MYP Integrated Sciences Atomic Structure Notes

The Modern Quantum Model of the Atom

1. The structure of the atom has fascinated scientists for centuries.
2. Over time, as new experiments and technologies were developed, our ideas about atoms have changed and improved.
3. The current quantum model of the atom is based on over a hundred years of experimental evidence.
4. It gives us a clearer picture of the tiny building blocks of matter and how they behave.

Early Atomic Models and Subatomic Particles

Dalton’s Atomic Theory – The First Modern Atomic Model

In the early 19th century, John Dalton proposed one of the first scientific models of the atom.

Dalton suggested that:

1. All matter is made up of tiny particles called atoms.
2. Atoms are indivisible – they cannot be split into smaller parts.
3. Atoms of the same element are identical and have the same mass and properties.
4. Atoms of different elements are different and have different masses.
5. A chemical reaction is simply a rearrangement of atoms; atoms are not created or destroyed in the process.

Discovery of Subatomic Particles

1. Later experiments in the late 19th and early 20th centuries showed that Dalton’s model was incomplete:
   1. Electrons were discovered (showing atoms are not indivisible).
   2. Protons and neutrons were discovered in the nucleus.
2. We now know that a typical atom contains three main subatomic particles:
   1. Protons
   2. Neutrons
   3. Electrons
3. These particles are the key building blocks of atomic structure.

The Three Main Subatomic Particles

1. The proton has a positive charge and is found in the nucleus.
2. The neutron has no charge (it is neutral) and is also found in the nucleus.
3. The electron has a negative charge and is found outside the nucleus, in regions we call energy levels or electron shells.

Atomic Mass Unit (u)

The masses of subatomic particles are extremely small, so we use a special unit called the atomic mass unit, symbol u.

Summary Table of Subatomic Particles

Atomic Number and Mass Number

Two key numbers describe an atom:

1. Atomic number (Z)
2. Mass number (A)

Atomic Number (Z)

1. If two atoms have different numbers of protons, they are different elements.
2. In a neutral atom, the number of electrons is equal to the number of protons, so: $$\text{Neutral atom} \implies \text{electrons} = \text{protons} = Z$$

Mass Number (A)

$$A=\text { number of protons }+ \text { number of neutrons }$$

We often call the number of neutrons $N$, so:

$$A=Z+N \quad \text { or } \quad N=A-Z$$

Knowing $Z$ and $A$ helps chemists to:

1. Describe the composition of an atom (how many protons, neutrons and electrons it has).
2. Predict how atoms might behave in chemical reactions.

The Nucleus

At the centre of every atom is a tiny region called the nucleus.

1. The nucleus contains protons and neutrons only.
2. These particles together are called nucleons.
3. The nucleus is very small compared with the whole atom, but it contains almost all of the atom’s mass.
4. Because it contains positively charged protons, the nucleus has an overall positive charge.

Nuclides and Isotopes

What Is a Nuclide?

1. A nuclide is a specific kind of atom defined by:
   1. its number of protons (atomic number, Z)
   2. its number of neutrons (which together with protons gives the mass number, A)
2. So a nuclide is a specific isotope of an element with a particular combination of protons and neutrons.
3. Atoms of the same element have the same number of protons, but can have different numbers of neutrons.
4. These different versions are called isotopes, and each isotope is one or more specific nuclides.

Nuclide Notation

We often write nuclides using the notation:

$$^A_Z E$$

where

1. $E$ is the chemical symbol of the element
2. $Z$ is the atomic number (number of protons)
3. $A$ is the mass number (total number of nucleons in the nucleus, namely, protons + neutrons)

Chemical Properties of Isotopes

The chemical properties of isotopes of the same element are almost identical because:

1. Chemical properties are mainly determined by the number and arrangement of electrons.
2. For a given element, all isotopes have the same number of protons and (in neutral atoms) the same number of electrons.

Isotopic Abundance and Relative Atomic Mass

Because elements usually exist as a mixture of isotopes, the relative atomic mass takes into account:

1. the mass of each isotope
2. the abundance (how common) each isotope is

Spectroscopy as Evidence for Quantized Energy Levels

What Is Spectroscopy?

1. Spectroscopy is the study of how light (electromagnetic radiation) interacts with matter.
2. Electrons in atoms can absorb or emit energy in the form of light when they change energy levels.
   1. When an electron absorbs energy, it moves from a lower energy level to a higher one (an excited state). This can produce an absorption spectrum.
   2. When an electron falls back from a higher energy level to a lower one, it releases energy as light. This produces an emission spectrum.
3. Each element produces a unique set of spectral lines because it has a unique set of allowed energy levels.