2.1: Inorganic Chemistry Overview

Atoms are made up of particles called protons, neutrons, and electrons, which are responsible for the mass and charge of atoms.  ↵

  

Master this section and you'll be able to

• Define the term element and distinguish between atom and element.
• Describe the basic structure of an atom, including the relative location, charge, and mass of its subatomic particles.
• Distinguish between the terms atomic number and mass number.
• Explain what an ion is.

Atoms and Subatomic Structure: The Foundation of Matter

Matter is anything that has mass and takes up space. All things in the universe (i.e. living and non-living things) are considered to be "matter". The smallest component of matter normally found is the atom. Therefore, all things in the universe (including humans) are made of atoms.

Atoms themselves are composed of even smaller subatomic particles (extremely tiny things) called neutrons, protons, and electrons. 

• Protons have a positive electrical charge (+), 
• electrons have a negative electrical charge (-), and 
• neutrons are electrically neutral, meaning they have no charge.

The nucleus at the center of the atom contains protons (positively charged) and neutrons (no charge). The electrons (negatively charged) are found in regions called orbitals around the nucleus. Electrons do not travel in fixed paths but are described by a probability distribution. The orbitals define the regions where an electron is most likely to be found at any given time. The different orbitals are associated with specific energy levels. See the images below for a simplified view of an atom.

Figure \(\PageIndex{1}\): Structure of an Atom — Common Planetary Representations of an Atom.  The helium (He) atom in the image on the left is made up of 2 protons and 2 neutrons located within the nucleus. The two electrons are in orbitals surrounding the nucleus.  The image on the right depicts lithium (Li), which has 3 protons, 3 electrons, and typically 4 neutrons in its most common isotope (= version of an element that has the same number of protons but a different number of neutrons).

An atom in its neutral state has the same number of protons and electrons. Because protons carry a positive charge (+) and electrons carry a negative charge (–), their charges cancel each other out, making the atom electrically neutral.

For example, a neutral sodium atom (Na) has 11 protons and 11 electrons. The 11 positive charges balance the 11 negative charges, resulting in an overall charge of zero. The most common isotope of sodium has 12 neutrons. 

All atoms are made of subatomic particles — protons, neutrons, and electrons — but differ in the number of these particles, giving rise to 118 unique elements.

 

🔢 Atomic Number: The Element’s ID Card

All known elements in the universe are organized in a chart called the Periodic Table of Elements. The table arranges elements according to their chemical properties, their number of protons, and the way electrons are organized in orbital shells. Elements are symbolized by a chemical symbol (one- or two-letter abbreviation). Every element on the periodic table has a special number called the atomic number — it is usually shown above the element’s symbol (like the "7" above "N" for nitrogen).
That number tells us how many protons are in the nucleus of each atom of that element.

💡 Fun fact: Because atoms are electrically neutral, the number of electrons always equals the number of protons. So once you know the atomic number, you also know how many electrons there are!

For example:

• Nitrogen (N) has an atomic number of 7, so it has 7 protons and 7 electrons.

• Sodium (Na) has an atomic number of 11, so it has 11 protons and 11 electrons.

✅ This rule works for every element in the periodic table!

Figure \(\PageIndex{2}\):  Periodic Table of Elements. The periodic table shows the element symbols, atomic numbers, and mass numbers. The mass of an atom is often referred to as its atomic weight on Earth — since in most cases, we can think of atomic weight and mass number as being roughly the same.

Each element has its own unique physical and chemical properties — like how it looks, how it reacts, or what it can do. These properties come from the number of protons in the atom.

An atom with a specific number of protons is the smallest unit that still shows the characteristics of that element. That means all atoms of the same element have the same number of protons.

The figure below shows three different elements, each with its own number of protons — and its own personality, so to speak!

       

Figure \(\PageIndex{3}\): From left to right, images of the elements sodium (solid), nitrogen (liquid), and oxygen (colorless gas bubbles in the pale blue liquid). The element sodium is made of sodium atoms, the element nitrogen is made of nitrogen atoms, and the element oxygen is made of oxygen atoms.

  

⚖️ Atomic Mass: The Weight Behind the Atom

You now know that atoms are made up of three main subatomic particles: protons, neutrons, and electrons. But believe it or not, scientists have discovered many more subatomic particles beyond these three. Do not worry though — we will not dive into those here! 

Protons and neutrons each have a mass of approximately one atomic mass unit (amu) and are located at the center of the atom, forming the nucleus. Electrons occupy regions called orbitals or electron clouds surrounding the nucleus, organized into energy shells. Because electrons are so small — about 1/1836 the mass of a proton — their mass is often considered negligible (effectively zero) in most calculations.

When it comes to atomic mass (how “heavy” an atom is), only protons and neutrons really matter. We can therefore say, the number of neutrons affects the mass of the atom, but not its charge.

• A proton and a neutron each weigh about 1 atomic mass unit (amu) — basically the same weight.

• Electrons are like tiny lightweights — about 1/1800 the mass of a proton or neutron! Because they are so small, we usually ignore their mass when calculating the atom’s total mass.

 

➡️ Summary:

Atomic Number = Number of protons  

Atomic Mass =  Atomic Weight = Number of protons + Number of neutrons 

Charge of an Atom = protons (+) and electrons (−)  

Charge & Mass of Subatomic Particles
	Charge	Mass (amu)	Location
Proton	+1	1	nucleus
Neutron	0	1	nucleus
Electron	−1	0	orbitals

 

 

Atoms Are Mostly Empty Space… So Why Don't We Fall Through Things?

Believe it or not, over 99% of an atom is empty space! Protons and neutrons are packed into the tiny nucleus, and the much smaller electrons zoom around it — but most of the atom is just... nothing.

So, if atoms are mostly empty, why do solid things feel solid?

It all comes down to electrons. Electrons are negatively charged and hang out in the outer regions of atoms. When atoms get close, their electrons repel each other — kind of like trying to push the same poles of two magnets together.

These repelling forces (called intermolecular forces) keep atoms from overlapping. That is why you do not fall through your chair, even though both you and the chair are made mostly of empty space!

🔋 Sometimes atoms gain or lose electrons, becoming ions — charged versions of atoms. This changes how strongly they interact with other atoms and affects many things in biology, like nerve signals and muscle movement.

Interactive: Build an Atom: Build an atom out of protons, neutrons, and electrons, and see how the element, charge, and mass change. Then play a game to test your ideas!