The periodic table: how the elements are arranged by atomic number into groups and periods, the development of the table by Mendeleev, metals and non-metals, and electronic configurations of the first 20 elements.

What this dot point is asking

Edexcel wants you to describe how the modern periodic table is built from the elements arranged in order of increasing atomic number, explain how Mendeleev's earlier table led to it, define groups and periods and the metal and non-metal divide, and write the electronic configurations of the first 20 elements, linking the configuration to an element's position. This underpins the trends you meet in the groups topic.

How the modern table is arranged

The periodic table lists all the known elements in order of increasing atomic number (number of protons). The arrangement places elements with similar chemical properties in the same vertical column. There are two key features:

• Groups are the vertical columns, numbered 1 to 7 and then 0 (or 8). Elements in the same group have the same number of electrons in their outer shell, which is why they have similar chemical reactions.
• Periods are the horizontal rows. The period number equals the number of occupied electron shells in atoms of those elements.

Mendeleev and the development of the table

Several scientists tried to organise the elements, but Dmitri Mendeleev (1869) made the breakthrough. He arranged the known elements in order of atomic mass, but he made two clever decisions:

1. He grouped elements with similar properties together, even when this meant leaving a gap or swapping the order of two elements whose masses were out of sequence.
2. He left gaps for elements that had not yet been discovered, and used the pattern to predict their properties (for example eka-silicon, later found to be germanium).

When the predicted elements were discovered with the expected properties, his table was confirmed. The modern table improved on his by ordering elements by atomic number rather than atomic mass, which removed the anomalies in his ordering. Before Mendeleev, Newlands had spotted a repeating pattern every eighth element (his law of octaves) but did not leave gaps, so his arrangement broke down for heavier elements.

Metals and non-metals

The table is divided by a stepped line into metals and non-metals:

• Metals (the large majority, on the left and in the centre) lose electrons to form positive ions. They are typically shiny, good conductors of heat and electricity, malleable and have high melting points.
• Non-metals (the upper right) gain or share electrons. They are typically dull, brittle when solid, and poor conductors, with lower melting points.

Elements near the dividing line (such as silicon) can show some properties of both.

Electronic configuration of the first 20 elements

Electrons occupy shells (energy levels) around the nucleus. The shells fill from the inside out, and each can hold a limited number of electrons:

• 1st shell: up to 2 electrons.
• 2nd shell: up to 8 electrons.
• 3rd shell: up to 8 electrons (for the first 20 elements).

To write the configuration, place electrons in shells, filling each before moving outwards. For example, the configuration of chlorine (atomic number 17) is $2,8,7$.

Try this

Q1. State what all elements in the same group have in common. [1 mark]

• Cue. The same number of electrons in their outer shell.

Q2. Write the electronic configuration of an atom of aluminium (atomic number 13) and give its group and period. [3 marks]

• Cue. $2,8,3$; Group 3; period 3.

Q3. Explain why the modern periodic table places the elements in order of atomic number rather than atomic mass. [2 marks]

• Cue. Ordering by atomic number removes the anomalies in Mendeleev's table, so all elements fall into groups that match their chemical properties.