1.1 c & 1.2 a: Periodic Trends/Isotopes/Unstable Isotopes

Periodic Trends and Properties, and Isotopes ​

Year 11

​What we will cover

1.1 c : classify the elements based on their properties and position in the periodic table through their:

– physical properties

– chemical properties

1.2 a: investigate the basic structure of stable and unstable isotopes by examining:

– their position in the periodic table

– the distribution of electrons, protons and neutrons in the atom

– representation of the symbol, atomic number and mass number (nucleon number)

​Elements, compounds, mixtures etc.

• Elements: ​substance consisting of one type of atom. E.g. diamond

• Compound: pure substance consisting of two or more elements. E.g. water

• ​Mixture: substance containing two or more substances that aren't chemically combined.

  • heterogenous: visibly different substances or phases. E.g. soil, ice in water

  • homogenous: uniform in appearance and composition . E.g. cordial

​The Periodic Table

• there are changes in properties from group to group, across a ​period, and down a group

• the PT can help us determine the 'general' properties of an element based on others we know

  • i.e. if we are unsure what bromine may react with commonly, we can look to other elements in that group (like chlorine) and assume they will make similar compounds

​What can we determine from the PT

• Metallic character

  • increases as we go down a group , but decreases as we go across a period

  • shiny, good conductors, malleable​, higher melting point

• number of subatomic particles

• ​other trends like atomic radius, ionisation energy, electronegativity

Multiple Choice

How are elements on the periodic table arranged by?

• in alphabetic order

• simular physical & chemical properties

• their symbols

• Just simular physical properties

Multiple Choice

Periods on the periodic table are __________.

• Horizontal Rows

• Vertical Columns

Multiple Choice

The formula used to calculate the number of neutrons is:

• Mass Number - Atomic Number

• Protons + Electrons

• Atomic Number - Mass Number

• Protons x2

​Basic atomic structure

• small dense positively charged nucleus, with protons and neutrons

• surrounded by an electron cloud, which we represent in shells/orbitals/energy levels

• atomic number, Z = number of protons = number of electrons (in a neutral atom)

• ​mass number, A = number of protons + number of neutrons

• next lesson: energy levels ​

Multiple Choice

Which of the following particles is located outside of nucleus of the atom?

• proton

• neutron

• electron

Multiple Select

Which of the following particles are responsible for most of the mass of an atom? (You may choose more than one answer.)

• proton

• neutron

• electron

Multiple Select

Which of the following particles compete to establish the charge of an atom?

• proton

• neutron

• electron

​Isotopes

• ​differing numbers of neutrons in the nucleus

• isotopes of the same element will have the same atomic number, but different mass numbers

• named by their mass number

  • e.g. chlorine-35 and chlorine-37 (contains an extra two neutrons)​​

​Relative abundance of isotopes

• the percentage of that isotope in the naturally occurring element

• for example: ​

​Calculating relative atomic mass from isotopic data

Multiple Choice

Four isotopes of lead include lead-204, lead-206, lead-207, and lead-208.  The average atomic mass of lead is 207.2.  Which isotope of lead is likely to be the most abundant.

• 204

• 206

• 207

• 208

​Unstable isotopes

• when an isotopes emits radiation it is unstable

• instability comes from a an excess of protons or neutrons

• this emission is actually from the nucleus, and there are three different types of emission ​

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He nucleus = two protons + two neutrons ​

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​Alpha emitting isotopes

Emits a helium nucleus, therefore loses 2 protons and 2 neutrons

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For example, a common uranium ​isotope is

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Which has 92 protons and 146 neutrons.

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If it loses a He nucleus, it will have 90 protons and 144 neutrons, and will transform into:

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​Beta emitting isotopes

Emits an electron, but from the nucleus.

Confused? ​

A neutron is decomposed into a proton and an electron, and then the electron is ejected from the nucleus. ​

​For example: cobalt-60 (radiation therapy)

Multiple Choice

What is the symbol for an alpha particle?

• ⁴₂He

• ⁰_-1e

• ⁰₊₁ e

• ¹₀n

Multiple Choice

Solve this equation for beta decay.
⁶⁰₂₇Co = ___ + ⁰_-1e

• ⁵⁶₂₅Mn

• ⁶⁰₂₈Ni

• ⁵⁸₂₃V

• ⁵⁹₂₇Co

​Gamma emitting isotopes

• Gamma radiation (on the EM spectrum) is emitted as waves

• Generally occurs as a secondary form of decay after alpha or beta radiation, as the daughter particles are usually in an excited state

  • They need to release some energy to become more stable

• The particles remain the same, there is no loss of particles, charge, or mass

• Common usage: radiotherapy, sterilisation of equipment

Multiple Choice

Complete the nuclear equation and determine the type of decay that is occurring in this reaction. 

• alpha

• beta

• gamma

• none

Multiple Choice

What particle completes this reaction?

• alpha particle

• beta particle

• gamma particle

• neutron

​Why are some isotopes unstable?

• Generally elements with larger nuclei (larger than AN 83) will be unstable, as the forces keeping the nuclei intact are not stable. Or, with smaller nuclei, the ratio of neutrons to protons is unstable.

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Two forces keeping the nucleus together:

1. ​electrostatic force between positively charge protons

2. mass-mass attraction between all particles​ (like a tiny gravitational force)

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• If n:p ratio is too high = beta emitter

• If n:p ratio is too low = alpha emitter ​

• Half-life of isotopes varies (time required for half the atoms of a sample to undergo radioactive decay). E.g. iodine 131 is 8 days, and uranium-238 is 4.5 x 10^9 years​

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​Penetrating power of radiation types

What's next?

• energy levels in atoms

  • ​electron configuration

  • stability of atoms

• atomic emission