C.W. BAKER HIGH SCHOOL

 

Atomic Structure Study Guide

Prepared by M. Foster, 5/03

I         Matter

a       Is made up of tiny particles.  An atom is the smallest unit of an element having all of the characteristics of that element.

1        The nucleus – in chemistry we consider the atom to be made up of a central part called the nucleus.  The nucleus contains most of the mass of the atom and is made up of protons and neutrons.

·        Protons – have a mass of 1 atomic mass unit and a charge of +1.  The number of proton which are present in a nucleus is given by the atomic number of the atom

·        Neutrons – have a mass of 1 atomic mass unit and no charge (neutral)

·        The mass number of the atom is the sum of the protons and neutrons that are present in a nucleus (example: the most common atom of carbon has 6 protons and 6 neutrons, the mass number of this atom is equal to 12)

 

2        Surrounding the nucleus of the atom are the electrons. 

·        Electrons – have a much smaller mass than the protons and neutrons and have a charge of –1.

·        In the modern theory of the atom the electrons are thought to be “wave-particles”.  That is, they have some properties that are normally associated with waves, while also having some properties that are normally associated with particles.

·        In a neutral atom the atomic number also indicates the number of electrons.  (i.e.  the number of electrons = the number of protons in a neutral atom).

 

II                 Development of the Atomic Theory

a       Dalton’s Model

1         Theory - John Dalton is frequently referred to as the “Father of the Atomic Theory”.  His theory was the first theory to explain the behavior of matter based on the idea that matter was made up of tiny particles.  The main ideas in his theory were:

·        Matter is made up of tiny, indivisible* particles called atoms

·        Atoms of an element are alike (have the same mass*)

·        Atoms of an element are different (have different masses)

·        Atoms can join together to form compounds.

·        Atoms are rearranged in chemical reactions

* No longer considered to be entirely correct

2        Experimental evidence - The discovery of three scientific laws led to the development of Dalton’s model of the atom.  They were:

·        The law of conservation of mass – the total mass of the reactants always equals the total mass of the reactants.

·        The law of definite composition – the elements in any  compound are always present in a definite mass ratio.

·        The law of multiple proportions – different compounds will have different mass ratios.

 

b       Thomson’s Model of the atom ( The Raisin bun model)

1        Theory - Thomson advanced the idea that the atom was a solid particle with negative electrons embedded in the main component of the atom which was positively charged.  He compared the atom to a raisin bun where the main component of the atom was analogous to the dough in the bun, and the raisins represented the electrons.

2        Experimental evidence – Thomson’s model of the atom was supported by his work with cathode ray tubes.  In this work, he made observations that led him to believe that the electron was very small and negatively charged, and that the electrons from different atoms were all the same.

 

c        Rutherford’s Model of the atom

1        Theory - Rutherford concluded that the atom was composed mostly of empty space.  He also concluded that the mass of the atom was concentrated in the center of the atom (the nucleus), which was positively charged.

2        Experimental evidence - Rutherford based his model of the atom on his gold foil experiment.  In this experiment, he directed alpha particles (positively charged particles) at a sheet of gold foil.  His model of the atom allowed him to explain how the alpha particles were affected as they passed through or were deflected by the gold foil.

 

d       Bohr’s Model of the atom (The planetary model)

1        Theory – Bohr concluded that the electrons orbited around the nucleus in definite energy paths or orbits.  He thought of the electron as being a little ball or particle.

2        Experimental evidence – Bohr calculated theoretically the energies of the various orbits that an electron in a hydrogen atom could occupy.  Based on these energies he was able to predict the energy of a photon of light that would be given off when an electron went from an excited state (a higher than normal energy lever to the ground state (the lowest energy level the electron can occupy.  These predictions matched the observed energies of the bright line spectrum from hydrogen.

 

e        Quantum Mechanical Model of the atom (Electron cloud model)

1        Theory – differs from the Bohr model in that electrons are no longer thought of as simply particles circling the nucleus in definite orbits.  Instead, the electron is considered to be a “wave-particle”.  The electrons are still thought to be present in definite energy levels, which can be further sub-divided into sub-shells and orbitals.  An orbital is an area in the atom where it is highly probable to find an electron.  Orbitals are sometimes described as clouds of negative charge within the atom.

2        Experimental evidence – the quantum mechanical model allows chemists to explain the bright line spectrum of elements other than hydrogen.  It also allows chemists to explain many of the periodic functions found in the Periodic Table (such as ionization energies, atomic radius, etc.) For more information on the quantum mechanical model of the atom click here

 

III              Isotopes

a       Although John Dalton believed that all atoms of an element were exactly unlike, the modern theory recognizes that atoms of an element can have different masses (although behaving the same in chemical reactions).  These atoms are called isotopes of the element.

1        Isotopes of an element have different masses because they have different numbers of neutrons in their nuclei.  Because they have different number of neutrons the atoms will also have different mass numbers.  For instance, carbon atoms can exist as carbon-12 isotopes (containing 6 protons and 6 neutrons) or they can exist as carbon-14 isotopes (containing 6 protons and 8 neutrons).  These two atoms behave exactly the same way in chemical reactions but have slightly different physical properties based on their different masses.

 

IV              Atomic Mass

a        The mass of a neutral atom is measured in atomic mass units (amu).  The standard for the atomic mass system is the Carbon-12 isotope, which is defined as having a mass of exactly 12 atomic mass units.  All other atomic masses are determined relative to this standard.  For instance, an atom which is twice as heavy as a C-12 atom would have an atomic mass of 24 amu’s.  Since the mass of protons and neutrons are very close to 1 amu, the atomic mass of an atom is very close to the mass number of the atom.

b        Average atomic mass -  since not all atoms of an element have the same mass, it is necessary to calculate the average atomic mass of the element.  This mass is a weighted average which takes into account the mass of the individual isotopes and their relative abundance in nature.

1        Example – in nature copper atoms exist in two forms Copper-63 and Copper 65.  It is also found that 69.17% of all copper atoms exist as Cu-63, while 30.83% exist as Cu-65.  The mass of a Cu-63 atom is approximately 63 amu’s and the mass of a Cu-65 atom is 65 amu’s.

 

 

 

V                  Electron Structure

a        Based on the quantum mechanical model, we now believe that electons are located in energy levels, sublevels, and orbitals.  Quantum mechanics further predicts the maximum number of electrons which can be held in the various energy levels.

·        1st energy level – 2 electrons

·        2nd energy level – 8 electrons

·        3rd energy level – 18 electrons

·        4th energy level – 32 electrons

b        Valence electrons – the electrons that are located in the outermost energy level of an atom are called the valence electrons.  The outermost energy level is called the valence shell.  Due to the overlap of the energy levels a maximum of 8 electrons can be held in the valence shell of an atom.

1        The number of electrons present in the valence shell of an atom can be determined from the Periodic Table in the NYS Reference tables.  The electron configurations in the lower left corner of each element give the electron arrangement for that atom.  The valence electrons are the electrons in the outermost level (the number furthest to the right in the configuration).  The chemical properties of an atom are very closely related to the number of valence electrons that the atom has.

2       Electron Dot Diagrams (Lewis Diagrams) – since the chemical properties of an atom depend on the number of valence electrons, electron dot diagrams have become very important in showing the number of valence electrons an atom has.  Below are several examples of electron dot diagrams.