Download Notes – Atomic Structure

Notes – Atomic Structure
- The basic building block of all matter is the atom. Atoms are composed of a center called a nucleus. The
nucleus is made of protons (positive charge) and neutrons (neutral charge). “Zipping” around the
nucleus are much smaller particles called electrons (negative charge). In a neutral atom the positive
charges of the protons balance with the negative charge of the electrons as there are the same
number of each. This results in a net charge of neutral.
- Protons are the particle that controls what the identity of each atom is. That is the number of protons
controls if an atom is carbon or gold. Change the protons equals change the atom. The number of
protons is the atomic number on the periodic table. The mass of one atom is decided by the protons
and the neutrons as they both have mass where as electrons are essentially mass less. The mass of the
atom is called the mass number.
- For each atom the mass was said to be the found by putting the mass or the protons and neutrons
together. This is true. However, each atom has more than one mass. The same kinds of atoms with
different masses are called isotopes. This can occur by having different numbers of neutrons. WE
CAN’T CHANGE PROTONS OR THE ATOM CHANGES!!! An example is carbon-12 and carbon-14.
Carbon normally has a mass number of 12 (6 protons and 6 neutrons). However, a second isotope of
carbon exists which has a mass number of 14 (6 protons and 8 neutrons). The nucleus in an isotope is
called a nuclide.
- Different isotopes are now written such that the reader can tell which is which. One writes the symbol
of the element with the atomic number in front as a subscript and the mass number in front as a
superscript.
Ex. – Carbon-12 and carbon-14 1
Decay
- Some types of atoms have nuclei that are not stable. That is the nucleus is unable to hold itself
together and as such is shoots out particles or energy as it falls apart. Atoms that do this are said to
be radioactive or undergo radioactive decay.
- Further investigation into radioactive decay discovered that there was three types of radiation that
could be released from the nucleus:
1.) Alpha (α) – is a helium nucleus being ejected from the nucleus. The largest of the three
types of radiation and known as α decay. When an atom shoots out an alpha particle
the protons decrease by 2 and the mass number decreases by 4 as an alpha particle
we said was a helium nucleus.
Ex. -
***If you add the right side numbers it always equals the left!!
2.) Beta (β) – is an electron
being ejected from the nucleus. An electron can come from the
nucleus because a neutron breaks apart into a proton and an electron and the electron
is shot out (along with an antineutrino). This does not change the mass as the neutron
changed into a proton which essentially has the same mass. The number of protons
changed so the atomic number and the identity of the atom must have changed. This is
called transmutation.
Ex. -
***If you add the right side numbers it always equals the left!!
3.) Gamma (γ) – is a higher energy photon (light energy) being ejected from the nucleus. Because γ
decay is energy, there is no mass change to the atom. This transmutation is shown
using a star to denote energy.
Ex. -
***If you add the right side numbers it always equals the left!!
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