Here I will try to explain basically what makes elements unique, and some basic reactions they have with each other, it will help understand phrases I use later on. If you already have somewhat of a grasp on Atomic Physics, feel free to skip most of this.
To start, all of everything is composed of Atoms, which are the smallest possible particle of an element. Atoms consist of a core made of even smaller particles called Protons and Neutrons, and Electrons orbiting around this core. In general, a stable (not exploding or radioactive) Atom has one Electron for every Proton, and near the same number of Neutrons as Protons. The number of Protons alone defines what element an Atom is, because Neutrons and Electrons can be added or subtracted*. The number of Protons is called the "Atomic number". The numbers of Protons and Neutrons combined is called the "Atomic Mass"(Because electrons have practically no weight) , and defines how heavy one atom of the element is (However the weight you feel when picking up a bar of Iron is more defined by how close together the atoms are)Every element also has an "Atomic Symbol", which is just a short name composed of one, two, or three letters, example H is hydrogen, Li is Lithium, Cu is copper. Only the first letter is capitalized to avoid confusing in formulas like "KHO", one Potassium (Symbol is K) one Hydrogen (H) one Oxygen (O), as opposed to KHo, which would be one Potassium and one Holmium (Ho)
I'll give you some examples of basic atoms
Above is one of the most basic atoms, a Hydrogen atom. It has one Proton, (Atomic number of 1) no neutrons*, (Atomic mass of 1) and a single electron. Electrons will always arrange themselves into layers or "shells", and depending on how close to the core the shell is, a certain shell will always only be stable with a certain number of electrons in it. The comfortable number is the number of the shell (The first shell is the closest in), multiplied by itself, and then by two. Example shell 1, 1 times itself is 1, times two is 2. That's how many electrons it needs to be stable. Due to Hydrogen only having one electron, Hydrogen always wants one more electron, because of this, pure hydrogen is "diatomic", meaning it will bond to itself to form the H
2 molecule. If you add Oxygen and heat, the Hydrogen atoms will bond to the oxygen instead of each other(Oxygen will bond with 2 other atoms stably), forming water and an explosion! I'll do a longer post on Hydrogen later.
This is a slightly more advanced atom, Carbon. It's core is 6 Protons (Atomic number 6) and 6 Neutrons (Atomic mass 12), and because it's second shell which wants to have 8 electrons only has 4 available, it wants to share 4 electrons from someone. The electron shells (or layers) of this would be written 2, 4.
Now if you take 4 Hydrogen atoms (which want one shared electron each) and 1 Carbon (which wants 4 electrons, and has 4 to share), the Hydrogen will surround the Carbon and form a Molecule(any group of atoms) known as Methane gas. If you replace one of the Hydrogens with another Carbon, the added Carbon will take 3 more Hydrogens to fill it's bonds, and form Ethane gas. You can keep adding carbons and making longer chains basically forever! (You might wonder why this comes in handy for element collecting, I use it because I can't always get a pure metal, sometimes I have to settle for a compound(any group of elements) containing it instead.)
*If you add or subtract Neutrons, you're creating a different form (these forms are called "Isotopes") of an element that is either heavier or lighter. You can add(or subtract) Neutrons to any element, and this sometimes makes them Radioactive, meaning they slowly decay into a different element. There are some elements that have NO stable Isotopes, meaning they are always radioactive, no matter how many Neutrons. More on Radioactivity Here.