Doc Brown's chemistry notes on atomic and electron structure

1. The structure of atoms – three fundamental particles - the properties of the proton, neutron and electron

1b. WHAT ARE ATOMS? and WHAT DO WE MEAN BY FUNDAMENTAL PARTICLES? (sub–atomic particles)

An ATOM is the smallest particle of a substance, an element, which can have its own characteristic properties AND cannot be split into simpler substances.

However, why do we have different elements?

Is an atom the simplest particle we need to know about to understand chemistry?

In order to answer these questions we must look a bit deeper into the fundamental structure of matter, that is everything around you!

Atoms are the smallest particles of matter whose properties we study in Chemistry.

Every element or compound is comprised of atoms. All the atoms are the same in the structure of an element (ignoring isotopes - different numbers of neutrons, see later) and two or more different atoms/elements must be present in a compound.

Each element has its own chemical symbol (carbon C, oxygen O, sodium Na etc.), which with added numbers (e.g. right), can be used to indicate the composition of an atom in terms of protons, electrons and neutrons. All of this will be explained in detail below

Initially, once the concept of an atom was established, it was assumed that atoms were indestructible and not divisible into smaller particles, but merely combined in different proportions to give the range of compounds we know about e.g. Dalton's atom model.

From experiments done in the late 19^th and early 20^th century it was deduced that atoms are made up of three fundamental or sub–atomic particles called protons, neutrons and electrons, which are listed below with their relative masses and electrical charges.

In section 7. the history of the development of the atomic model is described in detail.

WHAT IS THE NUCLEUS? WHAT ARE NUCLEONS?

The three fundamental particles of which atoms are composed

The table gives the relative mass and electric charge of the three sub–atomic particles known as the proton, neutron and electron

Protons and neutrons are much heavier (~ x 2000) than electrons.

You can think of the mass of an electron as about ¹/₂₀₀₀th of the mass of a proton or neutron, so, a pretty small mass BUT they occupy most of the space of an atom!!!

You should also realise because of the relatively small mass of the electrons most of an atom's mass is in the nucleus.

You see values of 1/1836 quoted for the relative mass of an electron, but don't worry about it, there are different ways/scales on which an electron's mass has been calculated.

The actual mass of a proton or neutrons is ~1.67 x 10^-27 kg (~1.67 x 10^-24 g)

The mass of an electron is ~9.1 x 10^-31 kg (~9.1 x 10^-28 g)

The mass of an atom varies from about 1 x 10^-20 to 1 x 10^-18 kg (1 x 10^-23 to 1 x 10^-21 g) depending on the element

The radius of the nucleus ranges from about 1 x 10^-16 to 1 x 10^-14 m (1 x 10^-7 to 1 x 10^-5 nm) depending on the element

The diameter of atoms varies from about 1 x 10^-10 to 5 x 10^-10 m (0.1 to 0.5 nm) depending on the element

Generally speaking the radius of an atom is about 10,000 times that of the nucleus!

A typical relatively small molecule would be no bigger than ~1 x 10^-10 to 1 x 10^-9 m (~0.1 to 1 nm)

NEXT 2. for 'A 'portrait of an atom' and comparing relative size of particles

Quizzes - practice exam questions

• Some basic practice exam questions on atomic structure (at end of index page)

• GCSE level Foundation (easier) Atomic Structure multiple choice QUIZ

• GCSE level Higher (harder) Atomic Structure multiple choice QUIZ

• gap-fill worksheet on Atomic Structure

• matching pair quiz on Atomic and electronic structure 1. fundamental particles

• matching pair quiz on Atomic and electronic structure 2. periodic table