Matter

An atom is the smallest unit that retains the identity of an element. Different substances are made from different kinds of particles: an oxygen molecule has two oxygen atoms, while sodium chloride is an extended arrangement of ions. Inside an atom, a tiny positive nucleus contains protons and, except for the most common hydrogen isotope, neutrons. Electrons occupy quantum states around the nucleus. We describe their likely locations with orbitals, rather than little planetary paths.

Rutherford and Bohr together made the Rutherford-Bohr atomic model, which is still used today.
During the experiment, some alpha particles were reflected right back at the observer, leading Rutherford to comment on the shocking nature of the experimental results, that “it was like shooting a bullet at a piece of tissue paper, and the bullet bounced back and hit you.”

The Four States of Matter

All forms of matter, whether compounds, elements, or mixtures, exist in a state of being either solids, liquids, gases, or plasma.

Solids are rigid, have a fixed shape, and fixed volume.

Liquids have a fixed volume, but no fixed shape. They assume the shape of the container they are placed in.

Gases have neither a fixed shape nor a fixed volume. They can expand or contract depending on the pressure and volume, and follow their own set of rules, discussed in Chapter 3.

There is also a state called plasma. A plasma contains enough mobile electrons and ions to respond strongly to electric and magnetic fields. It may be only partly ionized; not every electron has to be stripped from every atom. Although it contains charged particles, a large plasma can be approximately neutral overall because positive and negative charges balance. Lightning and many stars give us examples.

Pure Substances and Mixtures

As discussed earlier, pure substances can be either elements or compounds. Chlorine gas is an example of an element. Table salt is an example of a compound. Both of them are pure substances.

This is because no matter how hard we try to break the salt, it will always stay NaCl. Sugar is also a pure substance.

Is Iron a pure substance? What about steel?

As opposed to pure substances, mixtures are a combination of two or more pure substances that do not react with each other. In our everyday life, most substances we encounter are mixtures. Depending on which state of matter these substances are in, each mixture can have a different name.

A solution is a homogeneous mixture: at the scale of a representative sample, its composition is uniform. The solvent is the medium in which solutes are dispersed. Water with dissolved salt is familiar, but solutions can also be gases or solids. Air is a gas mixture, and some alloys are solid solutions. A gas such as carbon dioxide can be a solute in liquid water. Do not use the word solution as a synonym for liquid.

• Term	Definition
  Suspension	A heterogeneous mixture where setting can occur over time (ex: sand in water)
  Emulsion	A suspension (usually liquid in liquid) with distinctly different phases (ex: oil in water)
  Colloid	A heterogeneous mixture that is so fine that it almost looks homogeneous (ex: milk, paint)

Different types of colloids

Colloids can be further subdivided based on their bulk material’s phase and suspended material’s phase.

Note: Some of the names are repeated (Sol, foam, and aerosol), so in order to distinguish the different types, they are often referred to by their distinguishing phase (solid or liquid).

Example Problem 1.1.1

What would a substance consisting of both sodium and chloride atoms be classified as?

Elements usually exist as a single atom. Simply put, they are all the “flavors” that atoms come in, and as far as we know, the ones that have been found are the only ones that exist in the universe. They each have their individual configurations and typically hold a neutral net charge. That means that there is an equal number of protons compared to electrons. However, there are cases where they are not equal. Atoms that experience this are called ions.

Let us suppose we have an atom that has 1 electron and 1 proton (for now we disregard the neutron because it would not affect the net charge). Think of it like this, -1 + 1 = 0, therefore no charge. If this atom lost that 1 electron, it would become positively charged - a cation. Similarly, if the original atom gained an electron, it would become negatively charged - an anion.

When an atom loses electrons, it becomes more positive.

Example Problem 1.1.2

What is the smallest component of any given substance?

This mystery atom, however, is that of hydrogen. It is an element that is found everywhere, and is theorized to make up 75% of the universe. It is in your body, it is in water, it is in the sun and is one of the most important ingredients in keeping it burning.

The negatively charged hydrogen ion is denoted as H-, also known as hydride. The positively charged hydrogen is similarly written as H+. We will review element naming when the periodic table is introduced, but the concept of writing the charge above the element symbol is easy to grasp. For example, an atom of chromium that lost 3 electrons is written as Cr+3.

Apart from ions, an element might also exist with a neutral charge (equal number of protons and electrons), but a differing number of neutrons. This will result in an isotope. These isotopes exhibit different properties, different uses, and have different masses.

We will later revisit the idea of ions, isotopes, and atomic particles (neutrons, protons, and electrons) and see that they control many properties of whatever piece of matter they are a part of.