Atomic structure and periodic table

1.5a Atomic structure and periodic table

1.5a Relative massesThe unit of mass is 1/12 of the mass of an atom of the isotope Carbon-12 (12C=12 exactly)

The Relative Atomic Mass (Ar)of an element is the ratio of the mass of an average atom of that element to 1/12 of the mass of an atom of the nuclide Carbon-12.

The Relative Isotopic Mass of a nuclide is the ratio of the mass of one atom of that nuclide to 1/12 of the mass of a Carbon-12 atom.

The Relative Molecular Mass (Mr)of a substance is the ratio of the mass of an average molecule of that substance to 1/12 of the mass of an atom of the nuclide Carbon-12.


Task1.5a Explain what is meant by

The relative atomic mass of helium is 4.002602.

The relative isopotic mass of deuterium is 2.0000.

The relative molecular mass of water is 18.

1.5b mass spectrometer

Uses of mass spectrometers


The mass spectrometer can detect the abundance of isotopes such as U-238 and Pb-206.

In any one place the Earth concentrates some minerals like uranium but not others like lead. So some rocks form containing uranium but not lead.

U-238 is radioactive and decays to form Pb-206 over time.

This process is controlled by the half life of U-238 which is 4.5*109 years. The relative abundance of U-238 to Pb-206 can be used to calculate the age of the rock.

Rock is found to have ages up to about 4 thousand million years, but not older. This is the age of the Earth when the first solid rock formed from the magma on the surface of the cooling planet.


Radiocarbon dating using C-14 can be used to similarly date organic matter.


The mass spectrum of an element shows the relative isotopic mass (m/e) and relative abundance of each isotope of the element being tested.

In a mass spectrum the height of each peak = the relative abundance.

The Relative Atomic Mass of an element can be found by finding the sum of the products of the relative abundance of each isotope and its relative isotopic mass and the dividing by the total relative abundance.



work sheets mass spetrometer ( AS level)

Write a sentence for each stage in the formation of a mass spectrum.


Isotopes are atoms of the same element with different numbers of neutrons in the nucleus

The Relative Atomic Mass of an element is the weighted (to take account of relative abundance) average of the the Relative Isotopic Masses of all of the isotopes of that element.E.g. Chlorine has two isotopes with mass numbers (and relative isotopic masses
35 and37

35 37

75% is Cl and 25% is Cl

17 17


Let there be 100 atomsTotal mass of 100 atoms = (75 * 35) + (25 * 37) = 3550

Average mass of an atom (relative atomic mass of chlorine) = Total mass /Number of atoms=3550/100so relative atomic mass of chlorine = 35.5

PRACTICE QUASTIONS

1. Bromine exists as two isotopes with mass numbers 81 and 79. If there is 50% of each isotope in a sample what is the relative atomic mass of bromine?
2. Calculate the relative atomic mass of the following:(a) gallium 60% Ga 69 and 40% Ga 71
(b) neon 90% Ne 20 and 10% Ne 22
(c) silver 50% Ag 107 and 50% Ag 109
(d) boron 20% B 10 and 80% B 11
(e) neon 90.92% Ne 20, 8.82% Ne 22 and 0.26% Ne 21.
(f) lead 52.3% Pb 208, 22.6% Pb 207, 23.6% Pb 206 and 1.5% Pb 204.

ionization energies


The first ionisation energy is the enthalpy change when 1 mole of gaseous atoms of an element each lose an electron to form gaseous ions each with a single positive charge.

It is the enthalpy change for the reaction: M(g) ---> M+(g) + e-

The second ionisation energy is the enthalpy change when 1 mole of gaseous ions of an element each with a single positive charge each lose an electron to form gaseous ions each with a double positive charge.

It is the enthalpy change for the reaction: M+(g)---> M2+(g) + e-

All of these ionizations involve the removal of a negative electron from a positive nucleus. The attraction between these must be over come by supplying energy. The process is always endothermic. Example 1st IE of Na = +500kJmol-1


Task1.5d.1 Define the third ionisation enthalpy for the element M and give an equation.

Task1.5d.2 Define the first ionisation enthalpy for sodium, magnesium, chlorine and neon.


Task1.5d.3 Define the second ionisation enthalpy for lithium, aluminium and oxygen.


1.5e Electronic structureThe graph of successive ionisation energies against ionisation number shows electrons grouped into three energy levels or quantum shells with similar energies.

Electron 1 (ionisation number = 1, removed first) from the third shell, electrons 2-9 in second shell, electrons 10 and 11(removed last) from first shell closest to the nucleus.

Electron 1 is on its own in the outer shell indicating that this element is in group 1. The graph of first ionisation energy against atomic number shows the grouping of electrons into s, p (and d) subshells (orbitals) within the (quantum shells) energy levels. The general trend is for ionisation energy to increase with increasing atomic number across a period but 1st IE B < 1st IE Be and 1st IE O < 1st IE N.

PRACTICE QUSTIONS

Label the diagram with symbols for the elements.

Label electrons removed as s or p.

Write two sentences using the frame: The ionisation energy (increases/decreases) (from left to right across a period/down a group) because (distance from outer electrons to nucleus is increasing/the nuclear charge in increasing)