Physics - Atomic structure

Gcse AQA

Isotope

An isotope is an atom of the same element that has the same amount of protons and electrons, but different amount of neutrons

Emission of EM radiation

When an electron has moved up an energy level, it is unstable.

Eventually it will move back down

When it moved back down, it emits EM radiation

Absorption of EM radiation

Electrons are arranged in energy levels:

• the further away from the nucleus = higher energy level

• the closer to the nucleus = lower energy level

These electrons absorb energy

As it moved up an energy level, it will absorb EM radiation

Why are ions often more chemically reactive than neutral atoms

It is because of their positive charge which neutral atoms do not have, however generally reactivity depends on the reaction taking place.

Mass number

Total number of neutrons and protons in the nucleus of an atom

Ion

A particle or atom with non zero charge

Gold foil experiment

• 1909, Ernest Rutherford directed alpha beam particles at gold foil

• Most bounced back = the atom is mostly empty

• Some deflected but still went through = the atom’s nucleus has a strong, positive charge

• Some went through = the atom has a positively charged nucleus

Plum pudding model

• JJ Thomson discovered the electron toward end of 19th century and created the plum pudding model

• The model= positively charged “dough” with negatively charged electrons

The bohr model

• proposed in 1913 by Niels Bohr

• There is a small concentrated nucleus

• The nucleus has a postive charge

• Electrons orbit the nucleus at fixed distances in shells

The nuclear model

• Proposed in 1911

• Positive nucleus in centre

• Electrons exist in cloud around nucleus

How can electrons be knocked out of the outer energy levels of an atom?

• friction

• When they gain enough EM radiation to leave the atom

• From chemical reactions

What happens when one or more electrons are removed from an atom?

It becomes a positive ion because there is a resultant positive charge due to negatively charged electrons being lost

Discovery of the proton

Ernest Rutherford discovered that the positively charged nucleus in all material can be broken down into smaller positively charged particles. He called them protons

Discovery of the neutron

• discovered by James Chadwick

Atom

Building block of all matter

Atom radius

1× 10 to the power of -10

Proton

Positively charged particles with a relative atomic mass of one unit

Neutron

No charge but with relative atomic mass of one unit

Electron

Negatively charged particle with almost no mass (1/2000 the mass of a proton or neutron)

How many protons, electrons and what is overall charge of alpha particle?

• 2x protons

• No electrons

• Overall charge of +2

Atomic number

• The number of protons found in the atom of a specific element.

• Each element has a different atomic number.

• Elements in periodic table ordered by atomic number therefore the number of protons determines which element an atom is

Unstable nuclei

• Some nuclei are unstable - that is because of an imbalance in forces within the nucleus

• Forces exist between particles in the nucleus

• Some nuclei are unstable because of too few or too much neutrons or their large size

Radiation

• Unstable nuclei can emit radiation to become stable

• It is usually in the form of a wave or high energy particle

• As radiation moves away, it takes some energy with it, which reduces the overall energy of the nucleus - this makes it stable

• Process of emitting radiation is called radioactive decay and is a radom process