2.2.1 Mendeleev's periodic table

Mendeleev ordered the elements by mass but let chemical behaviour fix the columns

1. Dmitri Mendeleev arranged the elements known in the 1860s using their properties and the properties of their compounds.
2. He worked mostly in order of increasing relative atomic mass, starting a new row whenever similar chemical behaviour appeared again.
3. Elements that reacted in similar ways and formed compounds of similar formulae were placed in the same column.
4. The columns showed that chemical properties repeat at regular intervals, which is what makes the arrangement periodic.
5. Where an element did not fit, he trusted the chemical evidence rather than force it into the wrong column.
6. He left a gap instead of filling a position with an element whose chemistry did not match its neighbours.

Gaps in the table became testable predictions

1. Properties change in a regular way across a row and then repeat when the next row begins.
2. A gap therefore sits between elements whose properties bracket the missing one.
3. Mendeleev estimated a missing element's relative atomic mass from the elements on either side of the gap.
4. He predicted physical properties such as density and melting point from the same pattern.
5. He predicted the formulae of the compounds that the missing element would form.
6. He named two predicted elements eka-aluminium and eka-silicon, after the elements directly above them.
7. Gallium and germanium were discovered later and fell into exactly those two positions.

Isotope abundance explains the pairs that sit out of mass order

1. A sample of an element contains its isotopes in fixed proportions, and those proportions set the value of $A_r$.
2. A common isotope pulls the average towards its own mass far more strongly than a rare one does.
3. For a few neighbouring pairs the averages come out in the opposite order to the one their chemistry demands.
4. In today's table argon has $A_r\approx 39.9$ and potassium $A_r\approx 39.1$, yet argon is an unreactive gas and potassium a reactive metal.
5. Argon was not isolated until 1894, so that pair belongs to the modern table rather than to Mendeleev's.
6. Mendeleev himself reversed two such pairs, putting cobalt ($58.9$) before nickel ($58.7$) and tellurium ($127.6$) before iodine ($126.9$).
7. Ordering by atomic number removes every one of these exceptions, but that idea arrived after Mendeleev's work.

Why the later discoveries counted as evidence

1. The predictions were published before the elements were found, so they could not have been fitted to the results afterwards.
2. Gallium and germanium matched the predicted masses and densities closely.
3. A table that predicts unknown facts carries more weight than one that only organises known facts.
4. The gaps closed one by one as more elements were isolated.
5. The pairs still out of mass order were explained once atomic number replaced relative atomic mass as the ordering rule.

2.2.2 Periods, groups, metals and non-metals

Atomic number fixes both the element and its place in the table

1. Every atom of an element carries the same number of protons, and no two elements share that number.
2. The elements run left to right and then down, in order of increasing atomic number.
3. An element's position therefore follows directly from its proton count.
4. Carbon has atomic number $6$, so it is the sixth element in that order.
5. A neutral atom holds as many electrons as protons, so the same number gives the electron count.
6. Reading a position off the table is enough to fix the number of protons in the nucleus of that element.

Periods run across the table and groups run down it

1. Each horizontal row fills one more electron shell than the row above it.
2. Each main-group column collects elements whose atoms have the same number of outer-shell electrons.
3. Shared outer-shell arrangements are why elements in one column react in similar ways.
4. Lithium and sodium sit in the same column, and both are reactive metals that form compounds of similar formulae.
5. Magnesium and chlorine sit in the same row, and their properties are quite different.
6. A position across a row says little on its own, while a position down a column predicts chemical behaviour.

Metals sit to the left and non-metals to the right

1. Most elements on the left and in the centre of the table are metals.
2. Most elements on the right are non-metals, separated from the metals by a staircase line running down from near boron.
3. Metals are typically shiny solids that conduct electricity, while non-metals are typically dull and conduct poorly.
4. Hydrogen is a non-metal despite being drawn above group $1$.
5. Position is therefore a reliable first sort, with hydrogen the exception worth remembering.

Outer-shell electrons explain the split

1. Metal atoms usually hold one, two or three electrons in the outer shell.
2. Losing those few electrons leaves a full shell beneath, so metal atoms form positive ions.
3. Non-metal atoms usually hold four to seven outer-shell electrons.
4. Gaining or sharing completes that shell more easily than losing does, and how many electrons are involved depends on the element, from one for chlorine to four for carbon.
5. The group $0$ elements already hold a full outer shell, which is why they react with almost nothing.
6. The difference in outer-shell electrons is what turns a difference in position into a difference in chemistry.

Reading an element's identity and type from its position

1. Find the element and read its atomic number to get the proton count.
2. Count the rows down from the top to name the period.
3. Read the group number printed above the column, since the modern table has eighteen columns and the transition block sits between Groups 2 and 3.
4. Judge metal or non-metal from which side of the staircase line the element sits on.
5. Support that judgement by stating the outer-shell electron count and whether the atom tends to lose or gain electrons.

2.2.3 Electronic configuration and position in the periodic table

Electron shells fill from the inside out

1. The electrons in an atom occupy shells at fixed distances from the nucleus.
2. The first shell holds at most $2$ electrons.
3. The second and third shells each hold at most $8$ electrons across the first $20$ elements.
4. The fourth shell begins to fill once the third holds $8$.
5. A neutral atom has as many electrons as its atomic number.
6. An electronic configuration lists those electrons shell by shell, separated by full stops, starting with the shell nearest the nucleus.
7. Sodium's configuration $\mathrm{2.8.1}$ means $2$ in the first shell, $8$ in the second and $1$ in the third.

The first twenty configurations, one electron at a time

1. Period 1: hydrogen $1$, helium $2$.
2. Period 2: lithium $2.1$, beryllium $2.2$, boron $2.3$, carbon $2.4$, nitrogen $2.5$, oxygen $2.6$, fluorine $2.7$, neon $2.8$.
3. Period 3: sodium $\mathrm{2.8.1}$, magnesium $\mathrm{2.8.2}$, aluminium $\mathrm{2.8.3}$, silicon $\mathrm{2.8.4}$, phosphorus $\mathrm{2.8.5}$, sulfur $\mathrm{2.8.6}$, chlorine $\mathrm{2.8.7}$, argon $\mathrm{2.8.8}$.
4. Period 4: potassium $\mathrm{2.8.8.1}$, calcium $\mathrm{2.8.8.2}$.
5. Each element adds one electron to the one before it, so any configuration can be built from its neighbour.
6. A shell diagram carries the same information: the nucleus at the centre, concentric circles around it, and the right number of electrons marked on each circle.

The period number counts the occupied shells

1. Count how many numbers appear in the configuration, because each one stands for an occupied shell.
2. Hydrogen and helium have one number each, so they lie in period $1$.
3. Lithium to neon have two numbers, so they lie in period $2$.
4. Sodium to argon have three numbers, so they lie in period $3$.
5. Potassium and calcium have four numbers, so they lie in period $4$.
6. A number written as $1$ still counts, because that shell does hold an electron.
7. The period changes at the point where a new shell starts, such as between argon $\mathrm{2.8.8}$ and potassium $\mathrm{2.8.8.1}$.

The group number comes from the outer-shell electrons

1. Read the last number in the configuration to find the outer-shell electron count.
2. One outer-shell electron places an element in group $1$, and two place it in group $2$.
3. Counts of $3$ to $7$ place an element in groups $3$ to $7$ respectively.
4. A full outer shell places an element in group $0$, which holds the noble gases.
5. Helium belongs to group $0$ because its single shell is complete at $2$ electrons.
6. Chlorine $\mathrm{2.8.7}$ therefore lies in group $7$, and magnesium $\mathrm{2.8.2}$ in group $2$.
7. Elements in one group behave similarly because the same outer-shell count takes part in their reactions.

Going both ways between configuration and position

1. Write the configuration from the atomic number, filling each shell before starting the next.
2. Count the numbers for the period and read the last number for the group.
3. Calcium $\mathrm{2.8.8.2}$ sits in period $4$, group $2$.
4. Argon $\mathrm{2.8.8}$ sits in period $3$, group $0$.
5. The same two counts work from a shell diagram: occupied circles give the period, and electrons on the outer circle give the group.
6. Working backwards, an element in period $3$ and group $6$ has three occupied shells and six outer-shell electrons, giving $\mathrm{2.8.6}$.