# What Is Matter?

Look around you right now. Your desk, your chair, the air you are breathing, the water in your bottle: all of these things have something in common. They are all examples of matter. In science, matter is defined as anything that has mass and takes up space (that is, it has volume). Mass is the amount of "stuff" in an object, and volume is how much room that stuff occupies.

Matter includes every solid, liquid, and gas you can think of. Your textbook is matter. The milk in your cereal is matter. Even the air around you, which you cannot see, is matter because air is made of gases that have mass and take up space. If you have ever felt a gust of wind push against you, you have felt proof that air is matter.

### What Is NOT Matter?

Not everything you experience in daily life is matter. Light is not matter: it has no mass and does not take up space. Sound is not matter either: it is a vibration that travels through matter, but it is not matter itself. Heat and emotions are also not matter. A good rule of thumb: if it does not have mass and does not take up space, it is not matter.

### The Tiniest Pieces

Here is the big idea that drives all of chemistry and much of physics: all matter is made up of incredibly tiny particles called atoms. Every solid, every liquid, every gas in the universe is built from atoms. Your body, the food you eat, the water you drink, the ground beneath your feet: atoms, atoms, atoms.

But atoms were not always an accepted idea in science. The concept has a long history.

# Inside the Atom

Atoms are unimaginably small. How small? Consider this: a single grain of sand contains roughly 50 quintillion atoms. Written out as a number, that is 50,000,000,000,000,000,000 atoms in one tiny grain. You could never count them, even if you counted one atom per second for your entire life.

For a long time, scientists believed atoms were the smallest things in existence: solid, featureless spheres that could not be broken down any further. But starting in the late 1800s, a series of brilliant experiments revealed that atoms themselves are made of even smaller parts. These are called subatomic particles, meaning "smaller than an atom." There are three subatomic particles you need to know: protons, neutrons, and electrons.

## The Nucleus

At the very center of every atom is a tiny, incredibly dense region called the nucleus (plural: nuclei). The nucleus contains two types of subatomic particles packed tightly together:

- Protons carry a positive electrical charge (written as +1). Every atom has at least one proton. - Neutrons have no electrical charge (they are neutral, which is where the name "neutron" comes from). Neutrons help hold the nucleus together.

The nucleus is extraordinarily small compared to the whole atom, but it contains almost all of the atom's mass. Protons and neutrons are each about 1,836 times more massive than an electron.

## The Electron Cloud

Electrons are tiny particles that carry a negative electrical charge (written as -1). Electrons do not sit still inside the atom. Instead, they move rapidly in a region around the nucleus called the electron cloud. You can think of the electron cloud as the zone where electrons are likely to be found at any given moment, much like a swarm of bees buzzing around a hive.

## Electrical Balance

Here is an important rule: in a normal atom, the number of protons equals the number of electrons. Because each proton carries a +1 charge and each electron carries a -1 charge, these charges cancel out. The result is that a normal atom is electrically neutral, meaning it has no overall charge. For example, a carbon atom has 6 protons (+6 total) and 6 electrons (-6 total), so its overall charge is zero.

## Subatomic Particles at a Glance

The table below summarizes the three subatomic particles, where they are found, their electrical charge, and their relative mass.

| Subatomic Particle | Location | Electrical Charge | Relative Mass | |---|---|---|---| | Proton | Inside the nucleus | Positive (+1) | 1 (baseline) | | Neutron | Inside the nucleus | None (neutral, 0) | ~1 (about the same as a proton) | | Electron | Electron cloud (around the nucleus) | Negative (-1) | ~1/1836 (almost zero compared to protons) |

# Elements: Nature's Building Blocks

Now that you know atoms are made of protons, neutrons, and electrons, here is the next big question: what makes one type of atom different from another? The answer is the element.

An element is a pure substance made of only one type of atom. Gold is an element because every atom in a bar of pure gold is a gold atom. Oxygen is an element because every atom in a tank of pure oxygen gas is an oxygen atom. You cannot break an element down into simpler substances using ordinary chemical methods.

## The Atomic Number

What makes a gold atom different from an oxygen atom? The answer is simple but powerful: the number of protons.

The atomic number of an element is the number of protons in the nucleus of every atom of that element. The atomic number is what defines an element. Every atom with exactly 6 protons is carbon, no matter where in the universe you find it. Every atom with exactly 79 protons is gold. Change the number of protons, and you change the element entirely.

This is a fundamental rule of chemistry:

- All atoms of the same element have the same number of protons and are identical in their chemical behavior. - Atoms of different elements have different numbers of protons and behave differently.

## How Many Elements Are There?

Scientists have identified about 118 elements so far. Some of these, like oxygen, carbon, and iron, are found naturally on Earth. Others, like oganesson (element 118), are human-made in laboratories and exist for only fractions of a second.

All 118 elements are organized on a chart called the Periodic Table of the Elements, which you will study in detail in later lessons. For now, just know that the Periodic Table arranges elements by their atomic number and groups elements with similar properties together.

## Elements You Already Know

You interact with elements every single day, even if you do not realize it. Here are some of the most common ones:

| Element | Symbol | Atomic Number | Where You Find It | |---|---|---|---| | Hydrogen | H | 1 | Water (H₂O); the most abundant element in the universe | | Carbon | C | 6 | All living things; pencil graphite; diamonds | | Nitrogen | N | 7 | Makes up 78% of the air you breathe | | Oxygen | O | 8 | Makes up 21% of air; essential for breathing | | Iron | Fe | 26 | Steel, cars, bridges; carries oxygen in your blood | | Gold | Au | 79 | Jewelry, electronics, coins |

Notice that some element symbols match their English names (H for Hydrogen, C for Carbon), while others come from Latin or other ancient names (Fe for Iron comes from the Latin "ferrum"; Au for Gold comes from "aurum").

## You Are Made of Elements

Here is something remarkable: your body is made almost entirely of just a few elements. By mass, the human body is roughly 65% oxygen, 18% carbon, 10% hydrogen, and 3% nitrogen. The remaining 4% is a mix of calcium, phosphorus, potassium, and many other elements in small amounts. Every atom in your body was once part of something else, perhaps a rock, a star, or another living creature. The atoms simply rearranged.