# Elements and the periodic table

The periodic table contains the list of all known elements, ordered in a particular manner. Let's take the element carbon as an example, the same element that makes up diamond. The carbon atom contains a nucleus which in turn contains protons and neutrons along with electrons outside of the nucleus. How is carbon different from any other element? Each element contains a specific number of protons. For carbon, this is 6 protons.

# Electric charges

Protons are positively charged whereas electrons are negatively charged, and neutrons are not charged. Elements or mixtures of elements (compounds) like to remain neutrally charged. Sometimes you do find charge imbalances. Think about static electricity. When you put clothes in the dryer, they rub each other and some clothes get an imbalanced positive charge, whereas others get an imbalanced negative charge. Due to this, when you touch a sweater from the dryer, you sometimes get shocked, as the charges flow through your fingers. Positive and negative charges are attracted to each other to mainain a net charge close to 0.

Since positive and negative charges are attracted to each other, electrons of an atom are attracted to the protons present inside the atomic nucleus. This attraction is called as Coulomb force. However, just as unlike charges (positive and negative) attract, like charges (positive-positive or negative-negative) repel. So how do protons in the nucleus manage to stay so compact and not fly away?

Turns out that protons and neutrons inside a nucleus are attracted to each other due to the presence of nuclear forces. The reason that a nucleus retains its identity as one unit is because the nuclear forces (binding the nucleons) are usually stronger than the electromagnetic forces (that repel the positively charged protons from one another).

# Chemical bonds

Until now, we talked about elements made of atoms, and how those atoms remain neutral by an equal number of protons and electrons. However, most materials do not remain in element form. In fact, there are only six occuring monoatomic (single atom) elements found under standard conditions. These are called noble gases and are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). So what about the rest?

Let's take water, one of the most popular chemical formulae out there - H2O. Water contains two Hydrogen and two Oxygen atoms bound together. While water is very common, it is actually a strange atypical liquid, and is one of the main reasons for life (that's why we are so obsessed to find water on other planets). Water expands when cooling contrary to other liquids, is extremely stable to external temperature fluctuations, the best solvent out there, and binds very well to surfaces such as the feet of insects. But I digress. The main point I'm making is water is a combination of elements, bound together through chemical bonds.

A chemical bond is a strong attraction between atoms, ions (charged atoms that are missing an electron or more) or molecules (already combined atoms for example Hydrogen gas H2) that results in the formation of chemical compounds/molecules. There are 4 main kinds of chemical bonds. An ionic bond results from an electrostatic force of attraction between oppositely charged ions. An example is common salt (NaCl)which is a combination of sodium (Na) and chlorine (Cl) atoms. A covalent bond is when the bonding involves sharing of pair of two electrons between atoms. An example of covalent bonding is Carbon dioxide (CO2) that contains Carbon (C) and Oxygen (O) atoms. The hydrogen bond is what connects water molecules together and gives water some of its unique properties. Finally, metallic bonds occur between positive charged metal ions attracted to a cloud of electrons, which cause large stable metal structures.

# Phases of matter

The last 2 sections talked about atoms and bonds between atoms. The arrangement of these bonded atoms or molecules leads to the emergence of matter that exists in three different phases, solids, liquids, and gases. Most matter can be thought of in terms of these phases. In addition, there is a fourth phase called plasma. Later in the textbook I'll dedicate a chapter to how most everyday materials are combinations of two (sometimes even all three) phases. But for now, let's focus on understanding distinct phases.

Generally, a material is considered to be a solid if it maintains a constant volume and shape and additionally, its constituent atoms/molecules do not easily change their relative positions when pressure is applied to the solid. Liquids are another category of matter which occupy a constant volume but changes its shape easily, liquids assume the shape of the container into which they are poured into. Matter in the gaseous state does not have constant volume or shape. Essentially, a gas state material assumes the share and volume of the container in which it is contained. Lastly, material in the plasma state also has a variable volume and shape (like gas) but it significantly differs from gas due to the presence of large number of freely moving charges (ions and electrons) in addition to neutral atoms. The presence of ions inside a plasma results in it being an electrically conductive (unlike gas). Common examples of solids include: rock, plastics, paper, wood, etc. and examples of liquids include tap water, milk, etc. Examples of gas include air (which is a combination of several other gas molecules like: nitrogen, oxygen, argon, carbon dioxide, etc.). The plasma state of matter is usually not present under normal environmental conditions on Earth. It is produced momentarily during events like lightning, electric sparks, or in plasma TVs. In order to go from one phase to another, you need to apply temperature or pressure. The phase diagram below shows how different phases exist at specific values of temperature and pressure.

In summary, materials are composed of atoms. These atoms are in turn bonded together through chemical bonds. Sometimes these atoms bind together to form molecules. Matter emerges from the arrangement of these bonded atoms or molecules. Finally, The reason materials are in different phases, is because of the combination of interatomic forces and temperature. At high enough temperatures, interatomic attractions are overcome by random thermal energy, and materials are liquids or gases. At lower temperatures or higher pressures, atomic forces overcome thermal energy, and tend to stay closer together, forming a solid. More will be discussed in the coming chapters.

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