P4 - Atomic Struture

What is the radius of an atom?

About 1 × 10⁻¹⁰

What is the basic structure of an atom?

A positively charged nucleus surrounded by negatively charged electrons

Where is most of the mass of an atom concentrated?

In the nucleus

How are electrons arranged in an atom?

At different distances from the nucleus in different energy levels

What happens to electron energy levels when electromagnetic radiation is absorbed?

Electrons move further from the nucleus to a higher energy level

What happens to electron energy levels when electromagnetic radiation is emitted?

Electrons move closer to the nucleus to a lower energy level

In an atom, how does the number of electrons compare to the number of protons?

Equal (so atoms have no overall electrical charge)

What is the atomic number?

The number of protons in the nucleus

What is the mass number?

The total number of protons and neutrons in the nucleus

What is an isotope?

Atoms of the same element with different numbers of neutrons

How do atoms form positive ions?

By losing one or more outer electrons

What did scientists think atoms were like before the electron was discovered?

Tiny spheres that could not be divided

Describe the plum pudding model

It proposed that the atom is a ball of positive charge with negative electrons embedded in it

Who proposed the plum pudding model?

J.J Thompson

What did the plum pudding model say about the distribution of mass?

The mass was spread evenly throughout the atom

What experiment led to the nuclear model replacing the plum pudding model?

The alpha particle scattering experiment

What was the alpha scattering experiment?

Alpha particles were fired at a very thin sheet of gold foil.

What observations were made?

• Most alpha particles passed straight through the foil, showing atoms are mostly empty space
  

• Some alpha particles were deflected by small angles, meaning the centre of the atom must carry a positive charge (repelling the positive alpha particles)
  

• A very small number were deflected backwards, showing there is a very small, dense region in the atom with most of the mass.

What did these observations lead to?

• It led to the rejection of the plum-pudding model, which could not explain the large angle deflections
  

• Provided evidence for the nuclear model: a tiny, dense, positively charged nucleus with electrons around it

What did the alpha scattering experiment show about the atom’s mass and charge?

The mass is concentrated at the centre (nucleus) and the nucleus is charged.

What is the nuclear model?

A model where the atom has a small, dense, positively charged nucleus with electrons orbiting it.

What did Bohr change about the nuclear model?

He suggested electrons orbit at specific distances from the nucleus

What particle was later discovered in the nucleus?

The neutron

Who discovered the neutron?

James Chadwick

Why do some nuclei give out radiation?

Some nuclei are unstable and change to become more stable

What is radioactive decay?

A random process where an unstable nucleus gives out radiation as it becomes more stable

What is activity?

The rate at which a source of unstable nuclei decays

What unit is activity measured in

Becquerel (Bq)

What is count-rate?

The number of decays recorded each second by a detector (Geiger-Muller tube)

What is an alpha particle (α)?

• Two protons and two neutrons and it has a 2+ charge
  

• Low penetrating power (Stopped by paper or skin) and strongly ionising
  

• They only travel a few cm in air

What is a beta particle (β)?

• Fast-moving electron and has a -1 charge
  

• It’s moderately penetrating (stopped by aluminium) and moderately ionising
  

• They travel a few metres in air

What is a gamma ray (γ)?

• Electromagnetic wave and has no charge
  

• It’s weakly ionising and highly penetrating (stopped by thick lead or concrete)
  

• It travels a very long distance in air

What happens to both the mass and atomic number in alpha decay?

The atomic number reduces by 2 and the mass number reduces by 4

What happens to the mass number and atomic number in beta decay?

The mass number stays the same and the atomic number increases by 1, as a neutron turns into a proton

What effect does gamma emission have on mass and charge of the nucleus?

There is no change to the mass or atomic number of the atom

Define half-life

The time taken for the number of radioactive nuclei in an isotope to halve

What is radioactive contamination?

The unwanted presence of radioactive atoms that get onto or into an object

Why is contamination hazardous?

Because the contaminating atoms may decay, and and they release radiation which can be harmful

What is irradiation?

Exposing an object to nuclear radiation