At the most basic level, your body - and, in fact, all of life, as well as the non-living world - is made up of atoms, often organized into larger structures called molecules. An atom is the smallest unit of matter that retains all of the chemical properties of an element. It is made up of smaller fundamental particles; protons, neutrons and electrons.
For example, a gold coin is simply a very large number of gold atoms molded into the shape of a coin, with small amounts of other, contaminating elements. Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold. A gold atom gets its properties from the tiny subatomic particles it's made up of.
In Greek the prefix a means not and the word tomos means cut. Atom is derived from the greek word atomos meaning uncuttable or indivisible.
Matter and elements
The term matter refers to anything that occupies space and has mass, All matter is made up of substances called elements, which have specific chemical and physical properties.
There are 118 elements, but only 92 occur naturally. The remaining elements have only been made in laboratories and are unstable.
Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols come from their Latin names; for example, the symbol for sodium is Na, which is a short form of natrium, the Latin word for sodium.
The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N), which together make up about 96% of the human body. All elements and the chemical reactions between them obey the same chemical and physical laws, regardless of whether they are a part of the living or nonliving world.
Structure of the atom
Two of the sub-atomic partices are held in the nucleus, which is in the center of the atom. It contains (positive) protons and (neutral, uncharged) neutrons. The much larger region of the atom holds electrons (negative) particles that move around the nucleus. Electrons are held in shells: the first shell occupies 2 electrons at most and the second, (third and so on) shells carry maximum of 8 electrons.
The attraction between the positively charged protons and negatively charged electrons holds the atom together. Most atoms contain all three of these types of subatomic particles however, Hydrogen (H) is an exception because it typically has one proton and one electron, but no neutrons.
The number of protons in the nucleus determines which element an atom is, while the number of electrons surrounding the nucleus determines which kind of reactions the atom will undergo.
Mass and space
Protons and neutrons have approximately the same mass. Since grams are not a very convenient unit for measuring masses that tiny, scientists chose to define an alternative measure, the dalton or atomic mass unit (amu). A single neutron or proton has a weight very close to 1 amu. Electrons are much smaller in mass than protons, only about 1/1800 of an atomic mass unit, so they do not contribute much to an element’s overall atomic mass. On the other hand, electrons do greatly affect an atom’s charge, as each electron has a negative charge equal to the positive charge of a proton.
Protons, neutrons, and electrons are very small, and most of the volume of an atom - greater than 99 percent - is actually empty space. With all this empty space, you might ask why so-called solid objects don’t just pass through one another. The answer is that the negatively charged electron 'clouds' of the atoms will repel each other if they get too close together, resulting in our perception of solidity.
Atoms of each element contain a characteristic number of protons. In fact, the number of protons determines what atom we are looking at (e.g., all atoms with six protons are carbon atoms); the number of protons in an atom is called the atomic number.
In contrast, the number of neutrons for a given element can vary. Forms of the same atom that differ only in their number of neutrons are called isotopes.
Together, the number of protons and the number of neutrons determine an element’s mass number: mass number = protons + neutrons. If you want to calculate how many neutrons an atom has, you can simply subtract the number of protons, or atomic number, from the mass number.
The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units (amu). By definition, an atom of carbon with six neutrons, carbon-12, has an atomic mass of 12 amu. Other atoms don’t generally have round-number atomic masses for reasons beyond basic gcse knowledge. In general, though, an atom's atomic mass will be very close to its mass number, but will have some deviation in the decimal places.
As mentioned, isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Many elements - such as carbon, potassium, and uranium - have multiple naturally occurring isotopes. Carbon-12 contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 (six protons plus six neutrons). Carbon-14 contains six protons, eight neutrons, and six electrons; its mass number is 14 (six protons plus eight neutrons).
These two alternate forms of carbon are isotopes. Some isotopes are stable, but others can emit, or kick out, subatomic particles to reach a more stable, lower-energy, configuration. Such isotopes are called radioisotopes, and the process in which they release particles and energy is known as decay. Radioactive decay can cause a change in the number of protons in the nucleus; when this happens, the identity of the atom changes (e.g. carbon-14 decaying to nitrogen-14).
The periodic table
Groups and periods
A vertical column of elements in the periodic table is a group. The elements in a group have similar chemical properties to each other. This is because their atoms have the same number of electrons in the highest occupied energy level. Group 1 elements are reactive metals called the alkali metals. Group 0 elements are unreactive non-metals called the noble gases.For example, group 1 contains sodium and other very reactive metals, while group 7 contains chlorine and other very reactive non-metals. Group 0 (also known as group 8) contains helium and other very unreactive non-metals.
The horizontal rows are called periods, there are only two elements in Period 1 (hydrogen and helium). The period number tells us the number of shells of an element.
New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds, which involves atoms transferring or sharing electrons.
For example, carbon and oxygen react together to form the compound we know as carbon dioxide. Compounds usually have different properties from the elements they contain.
Chemical bonds involve electrons from the reacting atoms. Compounds formed from metals and non-metals consist of ions. Ions are charged particles that form when atoms (or clusters of atoms) lose or gain electrons:
- metal atoms lose electrons to form positively charged ions
- non-metal atoms gain electrons to form negatively charged ions
The ionic bond is the force of attraction between the oppositely charged ions.
Compounds formed from non-metals consist of molecules. The atoms in a molecule are joined together by covalent bonds. These bonds form when atoms share pairs of electrons.
The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound. Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide. Word equation for the reaction: copper + oxygen → copper oxide
Copper and oxygen are the reactants, they are on the left of the arrow. Copper oxide is the product, it is on the right of the arrow. If we just replace the words shown above by the correct chemical formulas, we will get an unbalanced equation, as shown here:
Cu + O2 → CuO
Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides. Here is the balanced symbol equation:
2Cu + O2 → 2CuO
Conservation of mass
The law of conservation of mass says that matter is not created or destroyed in a closed system. That means if we have a chemical reaction, the amount of each element must be the same in the starting materials and the products. We use the law of conservation of mass every time we balance equations!