SPI Chemistry · Formula sheet
Atomic Structure and Periodic Classification formulas
2 formulas, 4 reference tables and 13 common traps for SPI Chemistry Atomic Structure and Periodic Classification, grouped by subtopic.
Structure of the Atom
Learn this subtopic in the notesAtomic number, mass number, isotopes and isobars
Mass number and neutrons
- mass number (protons + neutrons)
- atomic number (protons)
- number of neutrons
Electron shells, valence electrons and ions
Shell capacity and valency
The particles of the atom and the models that found them
| Discovery or model | Scientist | What it showed |
|---|---|---|
| Electron | J.J. Thomson | Cathode rays carry negative particles |
| Proton | E. Goldstein | Canal rays carry positive particles |
| Neutron | James Chadwick | A particle with no charge and a mass close to a proton's |
| Nucleus | Ernest Rutherford | Alpha-particle scattering from gold foil: a tiny, dense, positive centre The scattering experiment found the nucleus, not the proton or the neutron. |
| Fixed orbits | Niels Bohr | Electrons move in shells K, L, M, N without losing energy |
Energy rises with distance from the nucleus: K is the lowest shell.
Common traps
Crediting Rutherford with the proton
Rutherford's gold foil experiment showed the nucleus. The electron was found by Thomson, the proton by Goldstein in canal rays and the neutron by Chadwick.
Getting the energy direction backwards
An electron needs energy to move away from the nucleus and gives energy out when it falls closer. If an electron loses energy going from one orbit to another, the second orbit is the nearer one.
Taking the mass number as the neutron count
The mass number counts protons and neutrons together. The neutrons are the mass number minus the atomic number, so has 16 neutrons, not 31.
Thinking a heavier atom always has the larger atomic number
Order by atomic number and order by mass number can differ. Cobalt (atomic number 27, mass number 59) comes before nickel (atomic number 28, mass number 58).
Giving isobars the same chemistry
Isobars share only the mass number. Their atomic numbers differ, so they are different elements with different chemical properties. It is isotopes that share chemistry.
Taking valency as the valence electrons every time
Chlorine has 7 valence electrons but valency 1: it is easier to gain 1 electron than to lose 7. With 5 or more valence electrons, valency is 8 minus that number.
Overfilling the M shell
Potassium (19) is 2, 8, 8, 1, not 2, 8, 9. For the first 20 elements the outer shell holds at most 8, and the next electron starts the N shell.
The Periodic Table and Its Trends
Learn this subtopic in the notesEarly attempts: Döbereiner, Newlands and Mendeleev
| Attempt | Basis | Remembered for |
|---|---|---|
| Döbereiner's triads | Atomic mass, three at a time | Middle mass is the average of the other two |
| Newlands' octaves | Atomic mass | Every eighth element is like the first From hydrogen, counting it as the first, the eighth element is fluorine. |
| Mendeleev's table | Atomic mass and properties | Gaps left for gallium and germanium |
| Modern periodic table | Atomic number | 7 periods and 18 groups |
Newlands' pattern breaks down after calcium.
Periods, groups and shells in the modern table
| Feature | Fact | Example |
|---|---|---|
| Periods | 7 horizontal rows | Period 3 runs from sodium to argon |
| Groups | 18 vertical columns | Group 18 holds the noble gases |
| Period number | Equals the number of shells | Sulphur (2, 8, 6) has 3 shells: period 3 |
| Group number, groups 1 and 2 | Equals the valence electrons | Calcium (2, 8, 8, 2): group 2 |
| Group number, groups 13 to 18 | Valence electrons plus 10 | Phosphorus (2, 8, 5): group 15 |
| Elements in each period | 2, 8, 8, 18, 18, 32, 32 | Period 1 holds only hydrogen and helium |
| Same group | Same valency, similar formulae | Group 1: and A metal chloride means valency 2: group 2. |
Shells give the period, valence electrons give the group.
Trends across a period and down a group
| Property | Across a period, left to right | Down a group |
|---|---|---|
| Number of shells | Stays the same | Increases |
| Valence electrons | Increase from 1 to 8 | Stay the same |
| Atomic radius | Decreases | Increases Electrons are added across a period, but the radius still falls: the nucleus pulls harder. |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
| Ease of losing electrons | Decreases | Increases |
Every row of the down-a-group column is the reverse of the across-a-period column, except valence electrons.
Common traps
Counting the eighth element wrongly
In Newlands' octaves the starting element counts as the first. From lithium the order is Li, Be, B, C, N, O, F, Na, so sodium is the eighth, not fluorine.
Saying Mendeleev used atomic number
Mendeleev arranged the elements by atomic mass. The modern periodic table, based on atomic number, came later from Moseley's work.
Swapping periods and groups
The modern table has 7 periods (rows) and 18 groups (columns). An option of 18 for the number of periods is the count of groups.
Reading the period from the electron count
The period is the number of shells, not the number of electrons. Magnesium, 2, 8, 2, has three shells and is in period 3, even though its outer shell has 2 electrons.
Thinking more electrons means a bigger atom across a period
Across a period electrons go into the same shell while the nuclear charge rises, so the atom gets smaller. The radius grows only down a group, where a new shell is added.
Mixing up the direction of metallic character
Metallic character falls from left to right and rises from top to bottom. Among elements of one period, the one furthest left is the most metallic.