4.1 the atom and isotopes

Dr Pusey

www.puseyscience.com

Syllabus points “The nuclear model of the atom describes the atom as

consisting of an extremely small nucleus, which contains most of the atom’s mass, and is made up of positively charged protons and uncharged neutrons surrounded by negatively charged electrons”

“Nuclear stability is the result of the strong nuclear force, which operates between nucleons over a very short distance and opposes the electrostatic repulsion between protons in the nucleus”

Learning goals Define:

Nucleus, proton, neutron, electron, atomic number, mass number, atomic mass unit

State the mass of protons, neutrons and electrons in terms of the atomic mass unit

Calculate the number of neutrons, electrons and protons in an atom by using the atomic number and mass number

Convert between atomic mass units and kilograms Give the SI unit for: masses on the atomic or molecular scale Define:

Isotope, radioisotope, nuclide, nucleon, strong nuclear force, electromagnetic force

Recall that a large number of isotopes exist for each element Identify that isotopes have different physical properties Identify that the mass number on the periodic table represents the most stable

isotope/s of an element Explain how the strong nuclear force opposes the electrostatic repulsion between

the protons in a nucleus Identify that most isotopes of elements are radioisotopes Identify that stable isotopes are the most abundant isotopes

500 BC

Democritus

Leucippus

Atoms are indivisible

John Dalton

Different arrangements =

different compounds

J. J. Thomson

Plum pudding model

Early Ideas about matter

Ernest Rutherford

Nuclear model

Neils Bohr

Bohr model (orbiting electrons)

1920’sErwin

Schrödinger

Quantum model (electron orbits)

Current model Physicists have discovered that protons and neutrons are composed of even

smaller particles called quarks.

As far as we know, quarks are not made up of anything else. Fundamental particle, perhaps?

The Standard Model

Explains what the world is and what holds it together

All the known matter particles are composites of quarks and leptons, and they interact by exchanging force carrier particles

From www.livescience.com

Covered in detail in the Year 12 Physics course

Orbiting model of the atomFrom BoardWorks

The electrons orbit the nucleus in layers called shells.

The nucleus is where most of the mass of the atom is found. It contains protonsand neutrons.

Particles in an atom

Particle Mass Charge Location

Proton 1 amu(1.66 × 10-27 kg)

+ 1 Nucleus

Neutron 1 amu(1.66 × 10-27 kg)

0 Nucleus

Electron 5.5 × 10-4 amu(9.11 × 10-31 kg)

- 1 Electron cloud

Nucleon = components of a nucleus, protons and neutronsE.g. Each atomic nucleus consists of one or more nucleons

Nuclide = specific type of atoms or nucleiE.g. A set of nuclides with equal atomic number but a different number of neutrons, are called isotopes

Atomic Mass Unit Also represented by amu (atomic mass unit), u (unified atomic mass unit) or Da

(Dalton)

Standard unit that is used for indicating mass on an atomic or molecular scale

1 amu = the mass of a single proton or neutron

1 amu = defined as one twelfth of the mass of an atom of carbon-12

1 amu = 1.66 × 10−27 kg

Element Atomic Number

Mass Number

Number of protons

Number ofneutrons

Carbon

Use a periodic table to fill in the following table:

Element Atomic Number

Mass Number

Number of protons

Number ofneutrons

Carbon 6 12 6 6

Platinum 78 195 78 117

Use a periodic table to fill in the following table:

Mass number

Atomic number

Element symbol

Another way of representing atoms

Example 2 Rewrite the following elements in the format 𝑍

𝐴𝑋

Helium

Uranium-238

Carbon-12

Hydrogen

Example 2 Rewrite the following elements in the format 𝑍

𝐴𝑋

Helium 24𝐻𝑒

Uranium-238 92238𝑈

Carbon-12 612𝐻𝑒

Hydrogen 11𝐻

Isotopes The type of atom is determined by the number of protons in its

nucleus (e.g. all Carbon atoms have 6 protons in the nucleus).

Some of the Carbon atoms may have a different number of neutrons, these are called isotopes

The most common form of Carbon is Carbon-12 but it can

also exist as Carbon 14

Isotopes will have the same chemical properties

Different physical properties (e.g. density)

https://gcps.desire2learn.com/d2l/lor/viewer/viewFile.d2lfile/6605/48792/PeriodicTable/Ions_and_Isotopes/Ions_and_Isotopes_print.html

Radioisotopes

Isotopes = atom of an element with a different number of neutrons

The most abundant isotope of an element will be the stable isotopes

Radioisotopes = unstable isotopes which may spontaneously decay by emitting a particle (or waves of radiation) from the nucleus to become more stable

Radioactive elements (e.g. Uranium) have no stable isotope

Periodic table showing isotope abundance

From: http://commons.wikimedia.org/wiki/File:Periodic_Table_Radioactivity.svg

From: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-structure-of-matter/the-nuclei-of-atoms-at-the-heart-of-matter/what-holds-nuclei-together/

Forces holding nuclei together

Uses of isotopes

Carbon-14 used in ‘carbon dating’ to determine the age of organic material. It can date objects up to 60 000 years old.

Americium-241 used in smoke detectors in your home to detect smoke.

Technetium-99 and Iodine-131 patients are given a radioisotope before going into certain scanners/imagers

The case of the radium watches

In the 1900’s Radium (a radioactive element) was widely used in glowing clock and watch hands

Young women who worked in the watch factories used to 'point' their brushes by licking and shaping the bristles prior to painting the fine lines and numbers on the dials. This practice resulted in the ingestion of radium, which caused serious health problems.

The Radium Girls, a group of young women working for US Radium in 1917, sued their employer for contracting an occupational disease. In the wake of the case, industrial safety standards were demonstrably enhanced for many decades.

Radium has a half life of 1600 years, so even today the old watch faces are still radioactive!

Resources - Isotopes

Further Reading Interactive periodic table http://www.ptable.com/ Periodic table of isotopes http://www.ciaaw.org/ How atomic particles helped solve a wine fraud

AV Tyler DeWitt - What Are Isotopes? (7:55) Khan Academy – Four Fundamental Forces Khan Academy – Nuclear stability and nuclear equations (8:24) SciShow - Strong Interaction: The Four Fundamental Forces of Physics #1a (3:36) SciShow - Strong Interaction: The Four Fundamental Forces of Physics #1b (3:52) SciShow - Electromagnetism - Electrostatic Force: The Four Fundamental Forces of Physics

#4a (3:37)

How did you go? Define: