Atoms have a definite structure and are made up of protons, neutrons, and electrons.
An atom is the smallest unit of an element that still retains the chemical properties of that element. The structure of an atom is determined by its protons, neutrons, and electrons. This atomic structure gives rise to the chemical and physical properties of the element. Chemical properties refer to the properties that determine the chemical nature of matter. Examples of chemical properties are combustibility (how easily a material can burn) and the ability of a substance to react with other substances. For example, molecular hydrogen (H2) is combustible, carbon dioxide (CO2) is not. Physical properties do not involve chemical changes. Examples of physical properties are color, shininess, and freezing point. For example, water (H2O) is colorless and has a freezing point of 0°C.

The Structure of an Atom

Atoms consist of an atomic nucleus, containing the protons and neutrons, and an outer region, containing electrons.
Atoms have a nucleus. The atomic nucleus is the core of an atom that contains protons and neutrons. Hydrogen is an exception to this rule and sometimes has no neutrons in its atomic nucleus. A proton is a subatomic particle that has a positive charge. A neutron is a subatomic particle that has no charge. Atoms also contain electrons. An electron is a negatively charged subatomic particle that moves in orbitals around the atomic nucleus.
Protons and neutrons are the two types of subatomic particles found in the atomic nucleus. Electrons are the subatomic particles that move around the nucleus. Protons have a positive charge, while electrons have a negative charge. Neutrons do not have a charge.
Each subatomic particle has a defined amount of mass and electric charge. The mass is measured in atomic mass units (amu) or daltons. The mass of a single proton or a single neutron is about one dalton. Electrons have a mass that is much smaller than the mass of a proton and neutron. Thus, electrons contribute little to the overall atomic mass of an atom. But, because an electron is negatively charged, it does contribute to the atom's total charge.

The number of protons inside the nucleus of an atom determines the element. The atomic number of an element is the number of protons in an atom. For example, hydrogen's atomic number is one, and there is one proton in the hydrogen atom. An atom's mass number is a value that is equal to the number of protons and neutrons in an atom. Mass number is not the actual mass of an atom, but an approximation. For example, hydrogen's mass number is 1.00794. Some hydrogen atoms do not have neutrons, while some have more than one, so this number is an average of the mass of all hydrogen atoms.

Ions and Isotopes

Ions are charged particles formed when an atom gains or loses electrons. Isotopes are atoms of an element with different numbers of neutrons in their nucleus.
Atoms have the ability to gain or lose electrons. An ion is a charged particle formed when an atom gains or loses electrons. Atoms which neither gain nor lose electrons are neutral. When atoms gain extra electrons they become negatively charged ions. A negatively charged ion is called an anion. Chloride and fluoride are common anions. When atoms lose electrons they become positively charged ions. A positively charged ion is called a cation. Sodium and potassium are commonly found in their cation forms. Atoms are also able to form isotopes. Isotopes are atoms of an element with different numbers of neutrons in their nucleus; in other words, they have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are isotopes. Both isotopes have six protons, but carbon-12 has six neutrons, while carbon-14 has eight neutrons. Their mass numbers are not the same due to the different number of neutrons in each atom.
Atoms become ions by losing or gaining an electron. If an atom has more electrons than protons, it is a negatively charged ion-an anion. If an atom has more protons than electrons, it is a positively charged ion-a cation.

Electron Shells

Electron shells are the atomic location where electrons may be found. They fill from the nucleus outward and seek their most stable form.
Electron shells are the atomic location where electrons may be found. In 1913 Danish physicist Niels Bohr developed a model that represents the atomic structure of an atom and shows the location of the nucleus and electrons surrounding the nucleus. It also models the orbits where electrons are found. In the model, these orbits create electron shells, so the number of electrons in the outermost shell can be visualized. Electron shells are denoted with a number and the symbol "n."

Electron Orbits Using Bohr Model

A Bohr model of an atom shows the orbits where electrons are found in an atom. These orbits are known as electron shells. The letter "n" denotes the shell number. In this image, there are four shells surrounding the atomic nucleus.
Electrons fill each shell in a specific order. Electrons first begin filling shells closest to the nucleus. Next, electrons fill shells farther from the nucleus. Electrons found in the shell closest to the nucleus are at a lower energy state than electrons in the outermost shell. Each electron shell can only hold a certain number of electrons. The inner shell, closest to the nucleus, can only hold a maximum of two electrons. Subsequent shells can hold more. Atoms typically completely fill their outermost electron shell in order to achieve maximum stability. For example, sodium (Na) has 11 electrons. The first two completely fill the first shell. The next eight electrons fill the second shell. The remaining electron resides in the third shell, which is not completely filled.
Electrons fill electron shells from the one closest to the nucleus outward. Sodium's first shell is full, with two electrons, as is its second shell, with eight electrons. Its third shell, containing only one electron, is not full, as it can hold up to eighteen electrons.
The valence shell is the outermost electron shell of an atom, where valence electrons are found. Valence electrons typically follow the octet rule, which states that atoms react by gaining or losing electrons in order to form the most stable electron valence structure.

The Importance of Atomic Orbitals

Atomic orbitals are where the electrons are located at different energy levels.

Scientists are unable to determine the absolute location of an electron in space and time. However, electrons move within an atomic orbital—a region of space, with a specific shape, which surrounds the atomic nucleus. With the help of mathematical equations, scientists are able to use the orbitals to predict where an electron might be at any given time.

Electron shells are related to atomic orbitals. An electron shell consists of one or more subshells, designated by the letters s, p, d, and f. Oxygen, for example, contains s and p subshells. Subshells are further broken down into orbitals. The letter of the subshell indicates the shape of the orbital. For example, orbitals in s subshells are spherical, while orbitals in p subshells are shaped like dumbbells. Electron shells are the same for all elements, but their complexity increases as the number of electrons increases.

Electron Shells and Orbitals

Electron shells, where the electrons in an atom can be found, contain one or more subshells. Subshells are where the electron orbitals are found. An orbital's shape is indicated by a letter-s, p, d, or f. Here, the 1s orbital is shaped like a small sphere. The 2s orbital is a larger sphere. There are three 2p orbitals, each of which is shaped like a dumbbell. They align on each of the three different axes. The orbitals come together, and the space they occupy is where the electrons may be found in the atom.