1.2.1 the atomic model

chem modelsofparticulatenatureofmatter

Structure 1.2.1 - atoms contain a positively charged, dense nucleus composed of protons and neutrons (nucleons). negatively charged electrons occupy the space outside the nucleus

atoms:

• positively charged, dense nucleons composed of protons and neutrons (nucleons)
• negatively charged electrons occupy the space outside the nucleus

nuclear symbol denotes number of protons, neutrons, electrons, element

= mass number
= chemical symbol
= proton number/atomic number

e.g. represents an aluminium atom with 13 protons, 13 neutrons, and 13 electrons

since atoms have no overall charge, the positive charge of the protons must be balanced by the negative charge of the electrons. therefore, the atomic number is also equal to the number of electrons in an atom.

subatomic particles:

subatomic particle	relative mass	relative charge
proton	1	+1
electron	0.0005 (1/2000)	-1
neutron	1	0

the mass of the electron is small enough to be negligible in mass calculations, so it is ignored

previous atomic models

Dalton: fixed ratios / proportions of elements form molecules/compounds

he proposed that:

• all matter is composed of tiny indivisible particles called atoms
• atoms cannot be created or destroyed
• atoms of the same element are alike in every way
• atoms of different elements are different
• atoms can combine together in small numbers to form molecules

Thomson:

• cathode ray tube experiment
  • vacuum tube, low pressure gas between cathode and anode
  • when high voltage applied, a cathode ray (so called because it originated from the cathode) was produced, travelling towards the anode
• observed deflection of cathode ray towards positively charged metal plates
• observed deflection of cathode ray when in presence of magnets
• concluded that the cathode ray was composed of negatively charged particles
• observed that when a wheel was placed in the path, it spun
• concluded that the particles had some mass
• concluded that there was a positive counterpart since the atom is neutral

proposed the:

• plum pudding model of the atom
• negatively charged electrons positioned in a positively charged sponge-like substance

Rutherford:

• fired alpha particles at a piece of gold foil
  • done with a piece of radium that underwent alpha decay, shooting alpha particles (helium nuclei) at a piece of thin gold foil
  • sensors/detectors placed behind, all around gold foil
• observed a large number of undeflected particles
• observed small number of slightly deflected particles
• observed an even smaller number of particles that underwent large deflections
• concluded that an atom is mostly empty space

Bohr:

• pictured hydrogen atom as a nucleus with an electron moving in an orbit
• electrostatic force of attraction between oppositely charged subatomic particles prevents electron from leaving the atom
• nuclear radius and atomic radius - majority of atom is empty space
• without neutrons, the positively charged protons would mutually repel each other and the nucleus would fall apart

for limitations of the bohr model, see 1.3.3, 1.3.4, and 1.3.5 electron configuration

ions

• the number of protons identifies the element
• the number of protons and neutrons never changes during a chemical reaction

when the number of protons in a particle is no longer balanced by the number of electrons, the particle has a non-zero charge

when an atom gains electrons, it becomes an anion
when an atom loses electrons, it becomes a cation

e.g.

• an aluminium ion
• an oxide ion

challenge questions

1. It is now known that some of these substances are not elements but compounds. Lime, for example, is a compound of calcium and oxygen. Find any other examples of compounds in this list, and explain why the component elements had not been extracted at this time.

Potash - potassium metal is too reactive (will react with air and water)

2. The derivation of the Rutherford formula is based on the assumption that only electrostatic forces need to be considered during the scattering process. Suggest why the experimental results deviate from this model for high-energy alpha particles.

The kinetic energy of high-energy alpha particles may have resulted in contact with the positive gold nuclei the alpha particles reaching close enough to the gold nuclei for the strong nuclear force to come into action (with in ) which was not included in Rutherford’s model.

3. “Construct an equation for the collision of an electron and a positron to give two photons and explain why it is balanced.”

• charge is the same on both sides
• momentum will remain the same
• energy is the same since

4. “Explain why the 13 protons in aluminium stay in the nucleus despite their mutual repulsion.”

• strong nuclear force between protons and neutrons which is stronger than the electrostatic repulsive force
  • short-range, powerful attractive force
  • arises from interactions between quarks, gluons involved
  • much stronger than electromagnetic force at very short distances, but strength falls off after
  • acts only between immediate neighbours in the nucleus, not all nucleons simultaneously

5. “Experiments show the nuclear radius depends on the mass number according to the expression . Deduce an expression for the density of a nucleus and comment on your result.”

the density of a nucleus is the same regardless (independent) of the mass number, so it is the same for all atoms

6. We generally make the approximation that the mass of an ion is the same as that of the corresponding atom. To how many significant figures is this approximation valid for the ion?

The mass of the hydrogen atom would be so the approximation of the mass of its ion to would be accurate to 3 sig. figs.