Atomic structure

An atom consists of a small central nucleus composed of protons and neutrons surrounded by electrons.

​

An atom will always have the same number of electrons as protons.

​

​

Atomic and mass number

The atomic number (or proton number) of an atom is equal to the number of protons in its nucleus.

​

The mass number (or nucleon number) of an atom is equal to the number of protons plus neutrons in its nucleus.

This Lithium atom has:

atomic number = 3

mass number = 7

Properties of protons, neutrons and electrons

nucleus

nucleus

outside nucleus

1

1

0.0005

+ 1

- 1

0

Nuclear notation

An isotope of carbon consists of 6 protons and 8 neutrons. This can be written as:

OR:

carbon 14

Isotopes

The atoms of an element always have the same number of protons.

​

Isotopes are atoms of the same element with different numbers of neutrons.

Note: The number after ‘hydrogen’ is the mass number of the isotope.

Question 1

U

235

92

An isotope of uranium (chemical symbol U) consists of 92 protons and 143 neutrons. Give the two different ways of notating this isotope.

uranium 235

The mass number of the Uranium isotope:

= 92 + 143 = 235

AND

Question 2

Determine the number of protons and neutrons in the isotopes notated below:

protons = 7

neutrons = 6

p = 27

n = 33

p = 79

n = 118

p = 94

n = 145

Note: Apart from the smallest atoms, most nuclei have more neutrons than protons.

Ionisation

Ionisation occurs when an atom loses or gains one or more electrons.

​

When an atom loses electrons it becomes a positive ion.

​

When an atom gains electrons it becomes a negative ion.

​

Radioactivity and Ionising Radiation

The nuclei of some isotopes are unstable and when they decay they give of radiation that causes ionisation.

​

This phenomena is called radioactivity and the radiation produced is called ionising radiation

​

Radioactivity is a random process. When a particular nucleus decays cannot be predicted.

Alpha, beta and gamma radiation

An alpha particle consists of two protons and two neutrons.

It is strongly ionising.

​

A beta particle is a high speed electron.

It is produced when a neutron has decays into an electron and proton.

It is moderately ionising.

​

Gamma rays are very high frequency electromagnetic waves.

They are produced when an unstable nucleus loses energy..

They are weakly ionising.

The penetrating power of �alpha, beta and gamma radiation

Deflection by magnetic fields

Alpha and beta particles are deflected in opposite directions due to their opposite charges.

Due to their much larger mass alpha particles are deflected far less than beta.

Gamma rays are not deflected because they are not charged.

Magnetic south pole placed behind the rays

Deflection by electric fields

Alpha and beta particles are deflected in opposite directions due to their opposite charges.

Due to their much larger mass alpha particles are deflected far less than beta.

Gamma rays are not deflected because they are not charged.

Electric field produced by positively and negatively charged plates

Choose appropriate words to fill in the gaps below:

Atoms consist of a very small _______, containing protons and neutrons, surrounded by _______. Atoms of the same element will always have the same number of _______ but different ________ of the same element will have different numbers of _________.

The atoms of some substances are unstable and _________. They may give off alpha or ______ particles or gamma rays.

Gamma rays are the most penetrating type of radiation, _____ is the least.

isotopes

radioactive

nucleus

protons

electrons

alpha

neutrons

WORD SELECTION:

beta

isotopes

radioactive

nucleus

protons

electrons

alpha

neutrons

beta

Alpha decay

Alpha particles consist of two protons plus two neutrons.

They are emitted by some of the isotopes of the heaviest elements.

Example: The decay of Uranium 238

+

Uranium 238 decays to Thorium 234 plus an alpha particle.

Notes:

1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2)

2. The alpha particle can also be notated as:

Question

Show the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92).

Beta decay

Beta particles consist of high speed electrons.

They are emitted by isotopes that have too many neutrons.

One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle.

Example: The decay of Carbon 14

+

Carbon 14 decays to Nitrogen 14 plus a beta particle.

Notes:

1. The beta particle, being negatively charged, has an effective atomic number of minus one.

2. The beta particle can also be notated as:

Question

Show the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg).

Gamma decay

Gamma decay is the emission of electromagnetic radiation from an unstable nucleus

Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle.

​

Example: Cobalt 60

+

Cobalt 60 with excess ENERGY decays to

Cobalt 60 with less ENERGY plus gamma radiation.

Changing elements

Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number.

Complete the decay equations below:

Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!)

There are other correct solutions

Choose appropriate words to fill in the gaps below:

When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______.

Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged.

Gamma rays consist of ______________ radiation that is emitted from a nucleus when it loses ________, often after undergoing alpha or beta decay.

electromagnetic

energy

mass

two

four

one

WORD SELECTION:

neutrons

electromagnetic

energy

mass

two

four

one

neutrons