Atomic Structure & Isotopes

What experiment led to the nuclear model of the atom?

Rutherford alpha particle scattering experiment (1911)

Describe the modern atomic model

A small dense positively charged nucleus surrounded by electrons in shells

What particles are found in the nucleus?

Protons and neutrons

Where is most of the atom’s mass concentrated?

In the nucleus

Why is a neutral atom uncharged?

Number of protons equals number of electrons

Charge of a proton

+1

Charge of a neutron

0

Charge of an electron

-1

Relative mass of a proton

1

Relative mass of a neutron

1

Relative mass of an electron

1/1840

What is an electron shell?

A region of space around the nucleus where electrons occupy specific energy levels

Maximum number of electrons in a shell

2n^2 where n is the shell number

Maximum electrons in first shell

2

Maximum electrons in second shell

8

Rule for filling electron shells

Lower energy shells fill before higher ones

Define atomic number (Z)

Number of protons in the nucleus

Define mass number (A)

Total number of protons and neutrons

How to calculate number of neutrons

Mass number minus atomic number

Define relative atomic mass (Ar)

Weighted mean mass of an atom relative to 1/12 of a carbon-12 atom

Why is Ar a weighted mean?

It accounts for the relative abundances of isotopes

Define relative isotopic mass

Mass of one isotope relative to 1/12 of carbon-12

Define relative molecular mass (Mr)

Weighted mean mass of a molecule relative to 1/12 of carbon-12

How to calculate Mr

Add together the Ar values of all atoms in the molecule

Define relative formula mass

Relative mass of an ionic compound based on its formula unit

Why formula mass is used for ionic compounds

Ionic compounds exist as giant lattices not molecules

What is an ion?

A charged particle formed by gaining or losing electrons

How are positive ions formed

By losing electrons

How are negative ions formed

By gaining electrons

Electrons in an ion

Number of protons minus ionic charge

Define isotopes

Atoms of the same element with the same atomic number but different numbers of neutrons

Why isotopes have same chemical properties

Same electron configuration

Why isotopes have different physical properties

Different masses

Example of hydrogen isotopes

Protium, deuterium, tritium

Use of deuterium

Heavy water to control nuclear reactions

What technique identifies isotopes

Mass spectrometry

Why ions are used in mass spectrometry

They can be accelerated and deflected by electric fields

What does time-of-flight mass spectrometry measure

Time taken for ions to reach a detector

What does a mass spectrum show

m/z values and relative abundances

How to calculate Ar from a mass spectrum

Sum of (m/z × abundance) divided by total abundance

Why chlorine shows a 3:1 isotope ratio

Chlorine-35 is more abundant than chlorine-37

Chlorine isotopes present

Cl-35 and Cl-37

m/z values for Cl2 molecules

70, 72, 74

Reason for 9:6:1 Cl2 peak ratio

Combinations of Cl-35 and Cl-37 based on abundance

How mass spectrometry identifies elements

By comparing calculated Ar with periodic table values