An atom has structure
An atom has a tiny nucleus containing positively charged protons and, in most atoms, uncharged neutrons. Negatively charged electrons occupy regions around the nucleus. Nearly all of an atom’s mass is concentrated in its nucleus, while the atom as a whole occupies much more space. In a neutral atom, the number of electrons equals the number of protons, so their charges balance. Diagrams showing electrons on tidy circular tracks are useful models for some introductory tasks, but they do not describe the full quantum behavior of electrons.
Atomic number defines the element
The atomic number is the number of protons in an atom’s nucleus. Every carbon atom has 6 protons; an atom with 7 protons is nitrogen instead. The mass number is the total number of protons and neutrons in one atom. A carbon-14 atom has 6 protons and 8 neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Do not confuse mass number with the decimal atomic mass often printed on a periodic table: that value reflects isotope masses and their relative abundances.
Ions and chemical bonds
An ion is an atom or group of atoms with an overall electrical charge. If a neutral sodium atom loses one electron, it becomes a sodium ion with a +1 charge. Losing a negative electron makes the charge more positive. Atoms can also gain electrons and become negatively charged. In an ionic substance, oppositely charged ions attract one another. In a covalent bond, atoms share electron pairs. These are introductory models; bonding varies across substances, and not every material is made of separate molecules. Solid sodium chloride forms an extended ionic lattice.
Elements, compounds, and mixtures
An element contains only one type of atom, defined by its proton count. A compound contains two or more elements chemically combined in fixed proportions. Water, H₂O, contains hydrogen and oxygen atoms in a 2:1 ratio. A mixture contains substances together without requiring a fixed chemical ratio; air is a mixture of gases. A physical separation, such as filtering an insoluble solid from water, can separate some mixtures. Separating a compound into different substances requires chemical change. During ordinary chemical reactions, atoms are rearranged and total mass is conserved in a closed system.
Further reading
These resources offer a fuller treatment of the underlying concepts. The lesson, quiz wording, and flashcards here were created for this project.
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