Atomic Structure Textbook Pages 4-5

Lesson Objectives Appreciate that there are various models to illustrate atomic structure Know that early models of atomic structure predicted that atoms and ions with noble gas electron arrangements should be stable Describe the properties of protons, neutrons and electrons in terms of relative charge and relative mass Understand the importance of elementary particles in the structure of the atom Be able to recall the meaning of mass number (A) and atomic (proton) number (Z)

The History of the Atom As early as 400 B.C., some Greek philosophers proposed that matter is made of indivisible building blocks known as atomos. (Atomos in Greek means indivisible.) To these early Greeks, matter could not be continuously broken down and divided indefinitely. Rather, there was a basic unit or building block that was indivisible and foundational to its structure.

Boyle's studies (1661) of gaseous substances promoted the idea that there were different types of atoms known as elements. He suggested that elements were types of substances that could not be made simpler. Dalton (1803) conducted a variety of experiments to show that different elements can combine in fixed ratios of masses to form compounds. He suggested that elements were composed of indivisible atoms. All the atoms had the same mass. Henri Becquerel discovered radioactivity (1896). This showed that particles could come from inside the atom.

1897 English scientist J.J. Thomson's cathode ray experiments led to the discovery of the negatively charged electron and the first ideas of the structure of these indivisible atoms. Thomson proposed the Plum Pudding Model, suggesting that an atom's structure resembles the favourite English dessert - plum pudding. The raisins dispersed amidst the plum pudding are analogous to negatively charged electrons immersed in a sea of positive charge.

1911, Ernest Rutherford's famous gold foil experiments led to the nuclear model of atomic structure. Rutherford's model suggested that the atom consisted of a densely packed core of positive charge known as the nucleus surrounded by negatively charged electrons. While the nucleus was unique to the Rutherford atom, even more surprising was the proposal that an atom consisted mostly of empty space. Most of the mass was packed into the nucleus that was abnormally small compared to the actual size of the atom.

Neils Bohr improved upon Rutherford's nuclear model (1913) by explaining that the electrons were present in orbits outside the nucleus. The electrons were confined to specific orbits of fixed radius, each characterized by their own discrete levels of energy. While electrons could be forced from one orbit to another orbit, it could never occupy the space between orbits.

1916 Gilbert Lewis proposed that the inertness of noble gases was down to having a full outer shell of 8 electrons. Ions were formed by atoms gaining or losing electrons to attain full outer shells and thus become stable and that atoms could also bond by sharing electrons to form full outer shells Erwin Schrodinger devised an equation incorporating the idea that electrons had some of the properties of waves as well as those of particles 1932 James Chadwick discovered the neutron

Bohr's view of quantized energy levels was the precursor to modern quantum mechanical views of the atoms. Quantum mechanics suggests that an atom is composed of a variety of subatomic particles.

The modern idea of what an atom looks like (or what the exam board want you to describe it as)

The properties of subatomic particles

Extra notes A nucleon is a subatomic particle that is found in the nucleus (i.e. a proton or neutron) The force of attraction between the electrons and the protons is known as an electrostatic force Although you might expect the protons to repel each other as they are all positively charged, they are held together by the strong nuclear force We use relative masses and charges to describe subatomic particles as otherwise the numbers are ridiculously small