Matter and Energy: What are atoms?

Transcription

1 Matter and Energy: What are atoms? By Encyclopaedia Britannica, adapted by Newsela staff on Word Count 518 Level MAX An illustration of an atom. The nucleus, containing neutrons and protons, is at the center. Circling around it are electrons. Image from: Pixabay. The tiny particles called atoms are the basic building blocks of all matter. Atoms can be combined with other atoms to form molecules, but they cannot be divided into smaller parts by ordinary means. The word atom derives from the Greek word atomos, which means indivisible. The ancient Greeks were the first to think of the atom as the basic unit of all matter, but it was not until the early 1800s that scientists began to understand how atoms work. This article is available at 5 reading levels at 1

2 Structure Most of an atom consists of empty space. Its mass is concentrated in its center, which is called the nucleus. The nucleus consists of protons and neutrons. (The ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) Protons carry a positive electrical charge, while neutrons carry no electrical charge. Circling the nucleus is a cloud of negatively charged electrons. Scientists believe that subatomic particles protons, neutrons and electrons are themselves made up of smaller substances. The substances are called quarks and leptons. This article is available at 5 reading levels at 2

3 Properties The single most important thing to know about an atom is how many protons it has in its nucleus. This is known as its atomic number. The atomic number determines what kind of atom it is. Every atom is associated with a specific chemical element. An atom is the smallest unit of an element, and each chemical element has a unique atomic number. For instance, hydrogen has an atomic number of 1 because every hydrogen atom has one proton in its nucleus. No other element has an atomic number of 1. Two other related properties of atoms are the mass number and the atomic weight. The mass number is equal to the total number of protons and neutrons in an atom. The atomic weight is equal to the mass number divided by a certain number that scientists have come up with. The mass number and atomic weight are very similar. For example, for carbon, the mass number is 12, and the atomic weight is Atoms that have the same atomic number but different mass numbers are called isotopes. Carbon-12, the ordinary form of carbon, has six protons and six neutrons per atom. Carbon-14 is an isotope with eight neutrons per atom. It still has six protons. If it did not have six protons, it would not be carbon. An ordinary atom has an equal number of protons and electrons. So the positive and negative charges are balanced. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles. Ordinary atoms that either gain or lose electrons are called ions. If a neutral atom loses an electron, it becomes a positive ion. If it gains an electron, it becomes a negative ion. This article is available at 5 reading levels at 3

4 Quiz 1 Read the summary below. Choose the answer that BEST fits into the blank to complete the summary. Atoms are tiny particles, which are the building blocks of all matter. The number of protons an atom has is called its atomic number, which decides what kind of atom it is. Changes in the number of neutrons in an atom make an isotope and changes in the number of electrons make a positive or negative ion. An atom has a nucleus, made up of protons and neutrons, which is surrounded by a cloud of electrons. An atom has quarks and leptons, which make up the nucleus and are positively and negatively charged. An atom has an atomic mass, which measures the amount of electrons and an atomic weight, which measures protons. An atom has mostly empty space except for the nucleus, which contains nucleons, neutrons and electrons. 2 Which of the following paragraphs provides information that would be MOST useful for students to understand why protons are the most important subatomic particle for defining the atom? The nucleus consists of protons and neutrons. (The ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) Protons carry a positive electrical charge, while neutrons carry no electrical charge. The atomic number determines what kind of atom it is. Every atom is associated with a specific chemical element. An atom is the smallest unit of an element, and each chemical element has a unique atomic number. For instance, hydrogen has an atomic number of 1 because every hydrogen atom has one proton in its nucleus. No other element has an atomic number of 1. Atoms that have the same atomic number but different mass numbers are called isotopes. Carbon-12, the ordinary form of carbon, has six protons and six neutrons per atom. Carbon-14 is an isotope with eight neutrons per atom. An ordinary atom has an equal number of protons and electrons. So the positive and negative charges are balanced. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles.ordinary atoms that either gain or lose electrons are called ions. 3 Which paragraph from the section Properties helps the reader understand the MOST important role negatively charged subatomic particles play within the atom? This article is available at 5 reading levels at 4

5 4 Which sentence from the article helps the reader understand WHY isotopes have the same atomic number? Carbon-12, the ordinary form of carbon, has six protons and six neutrons per atom. If it did not have six protons, it would not be carbon. An ordinary atom has an equal number of protons and electrons. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles. This article is available at 5 reading levels at 5