B, Mg to Al: electron is in a higher-energy p sub-shell. N to O, P to S: electron is removed from a pair that repel.
Down a group	falls: new shell, more shielding, outweigh the extra nuclear charge (Explaining trends)
Why plot logs?	to compress a wide range of values onto one axis

Orbitals and configurations

Sub-shell	Orbitals	Electrons
s	1	2
p	3	6
d	5	10

Shell n holds up to 2n² electrons: 2, 8, 18, 32. s orbitals are spherical; p orbitals are dumbbell-shaped, three of them at right angles.

Fill order to krypton: 1s 2s 2p 3s 3p 4s 3d 4p.
Rules: lowest energy first; one electron in each orbital of a sub-shell before pairing; a pair has opposite spins.
Exceptions: Cr [Ar] 3d⁵ 4s¹, Cu [Ar] 3d¹⁰ 4s¹.
Positive ions of the d block lose 4s first: Fe²⁺ [Ar] 3d⁶, Fe³⁺ [Ar] 3d⁵, Cu²⁺ [Ar] 3d⁹.
Blocks: s (Groups 1 and 2, and He), p (Groups 3 to 0), d (transition region). Same outer configuration means similar chemistry (Blocks and chemistry).

Melting and boiling temperatures

Structure	Overcome on melting	Examples
giant metallic	metallic bonding (stronger with more delocalised electrons and a smaller, more highly charged ion)	Na, Mg, Al; Li, Be
giant covalent	many strong covalent bonds	Si; B, C
simple molecular	weak London forces between molecules; the covalent bonds stay	P₄, S₈, Cl₂; N₂, O₂, F₂
monatomic	very weak London forces	Ar; Ne

In Period 3, S₈ > P₄ > Cl₂, because more electrons give stronger London forces (Melting and boiling temperatures).

The ten traps

Giving electrons to a positive ion (it has lost them).
Saying isotopes have different numbers of protons.
Dividing by 100 when the peak heights do not add up to 100.
Taking a plain average of the mass numbers instead of a weighted mean.
Calling the tallest peak the molecular ion, instead of the peak with the largest m/z.
Forgetting the factor 2 for the mixed molecule in the Cl₂ pattern.
Writing an ionisation energy equation without state symbols, or removing two electrons at once.
Explaining a trend with "full shells are stable" instead of attraction, shielding and sub-shell.
Taking 3d electrons before 4s from a d-block atom, or writing Cr as 3d⁴ 4s².
Saying covalent bonds break when simple molecular substances melt.

Mixed test

Try these with no notes, then check the answers.

Give the number of protons, neutrons and electrons in ³⁷Cl⁻.
Iron has isotopes ⁵⁴Fe (5.8%), ⁵⁶Fe (91.8%), ⁵⁷Fe (2.1%) and ⁵⁸Fe (0.3%). Calculate its relative atomic mass to one decimal place.
A compound gives peaks at m/z 15, 29, 43 and 58. State its relative molecular mass and explain how you know.
State the m/z values and the relative heights of the molecular ion peaks in the spectrum of chlorine.
Write an equation, with state symbols, for the second ionisation energy of aluminium.
An element has successive ionisation energies of 590, 1145, 4912 and 6491 kJ mol⁻¹. Which group is it in? Explain.
Write the electronic configuration of Fe²⁺ in [Ar] form and draw its 3d electrons in boxes.
Explain why the first ionisation energy of sulfur is lower than that of phosphorus.
Explain why silicon has the highest melting temperature in Period 3.

Answers cover these until you have tried the questions

³⁷Cl⁻: 17 protons, 37 − 17 = 20 neutrons, 17 + 1 = 18 electrons.
Total = 100.0. (54 × 5.8 + 56 × 91.8 + 57 × 2.1 + 58 × 0.3) ÷ 100 = (313.2 + 5140.8 + 119.7 + 17.4) ÷ 100 = 55.911, so $A_\text{r}$ = 55.9.
58. The molecular ion is the peak with the largest m/z. The peaks at 15, 29 and 43 are fragments.
m/z 70, 72 and 74 in the ratio 9 : 6 : 1 (³⁵Cl–³⁵Cl, ³⁵Cl–³⁷Cl, ³⁷Cl–³⁷Cl), because the isotopes are in the ratio 3 : 1 and the mixed molecule can form two ways.
$\ce{Al+(g) -> Al^2+(g) + e-}$
Group 2. The big jump is between the 2nd and 3rd ionisation energies (1145 to 4912), so there are 2 electrons in the outer shell; the 3rd electron comes from an inner shell, closer to the nucleus and less shielded.
Fe is [Ar] 3d⁶ 4s². The 4s electrons are lost first, so Fe²⁺ is [Ar] 3d⁶, drawn [↑↓] [↑] [↑] [↑] [↑].
In sulfur the electron is removed from a 3p orbital that holds a pair of electrons. Repulsion between the pair makes it easier to remove. In phosphorus the three 3p electrons are in separate orbitals. This outweighs the extra proton in sulfur.
Silicon is giant covalent. Melting it requires many strong covalent bonds to be broken, which needs a lot of energy. In the other elements only metallic bonding or weak London forces are overcome.