Atomic structure, particles, charges and masses Notes

How Does The Internal Structure Of An Atom Explain Charge, Mass, And Electrical Behaviour Of Matter?

1. Matter is made of atoms, and understanding what is inside an atom explains both everyday materials and many modern technologies.
2. Atomic structure is also a good example of how physics uses models: ideas that start simple, then improve when new evidence appears.

Atomic Models Changed As Evidence Improved

1. Early thinkers such as the ancient Greeks proposed that matter might be made of tiny pieces (atoms), but they did not have experimental evidence or a correct picture of different kinds of atoms.
2. By the late 18th century, chemists noticed that substances react in fixed ratios by mass.
3. This supported the idea that matter comes in discrete units, atoms, which combine in set patterns.
4. As more elements were discovered, chemists organized them in the periodic table, a model that groups elements with similar chemical properties.
5. In the 19th century, Mendeleev left gaps in his periodic table and used patterns to predict undiscovered elements.
6. Those predictions were later confirmed by discoveries such as gallium and germanium.

Atoms Are Not Indivisible: The Key Subatomic Particles

1. For a long time, atoms were thought to be fundamental (unable to be split).
2. This changed in the late 19th century with the discovery of the electron, showing that atoms contain smaller parts.
3. Modern atomic structure is usually introduced using a simplified Rutherford-style model:
   1. A tiny central nucleus contains protons and neutrons.
   2. Electrons occupy the space around the nucleus.
   3. The atom is mostly empty space because the nucleus is extremely small compared with the whole atom.

The Plum Pudding Model of the Atom

1. Scientists proposed that:
   1. The atom was a sphere of positive charge
   2. Negative electrons were embedded throughout this sphere
2. Because this resembled fruit scattered in a pudding, it became known as the plum pudding model
3. The model was accepted as it successfully explained the presence of electrons inside atoms and accounted for the overall neutral charge of atoms.

Limitations of the Plum Pudding Model

1. The model assumed that positive charge was spread evenly throughout the atom.
2. It did not include any region of high mass or high density.
3. As a result, it could not explain the strong deflection of fast-moving particles.

Rutherford’s Nuclear Model

1. Ernest Rutherford investigated how positive charge was arranged inside atoms.
2. He proposed testing atomic structure by firing fast-moving particles at thin metal foil.
3. The results of this experiment could not be explained by the plum pudding model.
4. Most alpha particles passed straight through the gold foil, showing that atoms are mostly empty space.
5. Some alpha particles were slightly deflected, suggesting the presence of a positive charge.
6. A very small number of particles were deflected through large angles, indicating a dense region inside the atom.

Description of Rutherford’s Nuclear Model

1. Rutherford proposed that atoms contain a tiny, dense, positively charged nucleus.
   1. The nucleus contains almost all the mass of the atom.
   2. Electrons are located outside the nucleus and occupy most of the atom’s volume.
2. The majority of the atom consists of empty space.

What Actually Are Atoms?

1. An atom is the smallest unit of an element that still keeps the properties of that element.
2. All substances, whether solid, liquid, or gas, are made up of atoms.
3. Atoms are extremely small and cannot be seen directly using ordinary microscopes.
4. Even though atoms make up solid objects, most of an atom is empty space.

Overall Structure of the Atom

1. An atom has a central nucleus surrounded by electrons.
2. The nucleus is very small compared to the overall size of the atom.
3. Almost all of the atom’s mass is concentrated in the nucleus.
4. Electrons move around the nucleus in specific regions called shells or energy levels.

Charge And Neutral Atoms: Why Protons And Electrons Must Balance

1. Electric charge is a fundamental property of matter (like mass).
2. Charge is measured in coulombs (C). The key idea for atoms is:
   1. Each proton has charge $+1.6\times 10^{-19}\,\text{C}$.
   2. Each electron has charge $-1.6\times 10^{-19}\,\text{C}$.
   3. Each neutron has charge $0$.
3. An atom is usually neutral, meaning its total charge is zero. Therefore, in a neutral atom: $$\text{number of electrons} = \text{number of protons}$$
4. This equality matters because:
   1. If an atom gains or loses electrons, it becomes an ion (charged atom).
   2. The nucleus is positive (because of protons), so it attracts electrons by an electrostatic force.

Mass Distribution: Why The Nucleus Contains Almost All The Mass

1. The nucleus contains essentially all the mass of the atom, even though it is about 100,000 times smaller than the atom's overall size.
2. This happens because:
   1. Protons and neutrons are much more massive than electrons (about 2000 times heavier than an electron).
   2. The electron mass is so small that electrons contribute less than 0.1% of the total mass of an atom.

Atomic Number, Mass Number, And Simple Notation

1. Physicists and chemists use two key counting ideas:
   1. Atomic number, $Z$: the number of protons in the nucleus.
   2. Mass number, $A$: the total number of nucleons (protons + neutrons).
2. So: $$A = Z + N$$ where $N$ is the number of neutrons.
3. A common notation is: $$^{A}_{Z}\text{X}$$ where X is the element symbol.

Isotopes: Same Element, Different Mass

1. Atoms of the same element always have the same number of protons.
2. However, they can have different numbers of neutrons.
3. These are called isotopes.

Why do isotopes have (almost) the same chemistry?

1. Chemical properties are mainly determined by electrons, especially the outer electrons.
2. Changing the number of neutrons does not change the number of electrons in a neutral atom.
3. Therefore, electron arrangement (configuration) stays the same, so chemical behavior stays the same.
4. The mass changes because neutrons add mass.

What Happens When Atoms Gain Or Lose Electrons?

1. A neutral atom has equal numbers of protons and electrons.
2. An ion forms when an atom gains or loses electrons.
3. Losing electrons creates a positive ion.
4. Gaining electrons creates a negative ion.

Why Electrons Move but Protons Do Not

1. Electrons are much lighter than protons.
2. Electrons are located outside the nucleus and are less tightly bound.
3. Protons are held strongly inside the nucleus.
4. This explains why charging, static electricity, and current flow all involve electrons.