An atom has three main parts: protons, neutrons, and electrons. Protons and neutrons form the nucleus at the center of the atom, while electrons occupy the region of space around the nucleus called the electron cloud.
Each particle has a different charge, mass, and role in determining the properties of an atom. The number of protons identifies the element, the number of neutrons determines the isotope, and the number of electrons affects the atom’s electrical charge and chemical behavior.
Key Takeaways: Parts of an Atom
- The three main parts of an atom are protons, neutrons, and electrons.
- Protons have a positive charge, neutrons have no net charge, and electrons have a negative charge.
- Protons and neutrons are found in the nucleus, while electrons occupy the electron cloud around the nucleus.
- The number of protons determines an atom’s element and atomic number.
- Changing the number of neutrons produces a different isotope of the same element.
- Changing the number of electrons produces an ion.
- Nearly all of an atom’s mass is concentrated in its nucleus.
Parts of an Atom
Atoms are the building blocks of matter. There are 3 main components of atoms:
- Protons – The number of protons in an atom is often denoted by the letter Z. Each proton carries a positive electrical charge. The number of protons determines the type of atom. For example, an element with 1 proton is hydrogen. One with 2 protons is helium. The number of neutrons and electrons don’t have any bearing on the type of atom. In fact, an atom does not even need neutrons or electrons. The most common form of hydrogen consists of a single proton and nothing else. A proton consists of component elementary particles: 2 up quarks and 1 down quark.
- Neutrons – The number of neutrons in an atom is commonly indicated using the letter N. A neutron is about the same size as a proton, but it is electrically neutral. Each neutron consists of 1 up quark and 2 down quarks.
- Electrons – Electrons are extremely tiny, compared with protons or neutrons. The mass of an electron is only 1/1836th that of a proton. Each electron carries a negative electrical charge. Each electron consists of a single elementary particle.
| Particle | Symbol | Charge | Approximate Mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 amu | Nucleus |
| Neutron | n⁰ | 0 | 1 amu | Nucleus |
| Electron | e⁻ | −1 | 1/1836 amu | Electron cloud |
Protons and neutrons have similar masses, while an electron has only about 1/1836 the mass of a proton. Because electrons contribute very little mass, nearly all of an atom’s mass comes from its nucleus.
The atomic mass number of an atom is denoted by the symbol A and equals the sum of the number of protons and neutrons or Z + N.
Where Are the Parts of an Atom Located?
The three main subatomic particles occupy two general regions of the atom: the nucleus and the electron cloud.
- Atomic Nucleus – Protons and neutrons collectively are termed nucleons because they bind together to form the nucleus of each atom.
- Electron Cloud – The electron shells describe regions in which an electron is most likely to be located. The innermost shell, the K shell, holds up to 2 electrons. The next shell, the L shell, holds up to 8 electrons. In reality, electrons swiftly spin around the atomic nucleus, but they do not have a well-defined orbit, like the Earth around the Sun. An electron could be just about anywhere (including, briefly, inside the nucleus). The high kinetic energy of the particles forms a sort of outer layer or cloud around an atom. Atoms appear solid, like little balls, because of the motion of electrons. It’s like how a fan blade looks like a solid disc when the fan is in motion.
Opposites Attract
Opposite electrical charges attract each other, so the protons and electrons are drawn toward each other. They don’t meet because the electrons are moving too quickly. It’s sort of like the Moon and the Earth. The Moon is drawn toward the Earth by gravity, but the two bodies don’t crash into each other because of their motion. The Moon is constantly falling around the Earth like electrons are falling around the atomic nucleus.
Atomic Number and Mass Number
Three numbers describe the numbers of particles in an atom:
- Atomic number (Z) = number of protons
- Neutron number (N) = number of neutrons
- Mass number (A) = number of protons + number of neutrons
Therefore:
A = Z + N
and:
N = A − Z
For a neutral atom:
number of electrons = number of protons = Z
For an ion, the number of electrons differs from the number of protons.
Ions and Isotopes
Changing the number of parts of an atom produces ions and isotopes.
Ions – An ion forms when the number of electrons is different from the number of protons. If there are more protons than electrons, a positively-charged ion called a cation forms. If there are more electrons than protons, the net charge is negative and the species is called an anion. Ions can be formed from single atoms or bound groups of atoms. An ion of a single atom is called an atomic ion.
Isotopes – Isotopes are atoms with the same number of protons, but different numbers of neutrons. There are stable isotopes, where the numbers of protons and neutrons remains the same over time, or radioisotopes. A radioisotope is an unstable or radioactive isotope, which will decay into a more stable isotope, releasing energy and sometimes particles.
Example: Finding the Parts of an Atom
Consider a neutral atom of sodium-23. Sodium has atomic number 11.
Protons: 11
Neutrons: 23 − 11 = 12
Electrons: 11
So, a neutral sodium-23 atom contains 11 protons, 12 neutrons, and 11 electrons.
If the atom loses one electron to form Na⁺, the nucleus does not change. The ion still has 11 protons and 12 neutrons, but now it has 10 electrons.
How Much of an Atom Is Empty Space?
Most of an atom’s volume lies between the extremely small nucleus and the outer extent of its electron cloud. For this reason, atoms are often described as being “mostly empty space.”
However, this phrase can be misleading. The space around the nucleus contains electron probability distributions and electromagnetic fields. Matter also feels solid not because rapidly moving electrons form a solid barrier, but largely because of electromagnetic interactions and quantum effects between the electrons in neighboring atoms.
Common Misconceptions About the Parts of an Atom
Electrons orbit the nucleus like planets orbit the Sun.
The Bohr model often draws electrons this way, but modern quantum mechanics describes electrons using orbitals and probability distributions rather than fixed paths.
Protons and neutrons have exactly the same mass.
Their masses are similar, but a neutron is slightly more massive than a proton.
Every atom has neutrons.
The most common isotope of hydrogen, protium, has one proton and one electron but no neutron.
Atoms are solid balls.
Atoms have a tiny nucleus surrounded by a much larger electron cloud. Their apparent solidity results from interactions between atoms rather than atoms being solid spheres.
Changing any subatomic particle changes the element.
Only changing the number of protons changes the element. Changing neutrons changes the isotope, while changing electrons changes the ion.
Frequently Asked Questions
What are the three main parts of an atom?
The three main parts of an atom are protons, neutrons, and electrons. Protons and neutrons are in the nucleus, while electrons occupy the electron cloud around it.
Which part of an atom determines the element?
The number of protons determines the element. This number is also the element’s atomic number.
Which part of an atom has the most mass?
Protons and neutrons account for nearly all of an atom’s mass. Electrons have much less mass.
Which part of an atom determines its charge?
The balance between protons and electrons determines the net electrical charge. A neutral atom has equal numbers of protons and electrons.
Can an atom have no neutrons?
Yes. Protium, the most common isotope of hydrogen, has one proton and no neutrons.
Can an atom have no electrons?
A completely ionized atom can lose all of its electrons, leaving a bare nucleus. For example, H⁺ is simply a proton in the usual chemistry context.