FIND OUT MORE. Atoms 157 • Energy 166 • Light 178–179 • Motion 165
HOW DO SCIENTISTS USE QUANTUM THEORY?
Although quantum theory is hard to understand, it is
one of the most accurate scientific theories ever
developed. With quantum theory, scientists can
calculate precisely the properties of atoms, molecules,
and materials. Quantum theory is used to design
electronic components, new materials, and drugs.
Without it there would be no computers, mobile
phones, or many other recent inventions.
Quantum theory grew from the ideas
of Max Planck. He proposed that
atoms could only emit (give out)
energy in fixed units called quanta.
The theory has been developed to
explain the behaviour of particles
and the energy they emit.
WHAT IS THE UNCERTAINTY PRINCIPLE?
Quantum theory shows that the position and the speed
of a quantum, such as a photon or an electron, cannot
both be known exactly. The more accurately we know
the position, the more uncertain we are of the speed,
and vice versa. The uncertainty principle shows that
we can only calculate probabilities, not certainties.WHY IS QUANTUM THEORY SO STRANGE?
Quantum theory shows that energy behaves as both
waves and particles at the same time. Quanta are
packets of energy that exist in lumps or units like
separate particles – but when they travel, they spread
out like waves on the surface of a pond.
WHAT IS A QUANTUM?
A quantum is the smallest possible amount
of energy. A quantum of light (or other
electromagnetic radiation) is called a photon.
A photon may be emitted by an electron as it
makes a. QUANTUM LEAP.
Electrons in atoms occupy shells. In each
shell an electron has a certain energy. If an
electron moves to a higher or lower energy
shell, it is said to make a quantum leap.4 ABSORPTION AND EMISSION
When an atom absorbs (takes in) energy,
its electrons jump to higher energy levels.
As electrons fall back to their original levels
they emit (give out) photons. If an electron
emits a photon of blue light, it is losing more
energy than if it emits a photon of red light.HELIUM ATOM 3
This picture shows the
tracks left by leaping
electrons in a helium atom.
Quantum theory shows electrons as
fuzzy clouds, rather than in rings
orbiting a nucleus.Electrons leap to higher QUANTUM LEAP
energy levels, away from
the nucleus, absorbing
energy as they do soElectrons emit
energy as they
drop to lower
energy levels
Photon of
blue lightNucleusNucleusPhoton of
red lightQuantum Theory
MAX PLANCK
German, 1858-1947
Max Planck was born in
Kiel, Germany. He was
outstanding at all subjects at
school and was a talented
musician, but he decided to
devote his life to physics.
Planck’s constant (fixed
number) is used to calculate
the energy of quanta. He was
awarded the Nobel Prize for
Physics in 1918.quantum
theory