Atomic Structure

Atomic Structure Topic 2

2.1 The atom 2.1.1 State the position of protons, neutrons and electrons in the atom. 2.1.2 State the relative masses and relative charges of protons, neutrons and electrons. 2.1.3 Define the terms mass number(A), atomic number(Z) and isotopes of an element. 2.1.4 Deduce the symbol for an isotope given its mass number and atomic number 2.1.5 Calculate the number of protons, neutrons and electrons in atoms from the mass number atomic number and charge. 2.1.6 Compare the properties of the isotopes of an element. 2.1.7 Discuss the uses of radioisotopes.

This part of the Topic is mostly a repeat of what you learned at IGCSE. • If you can’t remember what you did, then you need to read through your notes or a textbook. • Make sure that you understand what ions are!

You should already know quite a lot about protons, neutrons and electrons. • You should know: • The relative charge on each of them. • The relative mass of each of them. • where in the atom each of them is located.

Some points to note: • The mass of an electron is so small that we often approximate it to zero. • The size and the sign are both important for the relative charges. • The absolute values for mass and charge are extremely small. We use the relative values because they are more convenient.

This gives us a model of an atom:

We represent this:

Notice we can use lots of different formats for this • But we can ALWAYS get the same information • The smaller of the 2 numbers is called The Proton Number or The Atomic Number, Z • This tell us how many protons are in the nucleus of an atom. • The bigger number tells us the total number of protons AND neutrons in the atom • This may be called The Nucleon Number or The Mass Number, A. • Make sure you know both names!

You need to learn these definitions: • The atomic number, Z, is the number of protons in the nucleus. • The mass number, A, is the number of protons plus the number of neutrons in an atom. • Notice that electrons are not used in either of these definitions!

How can we use these numbers to find the number of neutrons in a nucleus? • Nucleon number – atomic number • And how about the number of electrons? • For an atom, the number of electrons must be equal to the number of protons. • Why?

For positive ions, the atom loses one electron for each positive charge it gains. • For negative ions, the atom gains one electron for each negative charge it gains. • Note that the number of protons NEVER CHANGES when we form ions. • If a table only gives one number, it is may be the mass number or the proton number. TAKE CARE!

Notice that the mass number is sometimes called the “atomic mass” • Chemists are often very bad at language! • We use lots of different words to mean the same thing: • Atomic mass; atomic weight; relative atomic mass; RAM; Ar are all used to mean the same thing! • They are all number of protons + number of neutrons

Isotopes • How can the mass number be a fraction? • There are often different forms of the same element with different mass numbers. • The value that is given for the mass number is an average of the different forms that exist.

These different forms are called isotopes. • Learn the definition: • Isotopes are forms of the same element with different numbers of neutrons. • Isotopes of the same element always have the same number of protons. (The identity of an element is defined by its atomic number) • Isotopes are not all radioactive!

Some common isotopes • 11H 21H 31H • Hydrogen, deuterium, tritium • 3517Cl 3717Cl • 235U 238U • Notice it is not usually necessary to give the number of protons – as this is the same for all isotopes of an element.

Properties of Isotopes • Because isotopes of the same element always have the same number of protons, they also have the same number of electrons. • The chemical properties of an element are determined by the number of electrons, so . . . • Isotopes of the same element have the same chemical properties.

Many physical properties depend on the mass of atoms. • E.g. • Melting point • Boiling point • Rate of diffusion • Density • These do differ between isotopes of the same element. • Make sure you know what is meant by physical properties!

Radioisotopes • The stability of a nucleus depends on the balance between the numbers of neutrons and protons. • If there are either too many or not enough neutrons, a nucleus changes by giving out radiation. • This radiation may be in different forms • Already studied at IGCSE Physics!

Alpha particles consist of 2 neutrons and 2 protons. • They are sometimes referred to as “helium nuclei” Why? • Beta particles are high speed electrons • Gamma radiation is a form of electromagnetic radiation.

Radioisotopes can be used to: • Generate nuclear energy • Sterilize surgical instruments • Preserve food • Detect cracks in structural materials. • It would also be a good idea to know the specific isotopes and uses given below:

14C is used in in radiocarbon dating. • In this process use is made of the fact that living organisms take up carbon throughout their lives. The percentage of the isotope carbon 14 is fairly constant in our atmosphere as it is produced in the upper atmosphere by cosmic bombardment of naturally occurring carbon dioxide. • This means that the percentage of carbon 14 contained by all living organisms is also constant. However, when a living organism dies it stops taking up carbon 14. The isotope decays naturally with a half life of about 5,600 years. So a simple procedure involving counting the radioemissions due to carbon 14 from a sample of material that was once alive, can be used to estimate its date.

Cobalt 60 is used in hospitals as a gamma emission source in the treatment of cancer • Gamma rays are the most penetrating form of radiation. They can be focussed onto cancerous tissue to destroy it using a cobalt 60 source. This form of treatment is known as radiotherapy.

131I and 125I are used as medical tracers • In several conditions the body can be scanned for problems using iodine, which is easily taken up by the body and transported through the lymphatic system. The isotopes 131I and 125I are easy to detect and short lived in the body. • These isotopes emit both beta and gamma radiation. • They have short half lives (8 days for 131I, and 60 days for 125I)

Questions: • The calcium ion 40Ca2+ is essential for healthy teeth and bones. Identify the numbers of protons, neutrons and electrons present in it. • From the periodic table atomic number = 20 • Hence number of protons = 20 • No of neutrons = mass No – atomic No • Hence Noneutrons = 20 • As the ion has a charge of 2+ there are 2 fewer electrons than protons • Hence Noelectrons = 18

Identify the species with 17 protons, 18 neutrons and 18 electrons. • From the periodic table, the element with Z=17 is Cl • The mass number is Noprotons + Noneutrons • Mass No= 17 + 18 = 35 • As there is one more electron than protons, charge is -1 • Hence the species is 3517Cl-

Complete the table

Atomic Structure