The correct answer is: A. Electrons and protons
An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 picometers across. They are so small that accurately predicting their behavior using classical physics is impossible. Instead, quantum mechanics must be used to correctly describe and predict their behavior.
The center of an atom, called the nucleus, contains protons and neutrons. Protons have a positive electric charge, neutrons have no electric charge, and electrons have a negative electric charge. The number of protons in the nucleus is called the atomic number and determines the chemical element of the atom. For example, all carbon atoms have 6 protons in their nucleus. The number of neutrons in the nucleus is called the neutron number. Atoms of the same element with different numbers of neutrons are called isotopes. For example, carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.
The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force. The electrons are arranged in shells around the nucleus, with each shell having a maximum number of electrons. The first shell can hold up to 2 electrons, the second shell can hold up to 8 electrons, the third shell can hold up to 18 electrons, and so on. The electrons in the outermost shell are called valence electrons. The number of valence electrons determines the chemical properties of an atom.
For example, hydrogen has 1 proton in its nucleus and 1 electron in its outermost shell. This means that hydrogen has 1 valence electron. Oxygen has 8 protons in its nucleus and 8 electrons in its outermost shell. This means that oxygen has 8 valence electrons.
The electrons in an atom are constantly moving around the nucleus. They can move from one shell to another, but they can never be found in the nucleus. The electrons in an atom are what give it its chemical properties.