Elements and Atoms

Core Concept

Matter is anything that has mass and takes up space. All matter is composed of elements — substances that cannot be broken down into simpler substances by ordinary chemical reactions. There are 92 naturally occurring elements, but life is chemically lopsided: just four of them — oxygen (O), carbon (C), hydrogen (H), and nitrogen (N) — make up roughly 96% of the mass of living organisms. The smallest unit of an element that still retains that element's properties is the atom.

An atom has a tiny, dense core called the nucleus, which contains positively charged protons and electrically neutral neutrons. Around this nucleus orbits a cloud of negatively charged electrons. It is the number of protons that defines which element an atom is, and it is the arrangement of the electrons — especially the outermost, "valence" electrons — that determines how that atom will interact with other atoms. Everything else in biology, from the shape of a water molecule to the structure of DNA, traces back to this atomic-level accounting.

Key Concepts

Structure of the Atom

Protons carry a positive charge (+1) and sit in the nucleus. Neutrons carry no charge and also sit in the nucleus. Together, protons and neutrons account for nearly all of an atom's mass. Electrons carry a negative charge (−1) and occupy regions of space called electron shells surrounding the nucleus; they have only about 1/1836 the mass of a proton, so they contribute almost nothing to atomic mass. In an electrically neutral atom, the number of electrons equals the number of protons.

Atomic Number and Mass Number

The atomic number (Z) is the number of protons in the nucleus, and it uniquely identifies the element — every carbon atom has 6 protons, every oxygen atom has 8. The mass number (A) is the total number of protons plus neutrons. Because neutrons do not affect charge, atoms of the same element can have different mass numbers; these variants are isotopes. The atomic mass listed on the periodic table is the weighted average of an element's naturally occurring isotopes.

Electron Shells and Valence Electrons

Electrons occupy discrete energy levels, or shells. The first shell holds up to 2 electrons; the second holds up to 8. Electrons fill lower-energy (inner) shells before higher-energy (outer) shells. The electrons in the outermost shell are valence electrons, and they govern chemical reactivity. Atoms are most stable when their outer shell is full — 2 electrons for hydrogen and helium, 8 for most other atoms (the "octet rule") — so atoms tend to gain, lose, or share electrons to complete that shell.

Isotopes and Radioisotopes

Isotopes of an element have the same number of protons but different numbers of neutrons. Some isotopes are stable; others are unstable and decay spontaneously, releasing energy and subatomic particles — these are radioisotopes. The time it takes for half of a sample to decay is its half-life. Radioisotopes are powerful tools: carbon-14 dating estimates the age of fossils, and short-lived tracers injected into patients reveal active tissues in positron emission tomography (PET) scans or track thyroid uptake of iodine-131.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. Oxygen is highly electronegative, followed by nitrogen, while carbon and hydrogen are relatively weak. Differences in electronegativity between two bonded atoms determine whether a bond is nonpolar covalent, polar covalent, or ionic — the topic of the next note.

How It Works

Atoms interact by adjusting their valence electrons. An atom with an almost-full outer shell, such as chlorine (7 valence electrons), strongly attracts an extra electron, whereas an atom with a nearly empty outer shell, such as sodium (1 valence electron), easily gives one up. When sodium and chlorine meet, sodium donates its lone valence electron to chlorine: sodium becomes a positively charged ion (Na⁺) and chlorine becomes a negatively charged ion (Cl⁻), and the opposite charges bind them into table salt. This is one example of how valence electrons drive the cause-and-effect chain that produces molecules. More generally, the same drive to complete the outer shell produces sharing arrangements (covalent bonds), which we examine next.

How it works

Atoms interact by adjusting their valence electrons. An atom with an almost-full outer shell, such as chlorine (7 valence electrons), strongly attracts an extra electron, whereas an atom with a nearly empty outer shell, such as sodium (1 valence electron), easily gives one up. When sodium and chlorine meet, sodium donates its lone valence electron to chlorine: sodium becomes a positively charged ion (Na⁺) and chlorine becomes a negatively charged ion (Cl⁻), and the opposite charges bind them into table salt. This is one example of how valence electrons drive the cause-and-effect chain that produces molecules. More generally, the same drive to complete the outer shell produces sharing arrangements (covalent bonds), which we examine next.

Common confusions

• "Atomic mass equals mass number." Wrong. Mass number is a whole number for one specific isotope (protons + neutrons); atomic mass is a weighted average across all isotopes, which is why carbon's atomic mass is 12.01, not 12.
• "Neutrons are charged like protons." Neutrons are neutral (hence the name); only protons and electrons carry charge.
• "Adding an electron changes the element." It does not. Gaining or losing electrons creates an ion of the same element, because the proton count — the element's identity — is unchanged.
• "Electrons sit still in fixed orbits." Electrons occupy probability regions (orbitals/shells); the simple ring picture is a model, not a literal photograph.
• "All isotopes are radioactive." No — many isotopes are stable (e.g., carbon-12 and carbon-13); only unstable ones are radioisotopes.

Quick review

• Matter is composed of elements; an element's smallest unit is the atom.
• Protons (positive) and neutrons (neutral) form the nucleus; electrons (negative) form the outer cloud.
• Atomic number identifies the element; mass number = protons + neutrons.
• Valence electrons in the outermost shell govern bonding and reactivity.
• The octet rule explains why atoms gain, lose, or share electrons.
• Isotopes vary in neutrons; radioisotopes decay and are used in medicine and dating.
• Electronegativity predicts how strongly an atom pulls on shared electrons.