Electron arrangement of atoms

This page covers a single Form 4 content standard: the electron arrangement of atoms. It is the bridge between atomic structure and the Periodic Table and bonding, because the way electrons are arranged decides how an element behaves. Master the filling rules here and you will read group and reactivity straight off an electron arrangement for the rest of SPM Chemistry.

How electrons fill shells

Electrons move around the nucleus in shells (energy levels). They fill the shell closest to the nucleus first, because it has the lowest energy, and only start a new shell once the inner one is full. For the first 20 elements, the maximum number of electrons each shell can hold is:

• First shell (closest to nucleus): maximum 2 electrons.
• Second shell: maximum 8 electrons.
• Third shell: maximum 8 electrons (for the first 20 elements).

The electron arrangement (electron configuration) is written as the number of electrons in each shell, starting from the innermost, separated by dots or commas. For example, an atom with 11 electrons has the arrangement 2.8.1: two in the first shell, eight in the second, and one left over in the third.

Valence electrons and what they tell you

The electrons in the outermost shell are called the valence electrons. They are the most important electrons in chemistry because they are the ones involved in bonding, so they decide an element’s chemical properties and its reactivity. Two quick rules link the electron arrangement to the Periodic Table:

• The number of occupied shells tells you the period (row) the element is in.
• The number of valence electrons tells you the group the element is in.

Key idea. Elements with the same number of valence electrons are in the same group and have similar chemical properties. This is why the Group 1 metals all react in a similar way, and the Group 18 noble gases are all unreactive, their outermost shells are full and stable.

Worked example

Question. An atom has a proton number of 17. Write its electron arrangement, state the number of valence electrons, and state the group and period of the element.

Step 1, Find the number of electrons. In a neutral atom, number of electrons = proton number = 17 electrons.

Step 2, Fill the shells in order. Put 2 in the first shell (15 left), 8 in the second shell (7 left), then 7 in the third shell. The electron arrangement is 2.8.7.

Step 3, Read off the valence electrons. The outermost (third) shell contains 7 valence electrons.

Step 4, State group and period. Number of valence electrons = 7, so the element is in Group 17. Number of occupied shells = 3, so it is in Period 3. (This element is chlorine.)

Answer. Electron arrangement 2.8.7; 7 valence electrons; Group 17, Period 3.

Practice question

Question. An atom has a proton number of 20. (a) Write its electron arrangement. (b) State the number of valence electrons. (c) State the group and period of the element.

Answer. (a) 20 electrons fill as 2 (first shell), 8 (second shell), 8 (third shell), 2 (fourth shell), giving the electron arrangement 2.8.8.2. (b) The outermost shell has 2 valence electrons. (c) Two valence electrons means Group 2, and four occupied shells means Period 4. (This element is calcium.)

Exam tip

Fill the shells strictly in the order 2, then 8, then 8, and never put more than the maximum in any shell, the most common mistake is to write something like 2.9 or 2.8.9 by overfilling a shell. Read the group directly from the number of valence electrons and the period from the number of shells; if your group or period does not match a real element, recount your filling. Remember that an atom is described as stable when its outermost shell is full (2 for the first shell, 8 for the others), this is the “stable octet” idea that explains why the noble gases are unreactive and drives the bonding you will study next.

Building up the first 20 elements

It is worth practising the arrangements of the first 20 elements in order, because they follow a simple, repeating pattern. Hydrogen (1 electron) is just 1; helium (2) is 2 and has a full first shell. From lithium (2.1) to neon (2.8) the second shell fills up one electron at a time, and neon has a full second shell. From sodium (2.8.1) to argon (2.8.8) the third shell fills the same way. Then potassium (2.8.8.1) and calcium (2.8.8.2) begin the fourth shell. Notice how the pattern of valence electrons repeats, lithium, sodium and potassium all end in 1 valence electron, which is exactly why they share similar chemical properties and sit in the same group. Writing out this list once, in order, fixes both the filling rule and the group pattern in your memory.

Why a full outer shell means stability

An atom is chemically stable when its outermost shell is full, a duplet (2 electrons) for the first shell, or an octet (8 electrons) for the shells after it. The noble gases already have full outer shells, which is why they are unreactive. Every other atom tends to gain, lose or share electrons to reach the same full-shell arrangement, and that drive towards a stable duplet or octet is the reason chemical bonds form. Understanding this now means the ionic and covalent bonding in the next chapters will make immediate sense rather than feeling like new, unrelated rules.

Where this fits

This standard closes the Matter and the Atomic Structure chapter and leads straight into the Periodic Table and the Chemical Bond chapters, where valence electrons do the real work. Revise it with the revision notes and drill the filling rules with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that you fill shells in the correct order every time and can name a group and period from an electron arrangement instantly, because this skill underpins almost everything in the next two chapters.

Quick recap

• Electrons fill the innermost shell first; maxima are 2, 8, 8 for the first 20 elements.
• Write the electron arrangement as numbers separated by dots, e.g. 2.8.1.
• Valence electrons are in the outermost shell and decide chemical properties.
• Number of shells = period; number of valence electrons = group.