From WolfWikis

Atoms

Atoms consist of:

1. A tiny nucleus that despite its size carries virtually all the mass of the atom. It consists of
   1. protons (positive charge of 1+ for each proton)
   2. neutrons (no charge)
2. A cloud of electrons (negative charge of 1- for each electron)

Isotopes

Even the atoms of a single element are not always quite identical.

If two atoms belong to the same element they do have the same number of protons in their nucleus. This number is the atomic number. It is sometimes written as a subscript before the atomic symbol, e.g. ₃Li or ₁₇Cl. More often than not however it is omitted because it does not really give any new information. If we look up the element in the periodic table, we immediately see that lithium is element number 3!

More important is the number that is sometimes written as a superscript before the symbol, e.g. ³⁵Cl or ³⁷Cl. This mass number gives us the sum of the nucleons, i.e. the protons and the neutrons together. The numbers 35 and 37 tell us in this case that we have two atoms that are chemically pretty much the same but differ in mass. These are referred to as isotopes.

Because protons and neutrons have (almost) the same mass, we can roughly give them all a mass of 1 unit. A mole of only ³⁵Cl atoms would weigh about 35[g]. The molar mass of ³⁷Cl would be 37 [g/mol]. Chlorine as it occurs natually however is a 3:1 mixture of the two. This is why the molar mass of natural chlorine is [3*35+1*37]/4= 35.5 [g.mol]

Ions

Naked nuclei

A naked nucleus without electrons is an example of a charged particle. Charged particles are known as ions. To the top right of the atomic symbol is indicated what the charge of the particle is.
E.g. if we just have a proton,

its atomic number is 1 (it belongs to hydrogen ₁H),

its mass number is also one (no neutrons!) ¹H

and its charge is +1 (being a proton!). This is shown as H¹⁺ (or simply H⁺).

Together we could write for a proton: ₁¹H¹⁺ or ¹H¹⁺

There are two other isotopes of hydrogen that do have one and two neutrons respectively:

²H¹⁺: one proton plus one neutron

³H¹⁺: one proton plus two neutrons

For most elements the atomic symbol remains the same for all its isotopes but in the case of hydrogen they have special names and symbols:

²H¹⁺ = ²D¹⁺; a deuteron or a deuterium ion.

³H¹⁺ = ³T¹⁺; a triton or a tritium ion.

In natural hydrogen deuterium is quite rare. Tritium is not present at all: it is radioactive and needs to be produced in a nuclear reactor.

Naked nuclei of other elements have much higher charges as they have more protons:

³He²⁺, ⁴H²⁺

²³⁵U⁹²⁺, ²³⁸U⁹²⁺

Adding electrons

Except for hydrogen a naked nucleus is very, very rare in chemistry. E.g. the strongly positive ion ²³⁸U⁹²⁺ represents an extremely unstable species that would quickly pick up electrons from wherever it can get them. (In fact it would be so unstable that it has probably never been observed...)

For each electron it would pick up, its charge will go down by one unit, because the electron e^- has a negative charge.

²³⁸U⁹²⁺ + e^- ==> ²³⁸U⁹¹⁺

²³⁸U⁹¹⁺ + e^- ==> ²³⁸U⁹⁰⁺

²³⁸U⁹⁰⁺ + e^- ==> ²³⁸U⁸⁹⁺

This process is known as reduction.

After adding as many electrons as there are protons in the nucleus the particle is reduced to a neutral atom.

²³⁸U⁹²⁺ + 92e^- ==> ²³⁸U⁰ = ²³⁸U

So a uranium atom has both 92 protons and 92 electrons, which is why their charges cancel and the atom is neutral

For some elements the reduction process can sometimes go beyond the neutral, e.g. for chlorine:

³⁷Cl + e^- ==> ³⁷Cl^-

Or sulfur

³²S + 2e^- ==> ³²S^2-

These negative ions that have more electrons than protons in the nucleus are known as anions.

The positive ones as cations.

The reverse process (losing electrons) is known as oxidation.

Any process that forms ions (+ or -) is known as ionization.

In chemistry, we typically encounter ions with only a few electrons missing or in surplus. (Usually one or two but it can go up to about 8).

Exercises

Exercise 1

What is the number of protons, electrons and neutrons in the following particles.

²³⁴Th³⁷⁺

¹³C

²¹⁰Pb⁴⁺

⁸⁵Br^-

⁴⁰K⁺

⁶⁰Co³⁺

⁵⁵Fe²⁺

⁵⁶Fe²¹⁺

³¹P^3-

Exercise 2

Carbon and oxygen occur mostly as ¹²C and ¹⁶O. Other stable isotopes are ¹³C and ¹⁷O, they occur only in trace amounts. However there are (costly) ways of enriching the elements in the heavier isotopes. Departing from enriched elements that are 50:50 mixtures of ¹²C/¹³C and ¹⁶O/¹⁷O respectively, carbon dioxide is synthesized.

1. What is the molar mass of the reaction product?
2. Assuming the atoms of the isotopes react randomly with each other, what are the molar masses of a various combined species that are formed?

Exercise 3

How many moles of protons, neutrons and electrons are there in a mole of ³⁹Ca¹⁹F₂?

Exercise 4

Diffraction is a major way to study the structure of solids. Beams of both X-rays and neutrons have wavelengths of a size comparable to those of atoms. This makes them both suitable as a tool.

In such diffraction experiments the scattering power of an atom (or ion) is a measure for how strongly that particle will contribute to the total scattering of the radiation.

In X-ray diffraction the scattering power of an atom (or ion) is equal to the number of electrons of the particle.

In neutron diffraction the scattering power depends not on the number of electrons but differs from isotope to isotope because it is the nucleus that scatters, not the electrons.

Some neutron scattering powers:

1. What percentage of the total scattering power of the compound ²³⁸U²H₃ (uranium trihydride) is the result of the hydrogen for X-ray scattering? For neutron scattering?
2. What percentage of the total scattering power of the compound ⁷Li¹¹B²H₄ (lithium borohydride) is the result of the hydrogen for X-ray scattering? For neutron scattering?