CC4 · The periodic table · Topic 1 — Key concepts in chemistry | GCSE Edexcel Combined Science

Revise the key ideas

Mendeleev arranged elements mainly by increasing relative atomic mass, placing elements with similar chemical properties in the same columns.
He sometimes changed the mass order to preserve chemical groupings. Tellurium was placed before iodine so iodine joined the other halogens despite its lower relative atomic mass.
Mendeleev left gaps for undiscovered elements and predicted their properties from nearby elements. Later discoveries matching predictions supported his table.
Mendeleev worked before scientists knew about protons and atomic number. Ordering elements by mass could not explain every exception.
Moseley’s X-ray evidence helped show that elements should be ordered by atomic number. The modern table follows increasing proton number, solving the problems caused by ordering only by mass.
New evidence explained why proton-number order works.
Tellurium has atomic number 52 and iodine 53, so their modern order agrees with Mendeleev's chemical placement without reversing atomic-number order.

Rows in the periodic table are periods and vertical columns are groups. Atomic number increases across a period.
Elements in the same main group have similar chemical properties because their atoms have the same number of outer-shell electrons.
For main groups 1–7, group number gives the number of outer electrons. GCSE group 0 contains noble gases with complete outer shells; it is also numbered group 18 in another convention.
A simplified table: group 0 is at the right; omitted elements are not empty gaps in the full table.
Period number tells you the number of occupied electron shells in the simple model for these main-group elements.
Metals occupy the left and centre; non-metals are mainly on the right. Hydrogen is a non-metal despite being on the left above group 1.
Metals usually conduct electricity and form positive ions by losing electrons. Many non-metals form negative ions or share electrons; not every non-metal conducts like graphite.
The group predicts related chemical behaviour, not identical properties. Reactivity trends differ between groups and need the appropriate explanation.

Electrons occupy energy levels or shells around the nucleus. For a neutral atom, the electron total equals its atomic number.
For the first 20 elements, use a simple filling pattern of up to 2 in the first shell, then 8 in the second, then 8 in the third before starting the fourth. This is not a universal shell-capacity rule.
Electronic configurations list electrons from inner to outer shell. Sodium, Z = 11, is 2,8,1; the numbers add to 11.
11 electrons: period 3, group 1.
Helium is 2 and neon 2,8. Their full outer shells explain membership of group 0; helium's first shell is full with just two electrons.
Magnesium, Z = 12, is 2,8,2: period 3 and group 2. Chlorine, Z = 17, is 2,8,7: period 3 and group 7.
17 electrons: period 3, group 7.
Potassium, Z = 19, is 2,8,8,1 and calcium, Z = 20, is 2,8,8,2. Both have four occupied shells and are in period 4.
20 electrons: period 4, group 2.
Dot-and-shell diagrams show electron counts, not actual electron paths or scale. Include all electrons when asked for an atom's complete configuration.

An atom with configuration 2,8,3 has three occupied shells and three outer electrons: period 3, group 3, aluminium.
An atom with configuration 2,7 is fluorine: period 2, group 7. Chlorine below it has an extra occupied shell but the same number of outer electrons.
Elements below one another in a group have more occupied shells down the group. Their shared outer-electron count connects their chemistry.
To identify an element from a configuration, add all electrons for a neutral atom and find that atomic number. Do not use mass number as the electron count.
Use the table and configuration together: check electron total, occupied-shell count and outer-electron count. Group 0 is the exception to a literal outer-electron group-number rule.
Electronic arrangement relates to bonding: a group 1 metal commonly loses one outer electron, while a group 7 non-metal commonly gains one or shares one electron.