Introduction to the atom | Chemistry of life | Biology | Khan Academy
Khan Academy・2 minutes read
The atom is the fundamental unit of matter, composed of protons, neutrons, and electrons, with its identity determined by the number of protons it contains, reflecting a complex structure that challenges the traditional notion of solidity. Modern atomic theory highlights the electron's probabilistic nature and emphasizes that atoms are largely empty space, with the nucleus occupying only a tiny fraction of the total volume.
Insights
- The atom, once thought to be the smallest indivisible unit of matter, is actually made up of smaller particles—neutrons, protons, and electrons—challenging the traditional notion of solidity and highlighting that atoms consist mostly of empty space, with the nucleus occupying only a tiny fraction of the atom's total volume.
- The identity of an element is determined by its number of protons, known as the atomic number, which directly influences its chemical properties and interactions; for instance, carbon has six protons and is distinct from beryllium, which has four protons, illustrating how even slight differences in proton count can result in entirely different elements.
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Recent questions
What is an atom in simple terms?
An atom is the basic unit of matter.
How do electrons behave in an atom?
Electrons exist in probability distributions around the nucleus.
What defines an element's identity?
An element's identity is defined by its protons.
What are isotopes of an element?
Isotopes are variations of the same element.
Why is atomic mass important?
Atomic mass determines the weight of an atom.
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Summary
00:00
Understanding Atoms and Their Structure
- The concept of the atom is central to chemistry and is philosophically intriguing, as it represents the smallest indivisible unit of matter, derived from the Greek word meaning "uncuttable."
- Philosophers theorized that if you keep cutting an apple into smaller pieces, you would eventually reach a point where it could not be divided further, which they termed an atom, applicable to all substances.
- Modern science has revealed that atoms are not indivisible; they consist of fundamental particles: neutrons, protons, and electrons, with neutrons and protons forming the nucleus at the center of the atom.
- A helium atom, for example, contains two protons and two neutrons in its nucleus, with two electrons orbiting around it, although the term "orbit" is misleading in the context of quantum mechanics.
- The traditional model of electrons orbiting the nucleus like planets around the sun is incorrect; instead, electrons exist in probability distributions known as orbitals, which describe where an electron is likely to be found.
- An electron's position and momentum cannot be precisely determined simultaneously; instead, we can only describe the probability of finding an electron in a certain area around the nucleus.
- The charge of particles is fundamental: electrons have a negative charge, protons have a positive charge, and neutrons are neutral, which influences their interactions and the structure of atoms.
- The identity of an element is determined by the number of protons it contains, known as the atomic number; for instance, an element with four protons is beryllium, while one with six protons is carbon.
- The mass number of an atom is the sum of its protons and neutrons; for carbon, which has six protons and six neutrons, the mass number is twelve, with each proton and neutron contributing approximately one atomic mass unit.
- In a neutral atom, the number of electrons equals the number of protons; thus, a neutral carbon atom with six protons will also have six electrons, maintaining charge neutrality.
13:51
Understanding Atomic Structure and Isotopes
- A neutral atom consists of six protons and six electrons, with six neutrons in its nucleus, resulting in a total atomic mass primarily derived from the nucleus, as the mass of one proton is equivalent to 1,836 electrons, making the electron mass negligible for atomic mass calculations.
- The atomic number of an element indicates the number of protons it contains; for example, oxygen has an atomic number of 8 (eight protons), and silicon has an atomic number of 14 (fourteen protons).
- The atomic weight of carbon is 12.0107, which represents the average mass of all carbon isotopes found on Earth, factoring in the relative abundance of carbon-12 (six protons and six neutrons) and carbon-14, which is less common.
- Isotopes are variations of the same element that differ in neutron count; carbon-12 and carbon-14 are both isotopes of carbon, with carbon-12 being the predominant form on Earth.
- The nucleus of an atom occupies approximately 1/10,000 of the atom's total volume, meaning that 99.999% of an atom is composed of empty space, challenging our perception of solidity in matter.
- The scale of atomic structures is illustrated by the helium atom, where the nucleus is about one femtometer in size, and one angstrom (1 x 10^-10 meters) equals 100,000 femtometers, emphasizing the vast amount of empty space within atoms and the materials we perceive as solid.
