Electronic Structure & Electron Configuration

Atomic Structure

Atoms are the basic building blocks of matter. They consist of three main particles:

• Protons 6 found in the nucleus, have a relative charge of +1 and a relative mass of 1
• Neutrons 6 also in the nucleus, have no charge (0) and a relative mass of 1
• Electrons 6 orbit the nucleus in shells, have a relative charge of 61 and a very small relative mass (approximately 1/2000 of a proton)

The nucleus contains protons and neutrons, making it very dense and positively charged. Electrons move around the nucleus in fixed paths called electron shells.

Because electrons have a negative charge and protons have a positive charge, the overall charge of an atom is neutral if the number of electrons equals the number of protons.

For example, a helium atom has 2 protons and 2 electrons, so the positive and negative charges balance out, making the atom neutral.

Electron Configuration

Electrons are arranged in shells around the nucleus. Each shell can hold a limited number of electrons:

• The first shell can hold up to 2 electrons
• The second shell can hold up to 8 electrons
• The third shell can hold up to 8 electrons (for GCSE level)

Electrons fill the shells starting from the innermost shell moving outwards. This is called the filling order.

For example, an oxygen atom has 8 electrons. These fill the shells as 2 in the first shell and 6 in the second shell, written as 2,6.

The electron configuration helps explain an elements chemical properties because the electrons in the outer shell (called valence electrons) are involved in chemical reactions.

For instance, sodium has 11 electrons arranged as 2,8,1. The single electron in the outer shell makes sodium very reactive.

Learning example: The electron configuration of carbon (6 electrons) is 2,4 because the first shell holds 2 electrons and the second shell holds the remaining 4.

Note: Although the third shell can hold more than 8 electrons in higher levels of chemistry, for GCSE level it is limited to 8 electrons.

Simple Model of the Atom

Atoms are the smallest units of an element that retain its chemical properties. They are neutral overall because the number of protons (+ charge) equals the number of electrons ( 6 charge).

Electrons in the outer shell determine how atoms react with other atoms. Atoms gain, lose, or share electrons to become more stable, often achieving a full outer shell.

This simple model helps us understand why elements behave the way they do, such as why noble gases are unreactive (full outer shells) and why alkali metals are very reactive (one electron in outer shell).