Atomic Structure

What’s in an atom?An element is made up of one type of atom. For

example, the element gold is made up of only gold atoms.

Atoms are made up of smaller particles: protons, neutrons and electrons.

Protons and neutrons are found in the center of the atom, or the nucleus.

Electrons are found around the outside of the nucleus.

Protons have a positive charge.

Neutrons are neutral.

Electrons have a negative charge.

It is thought that protons and electrons are made up of even smaller particles called quarks.

As we learn more….The atomic model has changed a lot through the years. Democritus was the first scientist to propose that everything is made up of atoms. However, Aristotle disputed his theory and stated that matter was not made up of smaller particles. Perhaps Aristotle was more convincing or his ideas were just easier to believe, because for the next 2000 years, his incorrect theory was accepted.

Dalton’s TheoryIn the 1800’s, John Dalton was able to offer

proof that atoms exist. He thought of the atom as a solid sphere.

Thomson ModelJ.J. Thomson conducted experiments that

allowed him to infer that negatively charged particles (electrons) exist in an atom. He thought that they were evenly spaced, like chocolate chips in cookie dough.

Rutherford ModelRutherford suggested that most of the mass of an atom was found in the nucleus.

Bohr ModelNiels Bohr took things a step further by

hypothesizing that electrons are in motions and travel around the nucleus in fixed orbits.

Electron Cloud ModelCurrent research suggests that electrons do not

necessarily stay in fixed orbits; they just tend to stay in one area, but they can move around a bit!

Atomic MassThe nucleus of an atom is made up of protons

and neutrons, and makes up most of the mass.

The unit of measurement for atomic particles is atomic mass units (amu).

The mass of an individual proton or neutron is close to 1 amu.

A carbon atom has 6 protons and 6 neutrons, so its atomic mass is 12.

Atomic numberThe atomic number is the number of protons in

an atom.

A carbon atom has 6 protons, so its atomic number is 6.

Mass numberThe mass number of an atom is the number of

protons + the number of neutrons. This can be used to find the number of neutrons in an atom.

Number of neutrons = mass number – atomic number

IsotopesAtoms of the same element with different

numbers of neutrons are called isotopes.

Remember C-14 from last year, the radioactive element? It is an isotope of carbon. Normal carbon has 6 protons and 6 neutrons, but C-14 has 2 extra neutrons. This makes it unstable, or radioactive.